Effect of lung volume on the respiratory action of the canine pectoral muscles.

Lung volume influences the mechanical action of the primary inspiratory and expiratory muscles by affecting their precontraction length, alignment with the rib cage, and mechanical coupling to agonistic and antagonistic muscles. We have previously shown that the canine pectoral muscles exert an expiratory action on the rib cage when the forelimbs are at the torso's side and an inspiratory action when the forelimbs are held elevated. To determine the effect of lung volume on intrathoracic pressure changes produced by the canine pectoral muscles, we performed isolated bilateral supramaximal electrical stimulation of the deep pectoral and superficial pectoralis (descending and transverse heads) muscles in 15 adult supine anesthetized dogs during hyperventilation-induced apnea. Lung volume was altered by application of a negative or positive pressure (+/- 30 cmH2O) to the airway. In all animals, selective electrical stimulation of the descending, transverse, and deep pectoral muscles with the forelimbs held elevated produced negative intrathoracic pressure changes (i.e., an inspiratory action). Moreover, with the forelimbs elevated, increasing lung volume decreased both pectoral muscle fiber precontraction length and the negative intrathoracic pressure changes generated by contraction of each of these muscles. Conversely, with the forelimbs along the torso, increasing lung volume lengthened pectoral muscle precontraction length and augmented the positive intrathoracic pressure changes produced by muscle contraction (i.e., an expiratory action). These results indicate that lung volume significantly affects the length of the canine pectoral muscles and their mechanical actions on the rib cage.     

Neurovascular congruence results from a shared patterning mechanism that utilizes Semaphorin3A and Neuropilin-1

Peripheral nerves and blood vessels have similar patterns in quail forelimb development. Usually, nerves extend adjacent to existing blood vessels, but in a few cases, vessels follow nerves. Nerves have been proposed to follow vascular smooth muscle, endothelium, or their basal laminae. Focusing on the major axial blood vessels and nerves, we found that when nerves grow into forelimbs at E3.5-E5, vascular smooth muscle was not detectable by smooth muscle actin immunoreactivity. Additionally, transmission electron microscopy at E5.5 confirmed that early blood vessels lacked smooth muscle and showed that the endothelial cell layer lacks a basal lamina, and we did not observe physical contact between peripheral nerves and these endothelial cells. To test more generally whether lack of nerves affected blood vessel patterns, forelimb-level neural tube ablations were performed at E2 to produce aneural limbs; these had completely normal vascular patterns up to at least E10. To test more generally whether vascular perturbation affected nerve patterns, VEGF(165), VEGF(121), Ang-1, and soluble Flt-1/Fc proteins singly and in combination were focally introduced via beads implanted into E4.5 forelimbs. These produced significant alterations to the vascular patterns, which included the formation of neo-vessels and the creation of ectopic avascular spaces at E6, but in both under- and overvascularized forelimbs, the peripheral nerve pattern was normal. The spatial distribution of semaphorin3A protein immunoreactivity was consistent with a negative regulation of neural and/or vascular patterning. Semaphorin3A bead implantations into E4.5 forelimbs caused failure of nerves and blood vessels to form and to deviate away from the bead. Conversely, semaphorin3A antibody bead implantation was associated with a local increase in capillary formation. Furthermore, neural tube electroporation at E2 with a construct for the soluble form of neuropilin-1 caused vascular malformations and hemorrhage as well as altered nerve trajectories and peripheral nerve defasciculation at E5-E6. These results suggest that neurovascular congruency does not arise from interdependence between peripheral nerves and blood vessels, but supports the hypothesis that it arises by a shared patterning mechanism that utilizes semaphorin3A.     

Morphological constraints in the digging apparatus of pocket gophers (Mammalia: Geomyidae)

The digging apparatus of pocket gophers offers a unique opportunity to examine morphological constraints within a historical context because relationships among extant taxa are well resolved and the features enhancing digging performance are relatively well understood. Structural and functional considerations suggest that the muscles associated with tooth- and claw-digging in pocket gophers are subjected to contrasting levels of morphological constraints. To assess this hypothesis, we analysed the bones and muscles of the jaws and forelimbs in four genera comprising five species of pocket gophers. Morphometric analyses were performed on 12 osteological measurements selected to reflect overall skull size, variation in rostral shape and procumbency, differences in overall length of the forelimbs and processes relating to the function of lever systems used in claw-digging. In addition, dissections were made of the jaw, hyoid, neck and all of the forelimb muscles excluding the intrinsic muscles of the manus. Results of our morphometric analyses corroborate the recent suggestion that pocket gophers encompass a wide range of morphological variation extending from claw-diggers to tooth-diggers. Myologically, however, we found structural variation only in the forelimb muscles, some of which may be advantageous for digging. No changes in jaw, neck and hyoid muscles, other than differences in muscle mass or those concordant with differences in rostral shape, were noted. These results support our hypothesis that constrasting levels of morphological constraint exist between the jaw and forelimb muscles of pocket gophers. We present a discussion of the structural and functional constraints on jaws and forelimbs in gophers as well as an analysis of historical constraints acting on this group, and perhaps on mammals in general.     

Asymmetric limb malformations in a new transgene insertional mutant,footless

Six to eight copies of a transgene integrated into mouse chromosome 15 resulting in a new transgene insertional mutant, Footless, presenting with malformations of the limbs, kidney, and soft palate. Homozygotes possess a unique asymmetric pattern of limb truncations. Posterior structures from the autopod and zeugopod of the hindlimbs are missing with left usually more severely affected than right. In contrast, anterior structures are missing from the right forelimbs. The left forelimb is usually normal except for the absence of the distal telephalanges and nails. These structures are absent on all formed digits. In situ hybridization assays examined the expression of Shh, dHand, Msx2, Fgf8, En1, and Lmx1b in mutant limb buds and indicated normal establishment of the anterior/posterior and dorsal/ventral axes of the developing limbs. However, dysmorphology of the apical ectodermal ridge was observed in the mutant limb buds.     

Directional asymmetry in the forelimb of Macaca mulatta

Asymmetry was investigated in the forelimbs of 150 rhesus monkey (Macaca mulatta) skeletons using measurements of right and left humerii, radii, ulnae, second metacarpals, and femora. Seven of the ten forelimb dimensions were larger on the right than on the left side. Paired t-tests revealed that the mean of the right side was significantly larger than that for the left for two measurements of the ulna and two of the humerus. No measurement was significantly larger on the left than on the right side. These results indicate a small but significant asymmetry in the forelimb bones of rhesus monkeys and, as is the case for humans, the direction of asymmetry favors the right side. Our findings are consistent with an interpretation of hypertrophy of certain muscles and opens the question of whether rhesus monkeys preferentially use their right forelimbs for manipulative tasks that require manual dexterity, as is the case for humans. These forelimb skeletal asymmetries are discussed in light of the recent literature on cortical asymmetry and handedness in nonhuman primates.     

Teratogen update: clinical aspects of thalidomide embryopathy--a continuing preoccupation

There is much misinformation in the medical community regarding the thalidomide syndrome. Some physicians and scientists are unaware of the fact that organs other than the limbs were frequently affected. Some believe that thalidomide could produce any type of limb reduction defect. Most were aware of the very narrow period of early organogenesis during which the thalidomide-type malformations could be produced. Important features include the fact that limb reduction defects were primarily preaxial, included concomitant girdle hypoplasia when limb reductions were severe, were almost universally bilateral and did not include distal transverse-type defects often called "hemimelia". While it can be said that some spontaneous (non-thalidomide) malformations can mimic the thalidomide syndrome, it can also be said that many limb reduction defects can be determined not to have been produced by thalidomide. The risks of the various defects can be estimated following exposure, with most certainty for limb defects, with less certainty for other defects. Many defects were not associated with exposure to thalidomide such as cleft lip and severe mental retardation.     

Congenital tibial aplasia with preaxial polydactyly: soft tissue anatomy as a clue to teratogenesis

We performed preoperative arteriography and postamputation dissection on a human limb having complete tibial aplasia and preaxial polydactyly. The tibia was replaced by a tendinous band with an attached connective tissue mass. In the leg, the muscles were normal. Muscles usually arising from the tibia arose from the tendinous band, Intrinsic muscles of the foot were absent where skeletal elements were absent and there were supernumerary muscles where there were supernumerary skeletal elements. The tendinous insertions of the muscles that originated in the leg followed the skeletal pattern of the foot. The nerves were normally distributed with the exception that extra branches innervated the supernumerary toes. Both the anterior tibial and peroneal arteries were smaller than normal and progressively reduced in size as they coursed distally and could not be identified below the ankle. The dorsalis pedis artery was absent. The other arteries of the leg and foot were normal with the exception of extra branches supplying the supernumerary toes. The anatomy indicates that the foot is probably a double posterior duplication. Such a defect implies abnormal specification of the foot pattern due to a prespecification event. The tibial remnant with normal leg soft tissues indicates normal specification of the leg pattern and subsequent interruption of tibial morphogenesis due to a postspecification event. The presence of these different types of malformations in the same limb may seem to be contradictory. We suggest, however, that a single teratogenic event occurring at one moment in time could cause combined pre- and postspecification malformations. We further suggest that a diminished anterior tibial artery reduced the number of vessels available for collateral circulation and thus put the limb at risk for subsequent malformation.     

Fibre type regionalization of forelimb muscles in two mammalian species, Galea musteloides (Rodentia, Caviidae) and Tupaia belangeri (Scandentia, Tupaiidae), with comments on postnatal myogenesis

To investigate structural differences between propulsory and antigravity muscles, the spatial distribution of slow (type I) and fast (type II) muscle fibres in forelimb muscles of two species of small mammals was studied, Galea musteloides and Tupaia belangeri. Serial sections through complete forelimbs were prepared. Following histochemical fibre typing, the forelimbs were reconstructed three-dimensionally using product design software. Most forelimb muscles of both species showed a homogenous distribution of type I fibres. In the supraspinatus and triceps brachii (capita longum et laterale) muscles, however, a segregation of fibre types into ”fast” superficial areas and ”slow” deep regions was observed. Slow regions contained at least 60% type I fibres and were positioned along intramuscular extensions of the tendons of insertion. The functional implications of fibre type regionalization are discussed. An analysis of intramuscular fibre type distribution during postnatal myogenesis revealed no significant differences in muscle fibre differentiation between altricial and precocial juveniles. Differences in locomotor ability probably arise from heterochronic development of connective tissue components (endo- and perimysium). Accepted: 10 June 1999     

The teratogenic activity of a thalidomide analogus EM12 in rats on a low-zinc diet.

The relationship between the teratogenicity of EM12, 2-(2,6-dioxopiperiden-3'-yl) phthalimidine, a stable analogue of thalidomide, and zinc status in the maternal animal was investigated using pregnant rats on a low-zinc diet (1 ppm zinc, days 0--14 gestation) as the experimental model. Previous studies with this compound in rats fed a commercial diet at oral doses up to 250 mg/kg per day for three days and intravenous doses up to 10 mg/kg per day for three days failed to produce "typical" thalidomide malformations. However, when a dose of 150 mg/kg was given intraperitoneally to rats on a low-zinc diet, typical thalidomide malformations occurred with an incidence of 57.5%.     

Afferents from a single muscle of the forelimbs and fastigial neurons of the cerebellum

Responses of neurons in the medial nucleus of cerebellum (CBM) were studied on stimulation of ipsilateral and contralateral homonymous muscles, in decerebrated cats. The aim was to find out to what extent information from homonymous muscles of the forelimbs converge on the same CBM neurons and whether the probability of such a convergence depends on location (axial, proximal, distal) or function (flexor, extensor) of the tested muscles. The analysis was limited to the neurons belonging to the rostral part of the nucleus which is known to control the ipsilateral muscle periphery. Neuronal activity was recorded extracellularly using tungsten microelectrodes (5-12 M omega) and muscle stimulation was performed by bipolar coated steel electrodes, with the exception of the tip. At least 6 pairs of homonymous muscles were generally stimulated: two axial, two proximal and two distal in both forelimbs. Care was taken that, when a muscle was stimulated, the others were not activated either directly or in a reflex way. Out of the 65 neurons studied, 60 (92%) were responsive to muscle stimulation. It was specifically observed that a high percentage of cells reacted to stimulation of distal muscles (74% to ipsilateral and 71% to contralateral ones). More than half (55%) of the neurons were responsive to activation of a pair of homonymous distal muscles and about one third of them (31%) to both the pairs of distal muscles. On the contrary the percentage of responses to proximal muscles was reduced foremost in the ipsilateral ones (23%) and only an exiquous percentage of cells (15%) received information from the homonymous proximal muscles.(ABSTRACT TRUNCATED AT 250 WORDS)     

Abnormal limb regeneration in adult newts exposed to a pulsed electromagnetic field

This investigation examined two questions: 1) whether or not forelimb regeneration results in a faithful replacement of the distal skeletal pattern and 2) what effect exposure to a pulsed electromagnetic field (PEMF), the type reported to facilitate healing of human non-united bone fractures, would have on the process of limb regeneration. Of the native forelimbs, 98% (132 out 134) had a skeletal pattern that showed little difference with the only variation being the range of carpal bones (5-8). Following a 4-5 month postamputation period, the skeletal pattern of the normal regenerates was examined. While 72% (135 out of 188) of these forelimbs resembled the native group, 28% (53 out of 188) were abnormal. These abnormalities consisted of the loss of a digit, fused carpals, and long bone defects which occurred singly or in combination with one another. Exposure to a PEMF for the first 30 days postamputation, followed by a 3-4 month postamputation period, produced in addition to the normal (60%, 144 out of 240) and abnormal forelimbs (28%, 67 out of 240), a group of forelimbs with unique gross defects (12%, 28 out of 240). These defects, occurring singly or in combination, included the loss of two or more digits with associated loss of carpals, absence of the entire hand pattern, and abnormalities associated with the radius and ulna. We can offer no explanation for these observations.     

A model for nonexercising hindlimb muscles in exercising animals

Nonexercising muscles appear to be metabolically active during exercise. Animal models for this purpose have not been established. However, we have been able to teach animals to run on their forelimbs while their hindlimbs are suspended above the treadmill with no visible limb movement. To document that indeed this mode of exercise does not provoke additional muscle activity, we have compared the levels of neural activation of the soleus and plantaris muscles using a computer analysis of the electromyographic interference pattern, recorded from bipolar fine wire electrodes implanted across each muscle. Via computer analyses of the electromyographic interference patterns the frequencies and amplitudes of motor unit action potentials were obtained. The data were sampled during 20 s of every minute of observation. Comparisons were made in four conditions: (i) resting on the treadmill while bearing weight on the hindlimbs (normal rest), (ii) running on the treadmill (15 m/min, 8% grade) on all four limbs (normal exercise), (iii) resting while the hindlimbs were suspended in a harness above the treadmill (suspended rest), and (iv) exercising with the forelimbs (15 m/min, 8% grade) while the hindlimbs were suspended above the treadmill (suspended exercise). All four experimental conditions were carried out for 90 min each and were performed by each animal. The results clearly show that muscle activities (frequencies and amplitudes), when the hindlimbs are suspended above the treadmill, at rest or during exercise, are lower than the activities in these same muscles when the animals are at rest, supporting only their body weight. Activities in the same muscles during exercise were from 300 to 2000% greater than during hindlimb suspension.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sexually dimorphic muscles in the forelimb of the Japanese toad,Bufo japonicus

During the breeding season, male anurans display clasping behavior by holding females with their forelimbs. This behavior is peculiar to males, and may require specializations in forelimb musculature. The present study revealed that five kinds of forelimb muscles were heavier in the male Japanese toad than in the female: the flexor carpi radialis (FCR), the flexor antibrachii medialis caput superius (FAMsup), the abductor indicis longus (AIL), the extensor carpi radialis caput superius (ECRsup), and the flexor antibrachii lateralis superficialis caput superius (FALSsup). In addition, one breast muscle, the coracoradialis (CR), was also heavier in males than in females. A quantitative analysis of muscle fibers processed for myosin ATPase activity showed that, in such “sexually dimorphic muscles” of the female, both fast (twitch) and slow (tonic) muscle fibers were of smaller diameter than in other forelimb muscles of both sexes (all male muscles plus “nondimorphic muscles” of the female). Moreover, both types of fibers were less numerous than in the corresponding muscles of the male. These results suggest that the “sexually dimorphic muscles” are used especially for clasping by the male and are degenerative or subnormal in the female. Slow muscle fibers were neither peculiar to, nor abundant in, these clasping muscles, although they may well be necessary for tonic and prolonged contractions of the forelimb muscles during clasping. The mechanism of sexual dimorphism may be a direct action of androgens on clasping muscles or an indirect action on clasping muscles via the innervating motoneurons.     

Positional information in the forelimb of the axolotl: experiments with double-half tissues

It has been demonstrated recently that upper forelimbs of axolotls comprised of symmetrically arranged soft tissues do not regenerate (P. W. Tank, 1978,J. Exp. Zool.204, 325–336). These double-half forelimb stumps contained skin, muscle, and loose connective tissues in symmetrical arrangement. The present study explores the roles of muscle, skin, and epidermis in the regeneration of double-half forelimbs by grafting them separately to create forelimb stumps bearing symmetrical arrangements of these individual tissues. Forelimb stumps bearing symmetrically arranged flexor and extensor muscles and normally arranged skin underwent complete regeneration (96%). Forelimbs comprised of double-half skin overlying normally arranged muscles and deep tissues formed hypomorphic structures and nonregenerates (56%) with some single and multiple regenerates. Limbs with double-half deep tissues and complete epidermis either regenerated distally incomplete patterns (47%), single patterns (33%), or multiple patterns (20%). Those forelimbs comprised of double-half skin and no muscle regenerated incomplete patterns in the majority of cases (56%) but single and multiple limbs also were formed. Based on these results it can be concluded that no single type of tissue is solely responsible for the regenerative failure experienced by double-half forelimbs in the earlier study. The complete failure of forelimb regeneration occurs only when all types of soft tissues tested (skin, muscle, and deep connective tissues) are present in symmetrical arrangement.     

Temporal analysis of the role of growth hormone in the initiation and maintenance of limb regeneration in the hypophysectomized newt Notophthalmus viridescens

This study was designed to investigate and determine for how long, after either hypophysectomy or the third (last) growth hormone injection (to previously hypophysectomized newts), the circulating and now declining titers of endogenous or exogenous hormone remained at a sufficient concentration to permit a morphologically normal forelimb regeneration response in the adult newt Notophthalmus viridescens. To examine the declining levels of endogenous hormone (hormone withdrawal series [HW]), left forelimbs were amputated at specific times following hypophysectomy. Right forelimbs were amputated 5 days prior to hypophysectomy. The declining levels of exogenous hormone (hormone replacement series [HR] were examined in newts whose left forelimbs were amputated at specific times following the last of three consecutive alternate-day growth hormone injections that were initiated 5 days post hypophysectomy. Right forelimbs were amputated immediately following the first hormone injection. All experimental animals were sacrificed when their right forelimbs regenerated to an advanced digitiform regenerate. In both series right forelimbs regenerated normally. In the HW series normal regeneration resulted only when forelimbs were amputated within 48 hours post hypophysectomy, whereas in the HR series normal regeneration occurred in only those newts whose forelimbs were amputated within 12 hours of the last hormone injection. The regeneration response of left forelimbs in both series gradually declined with the time interval between either hypophysectomy or hormone injection and forelimb amputation. As the hormone titer declined, fewer limbs initiated a normal response; they became progressively more hypomorphic and eventually failed to undergo typical regeneration.     

Sexual selection on forelimb muscles of western grey kangaroos (Skippy was clearly a female)

Studies of sexual selection have tended to concentrate on obvious morphological dimorphisms such as crests, horns, antlers, and other physical displays or weapons; however, traits that show no obvious sexual dimorphism may nevertheless still be under sexual selection. Sexual selection theory generally predicts positive allometry for sexually selected traits. When fighting, male kangaroos use their forelimbs to clasp and hold their opponent and, standing on their tail, bring up their hind legs to kick their opponent. This action requires substantial strength and balance. We examined allometry of forelimb musculature in male and female western grey kangaroos (Macropus fuliginosus) to determine whether selection through male–male competition is associated with sex differences in muscle development. Forelimbs of males are more exaggerated than in females, with relatively greater muscle mass in males than the equivalent muscles in females. Furthermore, while muscles generally showed isometric growth in female forelimbs, every muscle demonstrated positive allometry in males. The significant positive allometry in male forelimb musculature, particularly those muscles most likely involved in male–male combat (a group of muscles involved in grasping: shoulder adduction, elbow flexion; and pulling: arm retraction, elbow flexion), clearly suggests that this musculature is subject to sexual selection. In addition to contributing to locomotion, the forelimbs of male kangaroos can also act as a signal, a weapon, and help in clasping, features that would contribute towards their importance as a sexually selected trait. Males would therefore benefit from well‐developed musculature of the arms and upper body during competition for mates. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 923–931.     

Brachially innervated ectopic hindlimbs in the chick embryo. II. The role of supraspinal input in the loss of limb motility

Brachially innervated grafted hindlimbs display a progressive loss of motility as development proceeds. However, the virtually immobile grafted hindlimbs of E20 embryos exhibited strong, synchronous contractions of gastrocnemius and tibialis muscles upon intraperitoneal injection of strychnine nitrate (20 μg). This result indicated that the marked behavioral deficit was not due to an inability of the motoneurons that innervate the immobile grafted hindlimbs to initiate and propagate action potentials, but was probably the result of an effective loss of motoneuron excitation. To examine the hypothesis that interaction with the supraspinal nervous system is involved in the reduction of grafted hindlimb activity, the normal forelimb and grafted hindlimb movements of chronic spinal embryos were examined. The normal forelimbs of chronic spinal embryos exhibited the same number of movements as normal embryos at all stages examined. Thus the deficit in grafted hindlimb motility is not comparable to the behavior of the normal forelimb in chronic spinal embryos and is, therefore, unlikely to be due to a lack of excitation from the supraspinal nervous system. The possibility of an inhibitory influence via supraspinal projections was examined in chronic spinal embryos that had brachially innervated grafted hindlimbs. After E12, the grafted hindlimbs of chronic spinal embryos displayed significantly fewer movements than the normal forelimbs of chronic spinal embryos but significantly more movements than the brachial hindlimb of embryos with intact spinal cords. By E18, however, both spinal and nonspinal brachial hindlimbs were equally dysfunctional. Thus prevention of supraspinal communication transiently reduces but does not prevent the emergence of motor dysfunction in the brachially innervated hindlimbs, which appears to be due to motoneurons not receiving sufficient net excitation, from spinal circuits, to propagate action potentials to the muscles. © 1993 John Wiley & Sons, Inc.     

Antiemetic Role of Thalidomide in a Rat Model of Cisplatin-Induced Emesis

The efficacy of thalidomide to attenuate cisplatin-induced emesis was evaluated in a rat model. Four groups were utilized: control group (peritoneal injection and gastric lavage with normal saline), cisplatin group (peritoneal injection of cisplatin at 10 mg/kg and gastric lavage with normal saline), thalidomide group (cisplatin as above and gastric lavage with thalidomide at 10 mg/kg), and granisetron group (positive control for antiemetic effects; cisplatin given as above and gastric lavage done with granisetron at 0.5 mg/kg). The cisplatin-induced kaolin consumption (pica behavior) was used as a model of emesis in patients. The animals’ kaolin and food intakes were measured. Further, medulla and gastric tissues were obtained 5 and 33 h after peritoneal injections to quantify the levels of Substance P and Neurokinin-1 receptor (NK-1R). The cisplatin-induced kaolin consumption was significantly (p < 0.05 vs. cisplatin group) attenuated by thalidomide 72 h after the injection. The levels of Substance P in the medulla and gastric tissue were increased 5 h after the injection in both cisplatin and thalidomide groups, however, returned faster to normal levels in the thalidomide group (p < 0.05 vs. cisplatin group). Further, levels of NK-1R in the cisplatin, thalidomide, and granisetron group were significantly increased at both 5 and 33 h (p < 0.05 vs. control group), with no obvious difference among these three groups. In conclusion, thalidomide attenuates animal equivalent of cisplatin-induced emesis, and this beneficial effect is associated with decreased levels of Substance P levels in the medulla and gastric tissue.     

Morphogenetic interactions occurring between blastemas and stumps after exchanging blastemas between normal and double-half forelimbs in the axolotl, Ambystoma mexicanum.

Regeneration blastemas were exchanged between surgically constructed forelimbs comprised of symmetrical tissues (double-anterior and double-posterior) and normal, unoperated forelimbs. Normal blastemas grafted at the stage of medium bud (MB) onto double-half forelimb stumps regenerated normal skeletal patterns in nearly all cases. Double-half blastemas transplanted at the stage of MB onto normal forelimb stumps did not regenerate complete limb patterns. These results indicate that a double-half blastema cannot be “rescued” by transplantation to a normal stump and that a double-half limb stump does not interfere with the ability of a normal blastema to distally transform. The regeneration blastema possesses sufficient positional information at the stage of MB to permit it to develop autonomously. Supernumerary forelimbs resulted from several types of graft-stump combinations. The location and handedness of these supernumerary limbs are predicted by the rules of a recently presented model for pattern regulation in epimorphic fields [French, V., Bryant, P. J., and Bryant, S. V. (1976). Science193, 969–981].