Observations on the gastric epithelium of ascidians with special reference to Styela clava

Summary Transmission electron microscopy shows the gastric epithelium of Styela clava to comprise at least three distinct cell types. Ciliated mucous cells which form the crest of each stomach ridge produce mucus by an unexpected route. Vacuolated cells lining the ridge sides appear to be absorptive in function. Gastric enzymes are produced by typical protein secreting cells scattered amongst the vacuolated cells. Undifferentiated cells are found in the crypts between ridges. The structure and function of the gastric epithelium in Styela is discussed with special reference to the wider concepts of ascidian gut organization.The author is grateful to Mrs. L. Rolph for technical help with scanning electron microscopy and Mr. J. Calvert and Mr. R. Jones for assistance with the transmission electron microscopy. Animals were collected through the kind offices of Mr. J. Sturges and other staff of the Admiralty Marine Trials Station, Portsmouth. This research was carried out during the tenure of SRC grant No. B/RG 82919 -The localization of polypeptide hormones in the pharynx and gut of Protochordates     

Observations on the cytology and ultrastructure of Ochromonas tuberculatus sp. nov. (Chrysophyceae), with special reference to the discobolocysts

Ochromonas tuberculatus sp. nov. is described by a combination of light and electron microscopy from material in culture, special attention being given to the structure and ontogeny of the discobolocysts, the cross-banded flagellar root and its relationship with the Golgi body and nucleus, and an unusual type of vesicle associated with the contractile vacuole.     

Observations on the dental anatomy of piranhas (Characidae) with special reference to tooth structure

A study of the tissues of the teeth and jaws in piranhas, using the scanning electron microscope and various techniques of light microscopy, revealed many dental adaptations related to the specialized feeding habits of these carnivorous fishes. The dentition is primarily sectorial, although some anterior teeth may be used in grasping. The scissor-like rows of teeth are maintained by the specialized pattern of tooth replacement. The bones of the jaws and the tooth attachment support the teeth very firmly. In its structural organization, the enameloid covering the teeth closely resembles that on the sectorial teeth of sharks and is adapted to the probable stress patterns set up in biting.     

Observations on the cytology of Bipes (Amphisbaenia) with special reference to its lampbrush chromosomes

The positions and general anatomical and histological characteristics of the gonads of Bipes biporus and B. canaliculatus are described. The amounts of DNA per haploid chromosome set have been measured in both species, the values being 1.83 and 2.0 pg for biporus and canaliculatus respectively. The karyotypes of both species are described on the basis of data from mitotic and meiotic metaphase chromosome sets and from lampbrush chromosomes. B. biporus has 10 macrochromosomes and 11 microchromosomes. B. canaliculatus has 11 macrochromosomes and 11 microchromosomes. The karyotypes of the two species differ distinctly with regard to the shapes of 3 of the macrochromosomes. Chiasma distribution is described for male meiosis in B. biporus. Studies of the lampbrush chromosomes of both species show the chiasma distribution in the female to be generally similar to that found in the male biporus. In B. canaliculatus, lampbrush chromosomes with maximally extended lateral loops are found in oocytes that are oblate spheroids measuring 0.7×1.0 mm along their short and long axes respectively, these being well before the start of the major phase of vitellogenesis. Smaller oocytes have more distinct chromomeres and shorter loops. Microchromosomes take the form of typical small lampbrush chromosomes in oocytes. There are at the most 1,000 chromomeres per haploid set of lampbrush chromosomes in B. canaliculatus. Chiasmata are described from lampbrush preparations in which the two half-bivalents are firmly attached to one another without evident association of their axes, indicating the possibility of chiasmate association between the DNA axes of lateral loops. There are remarkably few extrachromosomal nucleoli in Bipes oocytes, and its is suggested that this may indicate a level of ribosomal gene amplification that is much lower than that found in fish and Amphibia. The observations are particularly discussed in relation to current ideas concerning the structure and function of lampbrush chromosomes.     

Observations on root and stem contraction in cycads (Cycadales) with special reference to Zamia pumila L.

Cycads are pachycaulous plants possessing either aerial and columnar (arborescent) stems or subterranean and tuberous fleshy stems. In some taxa, such as Zamia pumila and Stangeria eriopus , the vertical shoots are subterranean; the shoot apex usually remains underground even though new shoot increments arc added as a result of leaf development. The external surface of these stems has a winkled appearance in the distal regions and the height of the leaf scars decreases basipetally This is because both the fleshy tap root and the stem contract, pulling the shoot underground. This contraction results from the collapse of horizontal rows of cells in both the pith and cortex. The vascular cylinder which appears straight in longitudinal sections becomes sinuously distorted as this collapse of cells occurs.