Spur development in chick embryos

Normal development of spurs and horny squamae have been studied in histological preparations obtained from the skin of tarsometatarsus in 8--16-day-old embryos. During the first, initial stage of development, by means of rearrangement of cellular matter and cell migration, three main parts of the spur are layed down -- the horny cover, the spur body and the fibrovascular cushion. For the second stage, vigorous growth of the spur germ at the expense of proliferative activity of its cells is characteristic. At the third stage (after hatching) in males, the spur body outgrows and the bony core is formed. Morphogenesis of the horny squama can be devided into two stages. At the initial stages by means of condensing cellular elements, squamous papilla and horny shell are layed down. The fibrovascular cushion is absent. The second stage is similar to the spur one and is characterized by growth of all the germ parts at the expense of cell proliferation. Comparing morphogeneses of the squama and the spur, it is possible to conclude that phylogenetic transformation of the squama into the spur is performed by two means (modi) of phyloembryogenesis: by means of adding new signs of development to the initial terminal stages of its morphogenesis.