[The histogenesis of somites in the chick]

The somites of the thoracic region of chick embryos incubated for 2 (stages 13-15) and 3 days (stages 17-19) have been examined cytologically and ultrastructurally. Cell contacts and cell arrangement were studied first in spherical somites. The further development of the spherical somite is characterized by changes in the cell arrangement. According to these changes two somitic cell populations can be distinquished: a ventral one, which during the third day loses its epithelial arrangement and becomes disaggregated, and a dorsal one, the dermatome, which remains spithlial for a longer time. Some cells of the ventral group reaggregate on the apical surface of the dermatome cells, and constitute the myotome. The contacts between dermatome and myotome cells as well as between the myotome cells themselves are described. Although the myotome cells appear to be arranged in an epithelial-like order and develop a basement membrane at the sclerotome side, the characteristic contacts and cell polarity of epithelia were not observed. These observations are discussed in relation to the process of cell adhesion and cell communication during the morphogenesis of the secondary mesenchyme.