70 LEONARD W. AVILLIAMS 



lar vessels in the adjacent intersegmental clefts and with irregu- 

 lar vessels upon the parietal plate, »forms the oblique or third 

 portion of the anterior cardinal vein. 



The posterior part of the anterior cardinal vein is formed, as 

 Evans has shown to be the case with certain other veins, from 

 several separate branches of the aorta. The first intersegmental 

 artery gives rise to the posterior part of the vena capitis medialis, 

 leaving that of the second intersegmental fissure to serve as the 

 first definitive intersegmental artery. 



The dorsal wall of the second somite of embryos of fifteen seg- 

 ments is almost separated from the lateral plate by the enlarge- 

 ment of the vessels of the network which gives rise to the oblique 

 portion of the anterior cardinal vein. A very narrow bridge, 

 however, connects the dorsal wall of the somite and the parietal 

 plate and forms the roof of the much reduced communication be- 

 tween the cavity of the segment and the coelom. 



The floor and the lower part of the walls of the somite fuse with 

 its core and are quickly converted into sclerotomic tissue. The 

 upper part of the walls of the somite, however, has a different 

 history, for instead of fusing with the sclerotome this part of the 

 cortex of the somite forms a center of growth from which much of 

 the mesenchyma not only of the sclerotome but also of the dermis 

 proliferates. Moreover, in addition to mesenchyma this part of 

 the somite also produces the myotome. It is important, there- 

 fore, to distinguish the roof of the somite, with its bordering zone 

 of growth, from the sclerotome, and for this purpose Reinak's 

 name Ruckentajel and its English equivalent, dorsal lamella, will 

 serve. 



The lower or lateral edge of the myotome merges gradually with 

 the sclerotome at the bottom of the lower myotomic groove, but 

 I cannot find a center of growth here, nor can I determine whether 

 or not there is a migration of cells to or from the myotome. 



The plate of neural crest cells is migrating laterally and now 

 overlies the adjacent edges of the somite and parietal plate. 



I71 embryos of eighteen segments (fig. 7) one sees a continuation 

 of the several processes described above, namely, the deepening 

 of the lower myotomic groove and its further extension upon the 



