Artluir B. Lamb 



191 



process and the somite, since it seems to me that it might equally well 

 be derived from neural crest cells. 



As the embryo develops, the median stalk is pushed away from the 

 alimentary canal and around the end of the notochord by the inter- 

 vening aorta. The notochord thus shifts its position from the dorsal 

 wall of the median stalk to its posterior wall. Such is the compression 



iE >■ 



Fig. 4. — Reconstruction of optic vesicle, ganglia and somites of an Acautbias 

 embryo, 19 mm. total length. Right side, medial view. The Anlagen of the 

 muscles lined obliquely. 



that exists in this region that the median stalk is nearly severed at the 

 point of contact. The irregularly placed median cavities of the stalk 

 now fuse together and finally assume connection with the lumen of the 

 somite on either side. The lumen of the median stalk persists until 

 a late stage, when the walls become mesenchymatous and the cavity is 

 obliterated. The stalk persists, however, as connecting strand until 

 nearly a 30 mm. stage (C. st. in Figs. 1-7). 



