486 



DEVELOPMENT OF PRIMITIVE BODY FORM 



Fig. 236. Transverse sections through chick embryo of about 12 to 13 pairs of somites, 

 about 38 hours of incubation. (Approximately between stages 10 to 11 of Hamburger 

 and Hamilton, '51, slightly older than that shown in fig. 235.) Observe that the optic 

 vesicles are constricting at their bases; heart is bent slightly to the right; anterior neuropore 

 is evident. (A) Optic vesicles. (B) Stomodaeal area. (C) Anterior end of develop- 

 ing heart. (D) Caudal extremity of forming heart. (E) Anterior intestinal portal 

 and forming caudal portion of the heart. (F) Well-developed somites. (G) Open 

 neural groove. 



below the notochord to complete the roof of the midgut (figs. 201D; 

 209C; 21 OF; 213). This process is similar to that which occurs in 

 the Amphibia (cf. fig. 219D). 



(2) A dorsal arching or evagination of the entoderm toward the noto- 

 chordal area, comparable to that found in the frog and other Am- 

 phibia, is present also. A study of figures 213H-J; 217G; 234B; 

 237E-G; 241B-D demonstrates the marked dorsal upgrowth of all 

 the forming body layers in the trunk area. (Note: In the elasmobranch 

 fishes, a subnotochordal rod of cells of entodermal origin is formed 

 similar to that in the frog and other Amphibia.) 



(3) The ventro-lateral walls of the midgut area, in contrast to those found 

 in the frog, are established largely by actual ingrowth of the entoderm, 

 mesoderm, and ectoderm with subsequent fusion in the median line 



