462 DEVELOPMENT OF PRIMITIVE BODY FORM 



NEURAL PLATE A.B. C D. E F. G. 



H.I.J. 



LIVER DIVERTICULUM 



Fig. 220. Beginning neural fold stage of frog embryo from prepared material. (A) 

 Beginning neural fold stage as seen from dorsal view. (B) Sagittal section near median 

 plane of embryo similar to that shown in (A). (C) Same as (B), showing organ- 

 forming areas. (D) Midsagittal section of caudal end of frog embryo slightly younger 

 than that shown in fig. 223B. Observe that the blastopore practically is closed, while the 

 dorsal diverticulum of the hindgut connects with the neurocoel to form the neurenteric 

 canal. Observe, also, ventral diverticulum of hindgut. 



until later in development, and, as a result, a small opening remains, 

 the umbilicus, where the embryonic and extra-embryonic tissues are 

 continuous. This discontinuity of the embryonic layers permits the 

 blood vessels to pass from the embryonic to the extra-embryonic re- 

 gions. {Note: In the teleost fishes, although a typical, flattened, gas- 

 trular form is present, the formation of the ventral body wall of the 

 trunk through a general retraction of tissues resembles that of the 

 rounded gastrulae mentioned above.) 



5. Blastocoelic Space and Body-form Development 



During the terminal phases of gastrulation in such forms as Amphioxus 

 and the frog, the blastocoel, as a spacious cavity, disappears for the most 

 part. Its general area is occupied by cells which migrated into the blastocoel 



