TYPES OF CHORDATE BLASTULAE 



359 



separating the superficial cells from the deeper ones" (fig. 171B-D). He 

 further suggests that "the bilaminar embryo of birds is to be homologized 

 with the blastula of the Amphibia, the cleft separating the two layers being 

 equivalent to the blastocoele" (p. 13). The formation of the hypoblast (pri- 

 mary entoderm) by a process of delamination from the upper layer or epiblast 

 agrees with the observations by Peter ('38) on the developing chick and pigeon 

 blastoderm (fig. 172) and of Spratt ('46) on the chick. It also agrees with 

 some of the oldest observations, concerning the matter of entoderm formation, 

 going back to Ollacher in 1869, Kionka, 1894, and Assheton, 1896. Others, 

 such as Duval (1884, 1888) in the chick, and Patterson ('09) in the pigeon, 

 have ascribed the formation of the primary entoderm to a process of invagi- 

 nation and involution at the caudal margin of the blastoderm, while Jacobson 

 ('38) came to the conclusion that the entoderm of the pellucid area arose in 

 chick and sparrow embryos through a process of outgrowth of cells from the 

 primitive plate and from an archenteric canal produced by an inward bend- 

 ing of the epiblast and primitive plate tissue. The latter author believed that 

 the entoderm of the area opaca arose by delamination. 



The hypoblast of the chick gives origin to most of the tissue which lines 

 the future gut, and, therefore, may be regarded as the potential entodermal 

 area. As in the amphibia and Amphioxus, the epiblast is composed of sev- 

 eral, presumptive organ-forming areas (fig. 173A). (See Pasteels, '36c; 

 Spratt, '42, '46.) At the caudal part of the epiblast is an extensive region 

 of presumptive mesoderm bisected by the midplane of the future embryonic 

 axis. Just anterior to this region and in the midplane is the relatively small, 

 presumptive notochordal area. Between the latter and the mesodermal area 

 is located the presumptive prechordal plate of mesodermal cells. Immedi- 

 ately in front of the notochordal region lies the presumptive neural area in 

 the form of a crescent with its crescentic arms extending in a lateral direc- 



E PI BLAST 



'HYPOBLAST 



Fig. 172. Delamination of hypoblast (entoderm) cells from upper or epiblast layer 

 in the chick blastoderm. (A) Posterior end of blastoderm (cf. fig. 171A). (B) Anterior 

 end of blastoderm. 



