366 



THE CHORDATE BLASTULA AND ITS SIGNIFICANCE 



by the formative cells of the blastula. The edge of this disc of formative 

 cells is attached to the trophoblast or auxiliary cells (fig. 176D). Only the 

 formative cells give origin to the future embryonic body. 



c. Eiitherian Mammals 



The eutherian mammals as a whole present a slightly different picture of 

 blastocyst development from that described above for marsupial species. These 

 differences may be outlined as follows: 



( 1 ) During the earliest phases of blastocyst development in most eutherian 

 mammals, a distinct, inner cell mass is elaborated at the formative 

 or animal pole (fig. Ill A, B). This characteristic is marked in some 

 species (pig, rabbit, man, and monkey) and weaker in others (mink 

 and armadillo). It may be entirely absent in the early blastula of 

 the Madagascan insectivore, Hemicentetes semispinosus; however, in 

 the latter, a thickening corresponding to the inner cell mass later 



NEURAL ECTODERM 



NOTOCHORD 

 PRE-CHORDAL PLATE 

 ENTODERM 



Fig. 179. Presumptive organ-forming areas in the blastoderm of the shark embryo. 

 (A) Median section of the blastoderm of Torpedo ocellata. Hypoblast cells are shown 

 in black. Caudal portion of the blastoderm is shown at the right. Cf. (B). (This figure 

 partly modified from Ziegler, '02 — see Chap. 6 for complete reference.) (B) Map of 

 the presumptive organ-forming areas of the blastoderm of the shark, Scylliutn canicula. 



