370 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE 



mammals and the late blastula of birds. In both groups the trophoblast tissue 

 is attached to the edges of the formative tissue and extends below in such a 

 way that the formative cells and trophoblast tissue tend to form a hollow 

 vesicle. In both, the formative portion of the blastula is present as a disc or 

 mass of cells composed of presumptive, organ-forming cells closely associated 

 at its lateral margins with the trophoblast or food-getting tissue. A marked 

 distinction between the two groups, however, is present in that the future 

 entodermal cells in fishes are localized at the caudal margin of the disc, 

 whereas in mammals and birds they may be extensively spread along the 

 under margin of the disc. In reptiles the condition appears to be somewhat 

 similar to that in birds and mammals, with the exception possibly of the 

 turtles, where the future entoderm appears more localized and possibly may 

 be superficially exposed. Therefore, while great differences in particular fea- 

 tures exist between the fishes and the higher vertebrates, the essential funda- 

 mental conditions of the early blastulae in teleost and in elasmobranch fishes 

 show striking resemblances to the early blastulae of reptiles, birds, and 

 mammals. 



The blastulae of teleost fishes remain in this generalized condition until 

 about the time when the gastrulative processes begin. At that time the noto- 

 chordal and mesodermal, cellular areas begin their migrations over the caudal 

 edge of the blastodisc to the blastocoelic space below, where they ultimately 

 come to lie beneath the epidermal and neural areas. Associated with the mi- 

 gration of notochordal and mesodermal cells, an entodermal floor or sec- 

 ondary hypoblast is established below the notochordal and mesodermal cells 

 by the active migration of primary hypoblast cells in an antero-lateral direction. 

 In the elasmobranch fishes there is a similar cell movement from the caudal 

 disc margin, as found in teleost fishes, but, in addition, a delamination of 

 entodermal (and possibly mesodermal cells) occurs from the deeper lying 

 parts of the blastodisc. 



7. Blastulae of Gymnophionan Amphibia 



In the Gymnophiona, nature has consummated a blastular condition dif- 

 ferent from that in other Amphibia. It represents an intermediate condition 

 between the blastula of the frog and the blastodiscs of the teleost and elas- 

 mobranch fishes and of higher vertebrates (fig. 182). In harmony with the 

 frog blastula, for example, a specialized periblast or food-getting group of 

 cells is absent. On the other hand, the presumptive entoderm and the pre- 

 sumptive notochordal, mesodermal, neural, and epidermal cells form a compact 

 mass at one pole of the egg, as in teleosts, the ohick, and mammal. Similar 

 to the condition in the chick and mammal, the entodermal cells delaminate 

 (see Chap. 9) from the under surface of the blastodisc (Brauer, 1897). 



