356 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE 



of the mesodermal crescent comprising much of the neck portion of the "pear" 

 (fig. 167E). 



The blastula of Amphioxus thus may be regarded essentially as a bilaminar 

 structure (i.e., two-layered structure) in which the hypoblast forms the lower 

 layer while the epiblast forms the upper composite layer. 



2. Amphibian Blastula 



In the amphibian type of blastula, a spherical condition exists similar to 

 that in Amphioxus (fig. 169). The future entoderm is located at the vegetative 

 (vegetal) pole, smaller in amount in the frog, Rana pipiens, and larger in 

 such forms as Necturus maculosus (fig. 169A, B). The presumptive noto- 

 chordal material occupies an area just anterior to and above the future dorsal 

 lip of the blastopore. The dorsal lip of the gastrula, when it develops, arises 

 within the entodermal area (fig. 169C-F). Extending laterally on either side 

 of the presumptive notochordal region is an area of presumptive mesoderm 

 (fig. 169C-F). Each of these two mesodermal areas tapers to a smaller di- 

 mension as it extends outward from the notochordal region. The presumptive 

 notochordal and mesodermal areas thus form a composite area or circular 

 marginal zone which surrounds the upper rim of the entodermal material. 



Above the chordamesodermal zone are two areas. The presumptive neural 

 area is a crescent-like region lying above or anterior to the presumptive 

 notochord-mesoderm complex. Anterior to the neural crescent and occupying 

 the remainder of the blastular surface, is the presumptive epidermal crescent 

 (fig. 169C-F). 



In the various kinds of blastulae of this group, the yolk-laden, vegetal pole 

 cells actually form a mass which projects upward into the blastocoelic space 

 (fig. 169 A, B). The irregularly rounded, presumptive entodermal, organ- 

 forming area, therefore, is encapsulated partially by the other potential germinal 

 areas, particularly by the chordamesodermal zone (fig. 169B). In a sense, 

 this is true also of the protochordate group (fig. 167D). 



The amphibian type of blastula includes those of the petromyzontoid 

 Cyclostomes, the ganoid fishes with the exception of bony ganoids, the dipnoan 

 fishes, and the Amphibia with the exception of the Gymnophiona, where a 

 kind of solid blastula is present. 



It is to be observed that the amphibian and protochordate blastulae differ 

 in several details. In the first place, there is a greater quantity of yolk material 

 in the blastula of the Amphibia; hence the presumptive entodermal area or 

 hypoblast projects considerably into and encroaches upon the blastocoel. 

 Also, in Amphioxus, the presumptive notochordal area forms a distinct dorsal 

 crescent apart from the presumptive mesodermal or ventral crescent (fig. 

 167F), whereas, in the Amphibia, the notochordal material is sandwiched 

 in between the two wings of mesoderm, so that these two areas form one 

 composite marginal zone crescent (fig. 169D, E). 



