350 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE 



blastula in chordate animals. Although many different specific types and shapes 

 of blastulae are present in the group as a whole, all of them can be resolved 

 into two basic groups. These groups, as mentioned in the beginning of this 

 chapter, are: 



( 1 ) blastulae without auxiliary, nutritive tissue and 



(2) blastulae with auxiliary tissue. 



Moreover, if the auxiliary tissue of those blastulae which possess this tissue 

 is not considered, all mature chordate blastulae can be reduced to a funda- 

 mental condition which contains two basic layers, namely, hypoblast and epi- 

 blast layers. The epiblast possesses presumptive epidermal, neural, notochordal, 

 and mesodermal, organ-forming areas, while the hypoblast cells form the 

 presumptive entodermal area. The shapes and sizes of these blastulae will, 

 of course, vary greatly. Moreover, the hypoblast cells may be present in 

 various positions, such as a mass of cells at the caudal end of a disc-shaped 

 epiblast (teleost and elasmobranch fishes), an enlarged, thickened area or 

 pole of a hollow sphere (many Amphibia) , a single, relatively thin layer of 

 cells, forming part of the wall of a hollow sphere (Aniphioxus), a rounded, 

 disc-shaped mass of cells overlain by the thin, cup-shaped epiblast (Clavelina), 

 a thickened mass attached to the underside of the caudal end of the disc-shaped 

 epiblast (chick; certain reptiles), a thin layer of cells situated below the epiblast 

 layer (mammals), or a solid mass of cells, lying below a covering of epiblast 

 cells (gymnophionan Amphibia). Although many different morphological 

 shapes are to be found in the blastulae of the chordate group, the essential, 

 presumptive, organ-forming areas always are present, and all are organized 

 around the presumptive notochordal area. 



But the question arises: Why is a generalized blastular pattern developed 

 instead of a series of separate, distinct patterns? For instance, why should the 

 notochordal area appear to occupy the center of the presumptive, organ- 

 forming areas of all the chordate blastulae when this area persists as a promi- 

 nent morphological entity only in the adult condition of lower chordates? 

 The answer appears to be this: The notochordal area at this particular stage 

 of development is not alone a morphological area, but it is also a physiological 

 instrument, an instrument which plays a part in a method or procedure of 

 development. The point of importance, therefore, in the late blastular stage 

 of development is not that the notochordal area is going to contribute to the 

 skeletal axis in the adult of the shark, but rather that it forms an integral part 

 of the biolgical mechanism which organizes the chordate embryo during the 

 period immediately following the blastular stage. Thus, if the notochordal 

 material can play an important role in the organization of the embryo and 

 in the induction of the neural tube in the fish or in the frog, it also can fulfill 

 a similar function in the developing chid or human embryo. Whatever it does 

 later in development depends upon the requirements of the species. To use 



