352 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE 



Step of development or gastrulation. The particular shape of the blastula has 

 its importance. However, this importance does not lie in the supposition that 

 it conforms to a primitive spherical type but rather that the various, pre- 

 sumptive, organ-forming areas are so arranged and so poised that the cell 

 movements so necessary to the next phase of development or gastrulation 

 may be properly executed for the particular species. In most species, the 

 formation of a blastocoelic space also is a necessary function of blastulation. 

 In some species, however, this space actually is not formed until the next 

 stage of development or gastrulation is in progress. 



In summary, therefore, it may be stated that the importance of the blastula 

 does not reside in the supposed fact that it is a one-layered structure or 

 blastoderm having a particular shape. Rather, its importance emerges from 

 the fact that the blastoderm has certain, well-defined areas segregated within 

 it — areas which will give origin to future organ structures. Moreover, these 

 areas foreshadow the future germ layers of the body. In diploblastic Metazoa, 

 two germ layers are foreshadowed, while in triploblastic forms, three germ 

 layers are outlined. As far as the Chordata are concerned, the hypoblast is 

 the forerunner of the entoderm or the internal germ layer; whereas the 

 epiblast is composed potentially of two germ layers, namely, the epidermal, 

 neural plate areas which form the ectodermal layer and the chordamesodermal 

 or marginal zone cells which give origin to the middle germ layer. 



In the following pages, the chordate blastula is described as a two-layered 

 structure composed of various, potential, organ-forming areas. This two- 

 layered configuration, composed of a lower hypoblast and an upper epiblast, 

 is used to describe the chordate blastula for the dual purpose of comparison 

 and analysis of the essential structure of the various blastulae. The bilaminar 

 picture, it is believed, will enable the student to understand better the changes 

 which the embryo experiences during the gastrulative period. 



G. Description of the Various Types of Chordate Blastulae with an 

 Outline of Their Organ-forming Areas 



1. Protochordate Blastula 



The following description pertains particularly to Amphioxus. With slight 

 modification it may be applied to other protochordates, such as Clavelina, 

 Ascidiella, Styela, etc. 



As noted in the introduction to this chapter, the potential entodermal cells 

 of Amphioxus lie at the vegetal pole and form most of the floor or hypoblast 

 of the blastula (fig. 167D). The upper or animal pole cells form a roof of 

 presumptive epidermal, notochordal, mesodermal, and neural cells arched 

 above and around the entoderm. The latter complex of organ-forming cells 

 forms the epiblast. The blastocoelic cavity is large and insinuated between 

 the hypoblast and epiblast. The presumptive notochordal and mesodermal 



