342 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE 



of Amphioxus possesses five major, presumptive, origan-forming areas (fig. 

 167A). These areas ultimately give origin to the ectodermal, mesodermal, 

 entodermal, notochordal, and neural tissues. In the eight-cell stage of cleavage, 

 the cytoplasmic substances concerned with these areas are distributed in such 

 a way that the blastomercs have different substances and, consequently, differ 

 qualitatively (fig. I67B). Specifically, the entoderm forms the ventral part 

 of the four ventral blastomeres; the ectoderm forms the upper or dorsal portion 

 of the four micromeres, while the mesodermal, notochordal, and neural sub- 

 stances lie in an intermediate zone between these two organ-forming areas, 

 particularly so in the blastomeres shown at the left in figure 167B. In figure 

 167C and D is shown a later arrangement of the presumptive, organ-forming 

 areas in the middle and late stages of blastuiar development. These figures 

 represent sections of the blastulae. Consequently the organ-forming areas are 

 contained within cells which occupy definite regions of the hlastula. In figure 

 167E-G are presented lateral, vegetal pole, and dorso-posterior pole views 

 of the mature blastula (fig. I67D), representing the organ-forming areas as 

 viewed from the outside of the blastula. 



It is evident from this study by Conklin that the organization of the fertilized 

 egg of Amphio.xus passes gradually but directly through the cleavage stages 

 into the organization of the mature blastula; also, that the latter, like the egg, 

 is composed of five, major, presumptive, organ-forming areas. It is evident 

 further that one of the important tasks of cleavage and blastulation is to de- 

 velop and arrange these major, organ-forming areas into a particular pattern. 

 (Note: Later the mesodermal area divides in two, forming a total of six, pre- 

 sumptive, organ-forming areas. ) 



If we analyze the arrangement of these presumptive, organ-forming areas, 

 we see that the mature blastula is composed of a tloor or hypoblast, made 

 up of potential, entoderm-forming substance, and a roof of potential ectoderm 

 with a zone of mesoderm and chordoneural cells which lie in the area between 

 these two general regions. In fact, the mesodermal and chordoneural materials 

 form the lower margins of the roof of the mature blastula (fig. 167D). Con- 

 sequently, the mature blastula of Amphioxus may be pictured as a bilaminar 

 affair composed essentially of a hypoblast or lower layer of presumptive 

 entoderm, and an upper concave roof or e pi blast containing presumptive 

 ectoderm, neural plate, notochord, and mesodermal cells. It is to be observed 

 further that the blastocoel is interposed between these two layers. This is the 

 basic structure of a typical coeloblastula. Furthermore, this blastula is com- 

 posed entirely of formative tissue made up of certain definite, potential, organ- 

 forming areas which later enter into the formation of the body of the embryo; 

 auxiliary or non-formative tissue has no part in its composition. All coelo- 

 blastulae conform to this general structure. 



If we pass to the blastula of the early chick embryo, a striking similarity 

 may be observed in reference to the presumptive, organ-forming areas (fig. 



