A MAP OF THE EARLY GASTRULA 59 



Now it should be pointed out that this variation in the position of the 

 first cleavage plane normally involves no subsequent differences and the 

 whole egg with either type of first cleavage develops into a normal embryo. 

 The above results were obtained only when the egg was operated upon and 

 separated into two blastomeres. 



The experiments show very definitely that all the factors necessary for 

 the development of a normal embryo are localized in or around the gray 

 crescent. In fact, from this experiment it is clear that even half the gray 

 crescent contains all the factors necessary for development. 



A map of the early gastrula 



How does the gray crescent act to bring about the formation of the 

 embryo? To answer this question we must first show what happens to the 

 gray crescent during normal development, and we must determine the rela- 

 tion of the gray crescent to other parts of the egg. In other words, we first 

 need a map or a plan of the egg showing the relation of one part to another. 

 Such a plan may be obtained by staining the egg with vital dyes. Various 

 regions of the egg are stained, and the vital dyes are traced during develop- 

 ment in order to determine which structures are derived from the regions 

 stained. We can start with the gray crescent shortly after fertilization, stain- 

 ing this region with Nile blue sulfate, and we can then follow the develop- 

 ment of this egg into the gastrula stage (Fig. 24). 



The early gastrula is characterized by the dorsal lip of the blastopore. The 

 cells at this region leave the surface and migrate internally. This process 

 results in a sharp line of pigment which may be used as a marker in tracing 

 the ultimate fate of the cells which were stained blue. When the dorsal lip 

 of the blastopore appears it is found that this stain is located above the lip 

 of the blastopore. This shows, then, that the gray crescent of the egg gives 

 rise to the dorsal lip of the blastopore of the early gastrula. When the 

 gastrula develops into an embryo the stained gray crescent materials can be 

 seen in the head endoderm of the primitive gut, or archenteron. 



Now we shall continue to follow the parts of the egg into the embryo. In 

 so doing we shall not concern ourselves about the means by which the various 

 parts of the egg move to their final positions in the embryo. This subject 

 will be treated in some detail in Chapter 9. What we propose to do here is 

 to make a map of the regions of origin of the various structures. 



