248 GENERAL ZOOLOGY 



the beginning of the blastula stage in development. The walls 

 of the blastula bounding the blastocoel become several cell layers 

 in thickness as cleavage progresses. The cells forming the roof 

 and sides of the blastula are derived from animal pole cells and 

 are accordingly smaller than the yolk-laden cells forming the 

 floor of the blastula. The blastocoel becomes filled with fluid 

 absorbed from the outside or produced by metabolism of the 

 cells of the blastula (Fig. 152, 7, 8). 



Gastrulation. — Cleavage at first merely produces a number of 

 cells in the dark and light areas, respectively, so that for a time 

 the distribution of pigment is similar to that of the egg before 

 cleavage. Gradually the pigmented area encroaches on the 

 light area as the result of an overgrowth of the small pigmented 

 cells. The advancing edge of the pigmented cells, known as the 

 germ ring, slowly covers the vegetative cells, until the latter are 

 completely hidden, except in a very small area, known as the 

 yolk plug. At the beginning of the overgrowth a short horizontal 

 notch forms at the margin of the germ ring in the region of the 

 gray crescent and gradually extends around the entire margin of 

 the ring. Thus the advancing margin of the germ ring is marked 

 by a shallow groove which is more pronounced at its point of 

 origin. The germ ring may now be considered as a blastopore 

 whose dorsal lip lies in the region of the gray crescent. At the 

 dorsal lip an active invagination of cells takes place principally 

 from the animal-pole region, though yolk-laden cells are also 

 lifted from the floor of the blastocoel. As the germ ring is 

 reduced in size, the invagination becomes active throughout its 

 extent, but only in the region of the dorsal lip is a large cavity, 

 called the gastrocoel or archenteron, formed (Fig. 153). As this 

 cavity pushes into the interior of the gastrula, the blastocoel is 

 gradually obliterated. Externally the cells forming the yolk 

 plug are finally covered up by the complete closure of the blasto- 

 pore and gastrulation is completed. The region of the blastopore 

 may now be referred to as the posterior end of the embryo. 



Germ Layers. — The gastrocoel, produced largely as a result of 

 the invagination of the dorsal-lip region of the blastopore, lies in 

 the animal half of the egg. Its roof and side walls are derived 

 mainly from animal-pole cells and its floor is made up of yolk 

 cells. These cells represent the endoderm of the embryo except 

 for some cells lying in the mid-line of the roof which separate to 



