282 Cell as Ba^is of Organic Activity 



Gastrulation. — Due to the more rapid growth of the animal cells, 

 the vegetal cells gradually start inbuckling or invaginating. The ani- 

 mal cells at the same time more or less inroll around the margin of the 

 double-walled cup thus formed. These movements gradually result in 

 the obliteration of the blastocoel and the formation of a new cavity, 

 the primitive gut or archenteron. The mouth of this cavity is the blasto- 

 pore. This stage is known as the gastrula. 



Two layers of cells now are clearly visible. The outer layer is 

 the ectoderm; the inner one lining the archenteron is entoderm and, 

 in part, future mesoderm. These constitute the three germ layers of 

 the embryo. 



Dorsal to the blastopore, the lip formed by this overgrowth of cells 

 is known as the dorsal lip. It assumes a role of more importance as 

 development proceeds. 



Mesoderm Formation. — After the embryo becomes a gastrula, it 

 elongates and flattens. The anterior and posterior ends are clearly dis- 

 tinguishable. From the layer of cells surrounding the archenteron, 

 mesoderm buds off in the dorsolateral region on both sides. This leaves 

 the archenteron surrounded by only entodermal cells. These pockets 

 of mesoderm grow downward and outward, pushing between the ento- 

 dermal and ectodermal cells. The portion of the mesoderm next to the 

 ectoderm is known as somatic or parietal mesoderm.; that in contact 

 with the entoderm as the splanchnic mesoderm. The cavity between 

 these two layers is the body cavity or coelom. 



At this same time, a mass of cells proliferates and separates from 

 the dorsal wall of the archenteron. These become an elevated rodlike 

 structure, the notochord or primitive axial skeleton. 



The Effect of Yolk. — In all eggs, the protoplasm alone is involved 

 in the cleavages. The yolk is only incidentally involved. If it is pres- 

 ent in large amounts, it forms an impediment which interferes with the 

 divisions (Fig. 84). The egg of Amphioxus, as just described, has but 

 a very small amount of yolk which is evenly distributed ; thus it inter- 

 feres but very slightly with the cleavage of the entire zygote. Other 

 animals, however, have differing amounts of yolk, and hence modifica- 

 tion in the early cleavage stages occurs. The frog, for example, has a 

 moderate amount of yolk, and the cells formed at the animal pole are 

 smaller and more numerous than those at the vegetal pole. A large 

 amount of yolk is present in the eggs of birds, and in these, 



