66 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



6. Gastrula. Examine slide " Asterias gastula or larval stages." This 

 stage marks the end of the cleavage stage (although of course cell division con- 

 tinues) and the beginning of differentiation. The gastrula arises from the blastula 

 by the pushing in of one wall just as if one should thrust one's finger into the 

 side of a rubber ball. The gastrula appears on the slide as an oval body, contain- 

 ing a central darker projection. Find one in profile, obtain an optical focus on 

 it and draw. The outer layer of cells is called the ectoderm; the inner layer, 

 which has been pushed in or invaginated, the entoderm; the entire invaginated 

 structure is the archenteron, or primitive intestine; and the opening of the 

 archenteron to the outside is the blastopore. The next step in development is 

 the formation of a third layer of cells, the mesoderm, between the ectoderm and 

 entoderm, and these three layers are called germ layers because from them all the 

 structures of the adult organism are derived. 



B. DEVELOPMENT OF THE FROG 



Preserved material will be provided for this purpose. Remove the jelly 

 from the eggs. Consult Holmes, chapter v, pp. 89-103. 



1. Cleavage stages. The egg of the frog is much larger than that of the 

 starfish because it contains a considerable quantity of yolk, a semifluid food 

 material containing proteins and fats. The egg is black and white, the black 

 half constituting the animal hemisphere and consisting largely of protoplasm, 

 while the white half, the vegetative hemisphere, holds most of the yolk. Owing 

 to the presence of yolk, the vegetative hemisphere divides more slowly and 

 produces larger cells than the animal hemisphere. 



Study with the low power or with a hand lens and draw two-, four-, and 

 eight-cell stages. Note the inequality in the size of cells of the two hemispheres, 

 and the tendency of the vegetative cells to lag behind, so that six cell stages may 

 occur. 



2. Blastula. After numerous cleavages a blastula is produced consisting of 

 numerous very small black cells, and less numerous larger white cells. Draw. 

 Then with a sharp knife bisect the blastula by a cut which cleaves the black and 

 white hemispheres, and examine the cut surface with a hand lens. Compare 

 with Holmes, Fig. 18 (p. 93), and draw. The blastula of the frog differs from 

 that of the starfish in that its wall is composed of several layers of cells. 



3. Gastrula or yolk plug stage. Owing to the large amount of inert yolk 

 in the white cells, they cannot invaginate as in the case of the starfish, but the 

 gastrula is formed mainly by the growth and extension of a sheet of black cells 

 down over the white cells. The white cells are thus inclosed and become the 

 entoderm. The closure of the black cells over the white is not quite complete, 

 leaving a circular opening, the blastopore, through which some of the white cells 

 protrude, producing a circular white area, the yolk plug. Draw, showing the 



