DEVELOPMENT OF THE FROG 1 (1) 



1. Place an unsegmented Frog's egg in a watch glass containing water 

 and examine it with both the dissecting microscope and the low power 

 of the compound microscope. Note that the egg is enclosed in a trans- 

 parent jelly, and that it consists of a dark-colored portion (animal 

 pole), which tends to lie uppermost in the water, and a lighter-colored 

 portion (vegetal pole). Make a drawing of the egg from the side, one 

 inch in diameter, so as to show both poles and the surrounding layer of 

 jelly. 



2. Examine, in the same way, the following cleavage stages of Frog's 

 eggs : 2-4 cells, 8 cells, 20-30 cells, and many cells, or blastula. 

 Note in each stage the direction of the cleavage planes and the compara- 

 tive size of the cells in the animal and vegetal poles. Make a drawing of 

 each stage from the side, omitting the jelly. 



3. Examine, as before, a Frog's egg in which the growth of the dark 

 cells of the animal pole over the light cells of the vegetal pole has begun 

 (gastrula). As this process of gastrulation continues, the circular area 

 (blastopore) at the vegetal pole becomes smaller. Finally, when gas- 

 trulation is complete, the blastopore is very small, and only a tiny por- 

 tion of the cells of the vegetal pole is visible externally, which is known 

 as the yolk plug. Draw the gastrula from the vegetal pole, showing 

 the yolk plug. 



1 B. pp. 222-225. 



359 



