42 GENERAL BIOLOGY 



Make a drawing of a gastric pit and of the glands empty- 

 ing into it, as seen with the high power. 



Vm. Embryology. 



It will be found necessary to have a complete series of 

 eggs and tadpoles preserved in formalin, especially in large 

 classes. It is also desirable to have the living material at 

 hand when undertaking this study. 



1. The Egg. Examine the eggs in a mass, also compare 

 the masses of the toad and other amphibia. How thick is 

 the surrounding jelly? Distinguish in it three layers, w T ith 

 concentric layers in them. What is the color of the egg? 

 The darker side of the egg is the animal or protoplasmic pole, 

 the side opposite is the yolk or vegetative pole. 



Make a drawing of the egg and its envelopes, also draw 

 each of the following stages as studied. 



2. Early Cleavage Stages. What evidence do you find 

 that the egg differs from the ones previously studied? Find 

 one with a single groove surrounding it. In what direction 

 does the groove extend? Are the resulting parts equal? 

 Find an egg with a second groove at right angles to the first. 

 The first one gives the two-cell stage, the second one the 

 four-cell stage. Are the cells equal in size? The third 

 groove passes in a horizontal or equatorial direction. How 

 many cells result ? What is their relative size ? 



3. Later Cleavage Stages. Study eggs in the sixteen-cell, 

 and thirty two-cell stages. How are these stages produced 

 from the earlier stages? Are the cells equal in size? Ex- 

 amine both poles of the eggs and explain the differences 

 found. Does the relative size of the pigmented and un- 

 pigmented areas remain unchanged in later cleavage stages? 

 How is this to be explained? 



