8 LABORATORY OUTLINES FOR EMBRYOLOGY 



female pronucleus. Coincident with the changes in the 

 egg nucleus, the spermatozoan also changes into the so- 

 called resting stage and is called the male pronucleus. 

 These two pronuclei approach each other and fuse to form 

 ih& first segmentation nucleus. 



Draw a cell showing the male and female pronuclei. 

 Label all the parts. 



4. Cleavage 



The amount and distribution of the yolk is correlated 

 with the character of the division of the egg {segmenta- 

 tion, cleavage). In eggs, which, for the most part, are 

 small and contain a comparatively small amount of yolk 

 material {deuto plasm) , the segmentation is complete. 

 That is, the whole egg divides into two daughter cells. 

 This kind of cleavage is known as total or holohlastic 

 cleavage. 



If there is a very small amount of yolk and this yolk 

 almost uniformly distributed throughout the cytoplasm, 

 the daughter cells, in general, are of uniform size. This 

 kind of cleavage is known as equal cleavage. Examples of 

 holoblastic equal cleavage are found in the Echinoder- 

 mata, Amphioxus, and, in general, in the Mammalia. 

 If the yolk is concentrated more at one pole than at the 

 other and yet there is not a large amount of it, the cells 

 resulting from segmentation are unequal in size. This 

 kind of cleavage is known as unequal cleavage. Ex- 

 amples of holoblastic unequal cleavage are found in the 

 Cyclostomata and the Amphibia. 



In eggs where the amount of yolk is comparatively 



