322 GENERAL PRINCIPLES 



actually takes place in many animals, but the polar bodies 

 undergo no further development and are to be regarded as 

 parts ejected from the maturing egg cell as useless. It is also 

 reasonable to regard the first polar body of Ascaris as poten- 

 tially equivalent to two second polar bodies. This leads then 

 to the conclusion that the egg nucleus and the nuclei of the 

 polar bodies are the equivalents of the four sperm nuclei which 

 developed from one sperm mother cell. 



697. The essential difYerence between the ripe egg cell and 

 the sperm lies in the great size of the egg and the motility of the 



n/"^ Spermato- 



( J cyte I 



Oocyte II C; O () () %\Tu' 



Ovum OO 60 00 00 Spermatids 



Polar globules 



Fig. 185. — Diagram to show the similarity in the development of ova and sperm 



cells. (McMurrich.) 



sperm. The great size of the egg is due in part to the prolonged 

 growlh period, during which the quantity of protoplasm is 

 greatly increased and reserve food in the form of yolk granules 

 is stored up within the cell, and in part to the formation of two 

 (three) rudimentary cells (polar bodies) in the process of matura- 

 tion instead of four equals cell, which leaves one with the devel- 

 opmental material which would otherwise be divided among 

 the four. These are evidently provisions for the early develop- 

 mental period of the embryo. 



698. The consequences of these preparatory processes are 

 now readily seen. The second maturation division is called 



