Reproduction and Development. 



5i 



The ovum before fertilization is a simple spherical cell, 

 without any large amount of nutritive material in the 

 form of food-yolk 04.). It contains a nucleus. Previous 

 to fertilization, however, in many forms of life, portions of 

 the nucleus, amounting to three parts of its mass, are got 

 rid of in little "polar cells" budded off from the ovum. 

 The import of this process we shall have to consider in 

 connection with the subject of heredity. The sperm is also 



Fig. 11. — Diagram of development. 

 See text. The fine line across G. indicates the plane of section shown in H. 



a nucleated cell ; and on its entrance into the ovum there 

 are for a short time two nuclei — the female nucleus proper 

 to the ovum, and the male nucleus introduced by the sperm. 

 These two unite and fuse to form a joint nucleus. Thus the 

 fertilized ovum starts with a perfect blending of the nuclear 

 elements from two cells produced by different parents. 



