12 ANATOMY OF THE RABBIT 



of ingesting food-materials, of discharging waste, of changing its 

 form, and of reacting in one way or another to stimuli arising out- 

 side of the body. Moreover, the protozoan cell is capable of giving 

 rise to new cells by division of its substance into two parts, which 

 process originates in the nucleus, and is usually associated at some 

 stage with union or conjugation of two parent cells. 



All the cells of the body of a multicellular organism are products 

 of a single cell, the fertilized egg, but the latter is a product of 



Fig. 1. Photomicrograph of a developing ovum 

 within the ovary of a rabbit, from a section. 

 X 150. ch, chromatin; cy, cytoplasm; fe, fol- 

 licular epithelium; nm, nuclear membrane; tf, 

 theca folliculi; zp, zona pellucida. 



fusion of two primary elements, the spermatozoon of the male 

 parent and the ovum of the female. The fertilized egg does not 

 exhibit the functions of a one-celled body, but possesses the poten- 

 tial of these functions, and the latter appear, to a large extent 

 individually, in the differentiation of its division-products into 

 specialized tissue elements. 



In this way, the processes which go on within the body of a 

 multicellular animal and the structure underlying these processes 

 are all based upon the same elementary functions of life as those 

 appearing in one-celled organisms. But the repeated division of 



