40 Our Food Mollusks 



relative size. They are thus differentiated in structure 

 because they have become adapted to various uses. 



So far as we are able to judge, all cells, whether ani- 

 mal or plant, are essentially similar in their nature. Each 

 is a body of living protoplasm, usually with a jelly-like 

 consistency, but with a minute structure that is difficult 

 to determine. Each cell mass contains a small spherical 

 or ovoid body called the nucleus. This is a part of the 

 living substance, and is complicated in structure. It is 

 a constant and essential part of the cell. 



Among the cells of the bodies of animals and plants 

 are those set apart to perform the function of repro- 

 duction. The essential feature of sexual reproduction 

 is the union of two cells, usually one from each of two 

 parent individuals, to produce a new cell which, by mul- 

 tiplication, builds the body of the offspring. 



These two sexual cells that thus unite, differ from 

 each other in size and in structure. The female cell, or 

 ovum, as it is called, is usually spherical and often rela- 

 tively large from the presence of secreted yolk or food 

 substance. With this the body of the early embryo is 

 to be fed until it possesses a digestive tract, and has some 

 means of capturing its own food; or, in animals in 

 which the young develops within the body of the parent, 

 until it forms organs by means of which it can, in a 

 parasitic fashion, take its food from the body of the 

 mother. Because they carry the food substance, ova are 

 inert, and must be sought by the male cells. 



The spermatozoa, or male cells, are exceedingly minute. 

 In structure they are much the same in nearly all ani- 

 mals. Each is a single cell containing a nucleus. This 

 latter body forms what is known as the " head," and the 

 remainder of the cell is extended from it in the form of 



