52 Elementary Biology. 



Morphology (structure). cont. Physiology (function). cent. 



D Contractile system Contraction motion. 



E. Supporting ,, Support. 



F. Protecting ,, Protection. 



G. Nervous ,, Nervation. 



Sense organs. Sensation. 



SECTION II. TRIBAL LIFE. 



The various systems we have already discussed are 

 concerned entirely with the maintenance of individual life. 

 One system present in the individual, however, we have 

 yet to examine, viz. the reproductive system, which is con- 

 cerned entirely with the maintenance of tribal life, i.e. the 

 maintenance on the earth's surface of organisms of the 

 same kind as that in which the special reproductive elements 

 under consideration are developed. 



Reproduction may be either sexual or asexual. Asexual 

 reproduction consists in the separation of a part (usually a 

 single cell) of the individual, which part is capable (without 

 union with any other part) of becoming an adult organism 

 like its parent 



By sexual reproduction is meant the production by the 

 same or different individuals of two kinds of cells, one of 

 which is known as the male cell (sperm or spermatozoon), 

 the other as the female (ovum), the union of which results 

 in a product (embryo) capable of forming, after it has passed 

 through several developmental changes, an adult organism 

 like one or other of its parents. It is worthy of note at 

 this point, that the male reproductive cell is almost always 

 in the animal, and very generally in the vegetal king- 

 dom, an extremely minute and very active body, whilst 

 the female cell in all cases (with a few exceptions in the 

 very lowest forms) is comparatively large and immobile. 

 After union with the male cell the female cell begins to 

 undergo changes which are probably due to the instigat- 

 ing influence of the male cell. In other words the female 



