178 GENERAL PRINCIPLES OF ZOOLOGY. 



choose special expressions for those progressive occur- 

 rences whose purpose is to render fertilization possible. 

 Very often, but not in all cases, there is necessary an active 

 transmission of the sperm from the male to the female, a cop- 

 ulation. In case of many marine animals, particularly most 

 fishes, cchinodernis, ccelenterates, the eggs and the sperma- 

 tozoa are discharged into the water, and the union of these 

 (impregnation or fecundation) depends upon chance. One 

 can bring about then artificially what is accomplished by 

 nature, by obtaining from the sexual organs the ripe prod- 

 ucts and bringing them together. For example, from the 

 uterus of a female frog one may take the eggs and im- 

 pregnate them with sperm from the testes of the male ; or 

 by suitable pressure upon the body of sexually ripe fishes, 

 the eggs may be collected in one dish, the sperm in another, 

 and the contents of the latter poured over the former, and 

 thus in many cases an entirely normal development may 

 be obtained. Such a proceeding is called artificial fer- 

 tilization; it would be more correct to call it artificial 

 fecundation. 



Fertilization. Let us now pass to the process of fer- 

 tilization in the narrower sense ; this begins with the en- 

 trance of the spermatozoon into the egg. Usually the egg 

 is surrounded by a gelatinous envelope, the chorion, to the 

 surface of which the spermatozoa remain attached, and 

 through which they bore until they reach the surface of 

 the egg (Fig. 90). But since the chorion, particularly in 

 eggs which are laid in the open air, must be hard and re- 

 sisting, there exists in it very often a special arrangement, 

 the micropyle, rendering possible the entrance of the 

 spermatozoon ; this is a single canal extending through the 

 entire thickness of the chorion, as in the eggs of fishes, or 

 a group of such canals, as in those of almost all insects. 



Monospermy and Polyspermy. Through the gelatin- 

 ous envelope, or through the micropyle canal, many sper- 

 matozoa may pass, but under normal conditions never more 

 than one serves for fertilization. To meet that spermatozoon 

 which is in ever so slight a degree in advance of the others, 



