134 GENERAL PRINCIPLES OF ZOOLOGY 



the contrary, there ensues a pause after the first polar body has been formed, or 

 the egg may remain in the oocyte stage; the egg then requires the entrance of a 

 spermatozoon in order to complete the further changes, i.e., the formation of the 

 second polar body and reconstruction of the egg-nucleus. This dependence of 

 the last phenomena of maturation upon the beginning of fertilization led for a 

 long time to the error that the formation of the polar bodies was a part of the 

 fertilization process itself. 



Spermatogenesis. The maturation of the egg has its counterpart in the 

 formation (maturation) of the spermatozoa spermatogenesis. As the oocyte, 

 by division, gives rise to four cells (the polar globules and the ripe egg), so the 

 spermatocyte, a cell comparable to the oocyte, divides into four spennatids. 

 Yet the two differ in that usually all four spermatids become spermatozoa. 

 That three of the four sex-cells of the female remain rudimentary (polar globules) 

 the fourth alone forming an egg, is explained by the need of the egg to con- 

 tain all possible material for use in development. 



Reduction Division. In the maturation division of both male and female 

 sex-cells agree in the chromosome reduction. This is due to the fact that the 

 maturation spindles have but half the number of chromosomes characteristic 

 of the species. Usually these chromosomes are distributed in four groups 

 (tetrads] in the preparatory stages of the egg, the tetrads later being distributed 

 among the four products of the divisions (fig. 95). The significance of this 

 will be shown in connection with the phenomena of fertilization (p. 138). 



2. Fertilization. 



Copulation and Fecundation. The term fertilization refers to the 

 internal processes which, after the meeting of the egg and spermatozoon, 

 go on in the interior of the former and end with a complete fusion of the 

 two sexual cells; on the other hand, special expressions are necessary 

 for those preparatory processes whose purpose is to render fertilization 

 possible. Very often, but not in all cases, there is an active transfer of the 

 sperm from the male to the female, a copulation. In many marine animals 

 (most fishes, echinoderms, ccelenterates) the eggs and the spermatozoa 

 are discharged into the water, and the union of these (impregnation or 

 fecundation) depends upon chance. 



Fertilization. The process of fertilization begins with the entrance 

 of the spermatozoon into the egg. Usually the egg is surrounded by a 

 gelatinous envelope, the chorion, to which the spermatozoa adhere, and 

 through which they bore until they reach the surface of the egg (fig. 96). 

 But since the chorion, particularly in eggs laid in the air, may be hard and 

 resisting, there exists very often a special arrangement, the micro pylar 

 apparatus, for the entrance of the spermatozoon; this may be a single 

 canal extending through the chorion, as in the eggs of fishes, or a group 

 of such canals, as in most insects. 



Monospermy and Polyspermy. Many spermatozoa may reach the 

 egg but normally only one serves for fertilization. The spermatozoon 

 which is in the slightest degree ahead of the others is met by a process of 



