212 FERTILIZATION 



fore, the union of the two haploid groups of chromosomes forms an integrated 

 association in which pairs of similar genes are yolced together to perform their 

 functions in the development of the new individual. 



In most animal species aside from the union of the chromosome groups, 

 there is a coalescence of most of the cytoplasm of the male gamete with that 

 of the female gamete as the entire sperm generally enters the egg (figs. 115, 

 118). However, in some species the tail of the sperm may be left out, e.g., 

 rabbit, starfish, and sea urchin, while in the marine annelid. Nereis, the head 

 of the sperm alone enters, the middle piece and tail being left behind. 



The morphological fusion of the two sets of nucleoplasms and cytoplasms 

 of the gametes during fertilization is made possible by certain physiological 

 changes which accompany the fusion process. These changes begin the instant 

 that a sperm makes intimate contact with the surface of the oocyte (or egg). 

 As a result, important ooplasmic activities are aroused within the egg which 

 not only draw the sperm into the ooplasm but also set in motion the physico- 

 chemical machinery which starts normal development. The initiation of normal 

 development results from the complete activation of the egg. Partial activation 

 of the egg is possible, and in these instances, various degrees of development 

 occur which are more or less abnormal. Partial activation of the egg happens 

 in most instances when the various methods of artificial activation (see p. 217) 

 are employed. 



While the main processes of activation leading to development are con- 

 cerned with the organization and substances of the egg, one should not over- 

 look the fact that the sperm also is activated (and in a sense, is fertilized) 

 during the fusion process. Sperm activation is composed of two distinct phases: 



( 1 ) Before the sperm makes contact with the oocyte or egg, it is aroused 

 by environmental factors to swim and move in a directed manner and 

 is attracted to the oocyte or egg by certain chemical substances secreted 

 by the latter; and 



(2) after its entrance into the egg's substance, the sperm nucleus begins 

 to enlarge and its chromosomes undergo changes which make it pos- 

 sible for them to associate with the egg chromosomes in the first 

 cleavage spindle. Also, the first cleavage amphiaster in the majority 

 of animal species appears to arise within the substance of the middle 

 piece of the sperm as a result of ooplasmic stimulation. 



In the process of normal fertilization it is clear, therefore, that two main 

 conditions are satisfied: 



( 1 ) There is a union of two haploid chromosome groups, one male and 

 the other female, bringing about the restoration of a proper diploid 

 genie balance; and 



(2) an activation of the substances of the fused gametes, both cyto- 



