174 



REDUCTION OF THE CHROMOSOMES 



assumed an actual degeneration or casting out of half the chromo- 

 somes during the growth of the germ-cells — a simple and easily 

 intelligible process. The whole weight of the evidence now goes to 

 show, however, that this view cannot be sustained, and that reduction 

 is effected by a rearrangement and redistribution of tlie nnclear sub- 

 stance without loss of any of its essential constituents. It is true 

 that a large amount of chromatin is lost during the growth of the 

 It is nevertheless certain that this loss is not directly con- 



po'ir 



nected with the process of reduction ; for, as Hertwig and others 

 have shown, no such loss occurs during spermatogenesis, and even 

 in the oogenesis the evidence is clear that an explanation must be. 

 sought in another direction. We have advanced a certain distance 

 towards such an explanation and, indeed, apparently have found it 



Fig. 86. — Formation of the polar bodies before entrance of the spermatozoon, as seen in the 

 living ovarian egg of the sea-urchin Toxopneusfes (X 365). 



A. Preliminary change of form in the germinal vesicle. D. The first polar body formed, the 

 second forming. C. The ripe egg, ready for fertilization, after formation of the two polar bodies 

 (p.b.,1, 2); ^, the egg-nucleus. In this animal the second polar body fails to divide. For 

 division of the second polar body see Fig. 64. 



in a few specific cases. Yet when the subject is regarded as a 

 whole, the admission must be made that the time has not yet come 

 for an understanding of the phenomena, and the subject must there- 

 fore be treated in the main from an historical point of view. 



A. General Outline 



The general phenomena of maturation fall under two heads ; viz. 

 oogenesis, which includes the formation and maturation of the ovum, 

 and spermatogenesis, comprising the corresponding phenomena in 

 case of the spermatozoon. Recent research has shown that matura- 

 tion conforms to the same type in both sexes, which show as close a 

 parallel in this regard as in the later history of the germ-nuclei. Stated 



1 Cf. Figs. 71, 88. 



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