234 



KEDrC'JJOX OF THE CHROMOSOMES 



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. The attempts to find such an 

 explanation have led to some of the most interesting researches of 

 modern cytology ; and though only i)artially successful, they have 

 raised manv new questions which promise to give in the end a deeper 

 insight into some of the fundamental questions of cell-morphology. 

 For this reason they deserve careful consideration, despite the fact 

 •that taken as a whole the subject still remains an unsolved riddle in 

 the face of which we can only return again and again to Boveri's 

 remark that whatever be its theoretical interpretation the numerical 

 reduction of the chromosomes is itself not a theory but a fact. 



Fig. 114. — Formation of the polar bodies before entrance of the spermatozoon, as seen in the 

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



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

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

 ( p. b. I, 2) ; e. the egg-nucleus. In this animal the first polar body fails to divide. For its division 

 see Fig. 89. 



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 maturation 

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

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

 the most general terms, this parallel is as follows : ^ In both sexes the 

 final reduction in the number of. chromosomes is effected in the course 

 of the last two cell-divisions, or maturation-divisions, by which the 

 definitive germ-cells arise, each of the four cells thus formed having 

 but half the usual number of chromosomes. In the female but one 



1 The parallel was first clearly pointed out by Plainer in 1889, and was brilliantly demon- 

 strated by Oscar Hertwig in the following year. 



