1 82 REDUCTION OF THE CHROMOSOMES 



history of tJic chroma tin in these two divisions is exactly parallel to 

 that in the formation of the polar bodies (Figs. 91, 92). From the 

 chromatin of the spermatocyte are formed a number of tetrads equal 

 to one-half the usual number of chromosomes. Each tetrad is halved 

 at the first division to form two dyads which pass into the respec- 

 tive daughter-spermatocytes. At the ensuing division, which occurs 

 without the previous formation of a resting reticular nucleus, each 

 dyad is halved to form two single chromosomes which enter the 

 respective spermatids (ultimately spermatozoa). From each sperma- 

 tocyte, therefore, arise four spermatozoa, and each sperm-nucleus 

 receives half the usual number of single chromosomes. The par- 

 allel with the egg-reduction is complete. 



These facts leave, no doubt that the spermatocyte is the morpho- 

 logical equivalent of the oocyte or immature ovarian egg, and that 

 the group of four spermatozoa to which it gives rise is equivalent 

 to the ripe egg plus the three polar bodies. Hertwig was thus led to 

 the following beautifully clear and simple conclusion : " The polar 

 bodies are abortive eggs which are formed by a final process of 

 division from the egg-mother-cell (oocyte) in the same manner as 

 the spermatozoa are formed from the sperm-mother-cell (sperma- 

 tocyte). But while in the latter case the products of the division 

 are all used as functional spermatozoa, in the former case one of the 

 products of the egg-mother-cell becomes the egg, appropriating to 

 itself the entire mass of the yolk at the cost of the others which 

 persist in rudimentary form as the polar bodies." 1 



3. Theoretical Significance of Maturation 



Up to this point the facts are clear and intelligible. When, how- 

 ever, we attempt a more searching analysis by considering the origin 

 of the tetrads and the ultimate meaning of reduction, we find our- 

 selves in a labyrinth of conflicting observations and hypotheses from 

 which no exit has as yet been discovered. And we may in this case 

 most readily approach the subject by considering its theoretical 

 aspect at the outset. 



The process of reduction is very obviously a provision to hold con- 

 stant the number of chromosomes characteristic of the species ; for 

 if it did not occur, the number would be doubled in each succeeding 

 generation through union of the germ-cells. But why should the 

 number be constant ? 



In its modern form this problem was first attacked by Weismann 

 in 1885, and again in 1887, though many earlier hypotheses regarcl- 



1 '90, i, p. 126. 



