240 



HE DUCT! ox OF THE CHROMOSOMES 



while the other passes into the polar body. Both the (i^g and the 

 first polar body therefore receive each a number of dyads equal to 

 one-half the usual number of chromosomes. The e^*; now proceeds 

 at once to the formation of the second polar body without previous 

 reconstruction of the nucleus. Tlach dyad is halved to form two 

 single chromosomes, one of which, again, remains in the egg while 

 its sister pas.ses into the polar body. Both the <igg and the second 

 polar bodv accordingly receive two single chromosomes (one-half the 

 usual number), each of which is one-fourth of an original tetrad 

 group. From the two remaining in the (t^)^ a reticular nucleus, much 

 smaller than the original germinal vesicle, is now formed. ^ 



Primordial sjerm-cell. 



Spermatogonia. 



Primary spermatocyte. 



Secondary spermatocytes. 



Division-period (the number of divi- 

 sions is much greater). 



Growth-period. 



M aturation-period. 



Spermatids. 

 Spermatozoa. 

 Fig. ii8. — Diagram showing the genesis of the spermatozoon. [After BOVF.RI.] 



Essentially similar facts have now been determined in a consider- 

 able number of animals, though, as we shall presently see, tetrad- 

 formation is not of universal occurrence, nor is it always of the same 

 type. For the moment we need only point out that the numerical 

 reduction of chromatin-w^ri-jTi" takes place before the polar bodies 

 are actually formed, through processes which determine the number 

 of tetrads within the germinal vesicle. The numerical reduction is 

 therefore determined in the grandmother-cell of the Q,^g. The actual 

 divisions by which the polar bodies are formed merely distribute the 

 elements of the tetrads. 



^ It is nearly certain that the division of the first polar body (which, however, may be 

 omitted) is analogous to that by which the second is formed, i.e. each of the dyads is 

 similarly halved. Cf. Griffin, '99. 



