GENERAL OUTLINE 179 



matin of the germinal vesicle. 1 In the formation of the first polar 

 body the group of chromosomes splits into two daughter-groups, and 

 this process is immediately repeated in the formation of the second 

 without aii. intervening rcticular resting stage. The egg-nucleus 

 therefore receives, like each of the polar bodies, one-fourth of the 

 mass of chromatin derived from the germinal vesicle. 



But although the formation of the polar bodies was thus shown to 

 be a process of true cell-division, the history of the chromosomes was 

 found to differ in some very important particulars from that of the 

 tissue-cells. The essential facts, which were first accurately deter- 

 mined by Boveri in Ascaris ('87, i), are in a typical case as follows 

 (Figs. 88, 89) : As the egg prepares for the formation of the first polar 

 body, the chromatin of the germinal vesicle groups itself in a num- 

 ber of masses, each of which splits up into a group of four bodies 

 united by linin-threads to form a "quadruple group" or tetrad 

 (Vierergruppe). The number of tetrads is always one-half the usual 

 number of chromosomes. Thus in Ascaris (megalocephala, bivalens] 

 the germinal vesicle gives rise to two tetrads, the normal number of 

 chromosomes in the earlier divisions being four ; in the salamander 

 and the frog there are twelve tetrads, the somatic number of chro- 

 mosomes being twenty-four (Fleming, vom Rath), etc. As the first 

 polar body forms, each of the tetrads is halved to form two double 

 groups, or dyads, one of which remains in the egg while the other 

 passes into the polar body. Both the egg and the first polar body 

 therefore receive each a number of dyads equal to one-half the usual 

 number of chromosomes. The egg now proceeds at once to the 

 formation of the second polar body without previous reconstruction 

 of the nucleus. Each dyad is halved to form two single chromo- 

 somes, one of which, again, remains in the egg while its sister passes 

 into the polar body. Both the egg and the second polar body accord- 

 ingly 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 egg a reticular nucleus, much smaller than the 

 original germinal vesicle, is now formed. 2 



Essentially similar facts have now been determined in a consider- 

 able number of animals, though the form of the tetrads varies greatly, 

 and there are some cases in which no actual tetrad-formation has been 

 observed (apparently in the flowering plants). It is clear from the 



1 The early accounts asserting the disappearance of the germinal vesicle were based on 

 the fact that in many cases only a small fraction of the chromatic network gives rise to 

 chromosomes, the remainder disintegrating and being scattered through the yolk. 



2 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. 



