238 REDUCTION OF THE CHROMOSOMES 



A study of the nucleus during these changes brings out the follow- 

 ing facts. During the multiplication of the oogonia the number of 

 chromosomes is the same as that occurring in the division of the 

 somatic cells, and the same number enters into the formation of the 

 chromatic reticulum of the germinal vesicle. During the formation 

 of the polar bodies this number becomes reduced to one-half, the 

 nucleus of each polar body and the egg-nucleus receiving the reduced 

 number. In some manner, therefore, the formation of the polar 

 bodies is connected with the process by which the reduction is ef- 

 fected. The precise nature of this process is, however, a matter 

 which has been certainly determined in only a few cases. 



We need not here consider the history of opinion on this subject 

 further than to point out that the early observers, such as Purkinje, 

 Von Baer, Bischoff, had no real understanding of the process and 

 believed the germinal vesicle to disappear at the time of fertilization. 

 To Biitschli i^J^i), Hertwig, and Giard {'j^, '"jy) we owe the discovery 

 that the formation of the polar bodies is through mitotic division, the 

 chromosomes of the equatorial plate being derived from the chro- 

 matin of the germinal vesicle.^ 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 an intervening reticular 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 carefully studied 

 in Ascaris by Van Beneden ('83, 'Sj), and especially by Boveri {'^y, i), 

 are in a typical case as follows (Figs. 116, 117): As the &gg prepares 

 for the formation of the first polar body, the chromatin of the ger- 

 minal vesicle groups itself in a number 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). T/ie nnniber of tetrads 

 ts aliuays 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 mole-cricket there are six tetrads, the somatic number 

 of chromosomes being twelve ; in Cyclops the respective numbers are 

 twelve and twenty-four (one of the most frequent cases); while in 

 Artemia there are eighty-four tetrads and one hundred and sixty- 



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. 



