I74 CAUSES OF VARIATION 



The later discovery of the mechanism of maturation and of 

 the extrusion of the polar bodies was a startling confirmation of 

 Weismann's prediction, and went far to fix his theories of hered- 

 ity in the minds of many biologists. In this connection Wilson 

 very pertinently remarks : 1 



The fulfillment of Weismann's prediction is one of the most interesting 

 results of recent cytological research. It has been demonstrated in a manner 

 which seems to be incontrovertible that the reducing divisions postulated 

 by Weismann actually occur, though not precisely in the manner conceived 

 by him, . . . but it remains quite an open question whether they have the 

 significance attributed to them by Weismann. 



Just when the reduction occurs is not known. It was at first 

 assumed that it occurs in connection with the extrusion of the 

 second polar body, an assumption based upon the development 

 of parthenogenetic eggs. But plants do not form polar bodies, 

 and again there is great uncertainty as to whether the rods 

 (tetrads), rings, or V-shaped bodies (whose number is half the 

 usual number of chromosomes) are to be regarded as represent- 

 ing the usual number of chromosomes split and arranged in 

 pairs, in which case the second polar body would accomplish 

 the reduction ; or whether the chromosomes as formerly known 

 never emerge from the nucleus of the oocyte, so that the iden- 

 tity of the chromosomes is in some way lost and the reduction 

 is effected at this early stage by some sort ,of internal fusing, or 

 perhaps by an entire rearrangement of chromatin granules. On 

 this point the evidence is confusing, but on two significant 

 points there is no doubt, the loss of chromatin matter out of 

 the line of descent, and a reduction of the chromosomes in the 

 germ cells to one half the somatic number. 2 



Composition of the chromosomes. 3 In view of the important 

 office of the chromosomes and the many theories of heredity 

 based upon their nature and constitution, in view also of their 

 evident importance in all studies on inheritance and variation, 



1 Wilson, The Cell, p. 246. 



2 By "somatic" number is meant the number characteristic of the soma, 

 the. body in general as distinct from the germinal matter whose function is not 

 growth but reproduction. 



8 Wilson, The Cell, pp. 294-304. 



