THE PHYSICAL BASIS OF HEREDITY. 



159 



reproductive, become isolated from each other quite 

 early in development; in one case, indeed, the differen- 

 tiation of reproductive cells from the somatic ones has 

 been traced by Boveri back to the first division of the egg. 

 This case of Ascaris ffiegalocephala is so striking and of such 

 fundamental theoretical importance that it must not be 

 passed without notice, for in it we find marked differences 

 between the somatic and reproductive cells in their nu- 

 clear structure, their relative amount of chromatin, and 

 mode of division. The egg of Ascaris has been the 

 classical object for cytological studies on account of its 

 small number of chromosomes (two in variety umvalefzs^ 

 four in bivalens), their large size, and the diagrammatic 

 clearness of the changes which take place in division. 

 In the division of the fertilized egg cell we have two 

 (in univalens) long chromosomes handed over to each 



daughter cell. As these two cells in 

 Differentiation of ixivn divide, a Striking difference is seen 

 reproductive and j^ ^^^ karyokinetic figures. In Fig. 6, 

 somatic tissues . , ■ 



in Ascaris ' ^"^ ^ two-celled Stage is seen from 



the pole ; in Fig. 6, B, a slightly later 

 stage in side view of the spindle. In the upper cell 

 of Fig. 6, A, the division is of the usual form, the two 

 chromosomes split longitudinally, and their two halves 

 travel to opposite poles of the spindle (Fig. 6, £). But 

 in the lower cell this is not the case. The central por- 

 tion of the two chromosomes is broken up into a large 

 number of minute chromatin granules which divide, and, 

 as shown in Fig. 6, B, form the only portion of the chro- 

 mosomes drawn up to the poles and entering into the 

 structure of the resting nuclei after the division is com- 

 plete. The large swollen outer ends of the chromo- 

 somes are cast off into the cytoplasm and are eventually 

 absorbed, playing no further part as nuclear structures. 



Fig. 6, C, shows the four-celled stage, in which a marked 

 12 



