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 megalocephala 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 univalens, 
four in Jivalens), 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 turn divide, a striking difference is seen 
reproductive and in the karyokinetic figures. In Fig. 6, 
somatic tissues Z 
a ional: A, such a two-celled stage is seen from 
the pole; in Fig. 6, 4, a slightly later 
stage in side view of the spindle. In the upper cell 
of Fig. 6, 4, 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, 2). 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 
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