TRANSLATORS INTRODUCTION xv 



Form, size, and number are different in different 

 animals, but there is reason to believe that they 

 are normally the same in all the individuals of a 

 species. The fertilised nucleus, thus consisting of 

 chromosomes from male and female, then divides 

 by a complicated process known as karyokinesis, 

 in which each chromosome splits longitudinally, one 

 half passing to each daughter-nucleus. Through- 

 out the whole process of embryonic and post- 

 embryonic growth, the chromatin is gradually 

 increasing in bulk, and being distributed by 

 karyokinesis. The normal character of these 

 divisions is as follows : A daughter-nucleus, after 

 separation, passes through a resting phase, in 

 which the chromosomes, as detinite structures, 

 disappear, and in which growth of the nuclear 

 matter occurs. Then chromosomes of definite 

 size and form, and corresponding in number to 

 those present in the fertilised egg-cell, again 

 appear. These split longitudinally, and a half of 

 each passes to each daughter-nucleus. The simi- 

 larity of these processes among all living creatures, 

 vegetable and animal, and their extreme complica- 

 tion, suggests that karyokinesis is the chief factor 

 in distributing the hereditary mass to the growing 

 organism. Weismann and some others think that 

 there is evidence for a difference in the nature of 

 the process, which may in some cases correspond to 

 his distinction between doubling and differentiating 

 divisions, but it may be said at once that the record 

 of observations is yet too conflicting for any such 

 general interpretation. 



