252 EVOLUTION AND ANIMAL LIFE 



begins development the kind of cells or tissues or organs will 

 be best developed whose determinants happen to be the better 

 fed, stronger ones, while other parts of the body may be made 

 smaller or even not appear at all on account of the starvation 

 of their determinants; also the stronger determinants in the 

 better developed parts of the body will produce by multiplica- 

 tion more and stronger daughter determinants for the germ 

 cells of the new individual than the weak determinants in the 

 ill-developed body parts, and thus this disparity in develop- 

 ment of body parts will be passed on, cumulatively, to^ suc- 

 cessive generations : which is nothing more nor less than . de- 

 terminate variation. 



All the speculations about the ultimate structure of the 

 germ plasm are interesting, but none of them of course is really 

 convincing. As Delage has well said, the chances are too many 

 to one against the probability of anyone's guessing correctly 

 the actual facts concerning the complex structural detail of 

 the protoplasmic make-up. The structural or inherent factors 

 in ontogeny, then, are to be understood only in so far as ob- 

 vious results or effects may reveal them. Now there is one set 

 of phenomena in ontogem^, to which we have not as yet called 

 attention, which does seem to throw some light on certain 

 essential features or facts of germ-cell structure which other- 

 wise would not be obvious to us. This set of phenomena is 

 that called mitosis or karyokinesis , and occurs in connection 

 with each division or cleavage of the egg cells, and of their 

 daughter cells or blastomeres. It occurs also in the division 

 or multiplication of cells in all the tissues of the body, and is 

 a phenomenon normal to cell increase anywhere in the body 

 at any time in the life of the organism. 



Direct or amitotic cell division is much less common and 

 seems to be restricted to certain kinds of tissues or to certain 

 periods in the history of the life of certain tissues. However, 

 the recent investigations of Child and others show that cell di- 

 vision without mitosis is more common than is usually thought. 

 In this kind of division, the process consists simply of the con- 

 striction and equal (or unequal) splitting of the cell body into 

 two parts, the dividing of the nucleus usually being slightly 

 in advance of that of the cytoplasm. Each half of the parent 

 cell has then but to increase in size to become the counterpart 

 gf its progenitor, In the mitotic or indirect division, on the 



