14 INTRODUCTION. 



stages of development consist in repeated division of the egg, 

 whereby it becomes converted from the unicellular condition, 

 which is permanent only in the Protozoa, to the multicellular 

 state characteristic of all higher animals, or Metazoa. To these 

 early processes of development the name segmentation is given. 



Segmentation is essentially a process of cell division, in 

 which the segmentation nucleus plays the same part that the 

 nucleus of an ordinary epithelial cell does in the act of division 

 of the cell ; the nucleus dividing first, and then the body of the 

 cell ; and the process being repeated again and again, until from 

 the single cell or ovum a very large number of cells are pro- 

 duced, from which by further division all the component cells of 

 the adult animal are ultimately derived. 



Special attention has been paid to the behaviour of the male 

 and female elements of the segmentation nucleus during its 

 first division. In the Nematode genus Leptodera, Nussbaum 

 states that the two pronuclei, male and female, take up a posi- 

 tion parallel to the long axis of the egg, which is ovoid in shape, 

 and then fuse together lengthways to form the segmentation 

 nucleus. The first segmentation plane is a longitudinal one, 

 and passes along the axes of the fused pronuclei, so that each 

 of the two cells formed by the first cleft contains one half 

 of the male pronucleus and one half of the female pronucleus. 

 Inasmuch as all the cells of the body of the adult animal are 

 derived by division from the two primary ones, it follows, as Nuss- 

 baum points out, that if this equal division of male and female 

 nuclear elements obtains in the later stages of cell division, each 

 cell of the adult animal will possess a nucleus derived in precisely 

 equal proportions from the father and from the mother. 



This suggestion, the bearing of which on theories of heredity 

 is of great interest, has received striking confirmation from the 

 researches of Van Beneden on the eggs of Ascaris. Van Beneden 

 finds that after extrusion of the polar bodies, and entrance of 

 the spermatozoon into the egg, the two pronuclei, male and female, 

 which are precisely equal in all respects, come very close together, 

 but do not fuse directly. Each pronucleus contains at first a 

 single, much convoluted, and varicose thread of chromatin, which 

 soon divides transversely into two, each of which becomes bent 

 into a U-shaped loop. There are thus four loops in all, two male 

 and two female. Each loop now splits longitudinally into two 



