184 SOME RECENT DEVELOPMENTS 



nucleus to form a polar body the chromatin, which 

 is present rather in the form of spherules than of 

 threads, is exactly halved in amount. 



Concerning the fusion of the male and female pro- 

 nuclei in the act of fertilisation some points of great 

 interest have recently been brought to light. In some 

 cases, as shown by Nussbaum in Ascaris, a direct 

 fusion of the two pronuclei is described as occurring, 

 while in other forms and by other observers the 

 process is said to be of a more complicated char- 

 acter. In the Nematodes, genus Leptodera, Nussbaum 

 says that the two pronuclei, male and female, take 

 up a position parallel to the long axis of the egg, 

 which is ovoid in shape, and then fuse together 

 lengthways. The first segmentation plane is a 

 longitudinal one and passes along the axis 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 pro- 

 nucleus. Inasmuch as all the cells of the body of 

 the adult animal are derived by division from the 

 two primary ones, it follows, as Nussbaum 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, one half of which is derived from the 

 father and the other half from the mother. 



This suggestion, the bearing of which on theories 

 of heredity is of the greatest importance, has 

 received most striking confirmation from the 

 extraordinarily minute observations of van Beneden 

 on the eggs of Ascaris. These investigations, 



