THE FERTIL,ISATION OF THE EGG 17 



passage through the cortex, and it is this activation which 

 enables them to complete their further development. 



The best study of the processes which bring about the fusion 

 of the pronuclei was made by Fankhauser (1925-41) in newts. 

 In this group, there is so-called facultative polyspermy, i.e. 

 more than one sperm may penetrate into one egg without 

 disturbing the development. One of the male pronuclei combines 

 with the nucleus of the egg. The other sperms which have 

 penetrated show some initial development, but then they dis- 

 integrate. Fankhauser constricted eggs shortly after fertilisa- 

 tion, by means of a hair ligature, dividing them either com- 

 pletely or partially into two halves. One half contained the 

 egg nucleus, with or without one or more sperm nuclei. The 

 other half contained only sperm nuclei. 



Fankhauser observed that, if constriction was complete, a 

 number of sperms developed simultaneously, forming large 

 asters in the half without the egg nucleus. In the case of 

 incomplete constriction, what happened in this half of the now 

 dumb-bell shaped egg depended upon whether or not, in the 

 other half, the egg nucleus had meanwhile united with a sperm 

 nucleus. If this had taken place, development occurred mainly 

 in those sperms in the half without the egg nucleus that were 

 farthest removed from the peduncle which joined the two 

 halves (Fig. 5). If the egg nucleus had not fused (because there 

 were no sperms in its half of the egg), development took place 

 in the sperm closest to the ligature in the other half. This 

 sperm nucleus and the egg nucleus moved towards each other, 

 and met on, or close to, the bridge. Fusion of the pronuclei then 

 took place. Fankhauser concluded from these observations, (1) 

 that there is an attraction between male and female pronuclei, 

 (2) that the egg nucleus promotes the development of the 

 nearest sperm, and (3) that this "favoured" sperm, and later 

 the zygote nucleus originating from the fusion of the pronuclei, 

 inhibit the development of the remaining sperms. It seems 

 probable that these promoting and inhibiting influences are due 

 to substances secreted by the nuclei, and diffusing into the 

 protoplasm. 



Some observations on the egg of the fresh water snail 



Raven - Outline Physiologie 2 



