104 FERTILIZATION 



normal egg, but the cleavages are usually abnormal and retarded. 

 Nuclear and astral cycles are often out of phase in dividing super- 

 numerary sperm complexes. In the half of a previously fertilized 

 egg which contains the egg nucleus but no sperm nucleus, cleavage 

 is incomplete and anastral. When a half contains no egg nucleus, 

 cleavage is frequently abortive. If one half has neither egg nor 

 sperm nucleus, there is either no cleavage or abortive cleavage. 

 When a fertilized egg is constricted, not ligatured, so that it be- 

 comes shaped like a dumb-bell, the situation becomes more com- 

 plicated, but can be summarized as follows, calling one half of the 

 dumb-bell L and the other R. 



L, female nucleus and sperm; R, sperm; thin bridge betzveeti L 

 and R. Cleavage in L and R. 



L, female nucleus and sperm; R, sperm; thick bridge between L 

 and R. Cleavage in L but not in R. 



L, female nucleus; R, sperin. No cleavage in L, cleavage in R. 

 When the bridge is thick, the male or female nucleus sometimes 

 moves across the bridge and joins the other nucleus, in which 

 case a normal first cleavage furrow occurs in L or R. 



These experiments suggest that substances may diffuse out of 

 the female nucleus or the sperm nucleus under the influence of 

 proximity to the female nucleus, which cause degeneration of 

 supernumerary spermatozoa. Some information about the charac- 

 teristics of this substance might be obtained by continuing Fank- 

 hauser's studies with different bridge widths between dumb-bells 

 and by observing the variations in the times at which supernumer- 

 ary sperm nuclei degenerate according to their distance from the 

 fusion nucleus. These remarks apply particularly to the eggs of 

 Triturus helveticus. In those of Diemictylus viridescens, super- 

 numerary sperm nuclei which are at different distances from the 

 fusion nucleus begin to degenerate at the same time, a phenomenon 

 which seems to be incompatible with a diffusion mechanism. The 

 experiments of Allen (1954), on the behaviour of the egg nucleus 

 after fertilization of sea-urchin eggs sucked into narrow glass 

 capillaries, suggest that some substance diffuses out of the sperm 

 head or male pronucleus into the cytoplasm, where it affects the 

 female pronucleus. Allen found that the latter elongated or 

 bulged in the direction of the sperm head, which at the time was 

 located at the periphery of the elongated egg. To make distinctions 



