THE FERTILISATION OF THE EGG 15 



wave has passed. In completely normal eggs, this happens so 

 rapidly that the egg cortex becomes impenetrable practically 

 instantaneously. In eggs under unfavourable conditions, how- 

 ever, the process is retarded, and its advance can be followed. 

 In the frog's egg, e.g., such a retardation of the cortical reaction 

 can be produced by treatment with a sodium chloride solution 

 (Bataillon, 1919). It increases the chance that more than one 

 sperm will penetrate, and may easily give rise to polyspermy. 



We have seen above (p. 11) that one of the substances 

 secreted by the sea urchin egg paralyses sperms of other 

 species. This may be regarded as a barrier against hybrid 

 fertilisation, but it does not completely prevent it. Closely 

 related species can often be crossed easily. By means of such 

 artifices as high sperm concentrations, and modification of the 

 acidity of the medium, it is possible to combine eggs and 

 sperm of species which are very far apart taxonomically. In 

 this way, sea urchins' eggs have been successfully fertilised, 

 even with sperm of worms or molluscs (Kupelwieser, God- 

 lewski). We shall later return to these experiments. 



The sperm penetrates head first into the egg, but soon after- 

 wards it rotates through 180°, so that its head points towards 

 the egg surface, and its middle piece towards the centre. In 

 those cases where the tail of the sperm has entered into the 

 egg as well, this now drops off and dissolves in the egg cyto- 

 plasm. The head of the sperm swells strongly, and, regaining 

 the appearance of a normal nucleus, becomes the male pro- 

 nucleus. Originating from the middle piece of the sperm, a 

 star-shaped radiation, the sperm aster, appears in the egg 

 cytoplasm (Fig. 4, ^-^). Chambers has shown by microdissection 

 that this is due to local gelation of the cytoplasm. This process 

 of gelation spreads like a wave in all directions through the 

 protoplasm. At the same time, the centre of the sperm aster 

 liquefies again, forming an area of liquid protoplasm around 

 the male pronucleus. Simultaneously also, the male and female 

 pronuclei move towards each other, and meet in the central 

 area of the sperm aster. They may lie side by side for some 

 time, or, alternately, they may fuse immediately (Fig. 4, '*). 

 Now two new radiations appear, one on each side of the pair 



