l8 FERTILIZATION 



which is responsible for the two pronuclei taking up a central 

 position within the egg, is what used to be called the Cleavage 

 Path of the fusion nucleus. The movements of the female pro- 

 nucleus towards the male pronucleus are thought by Chambers to 

 be caused by cytoplasmic streaming towards the middle of the male 

 pronucleus. 



This description of the movements of the pronuclei may seem 

 complicated, but in fact it is over-simplified in several respects; 

 for example, little or no attention has been paid to the somewhat 

 conflicting results obtained by earlier workers such as Fol (1879) 

 or to the path of the male pronucleus in Classes 1-3 fertilization. 

 A more serious difficulty concerns the behaviour of the female 

 pronucleus in the absence of a male pronucleus. As is well known, 

 activation makes the egg nucleus swell. But, as Moore showed in 

 1937, the female pronucleus not only swells after parthenogenetic 

 activation but also moves to the centre of the egg under its own 

 steam. The same occurs after pseudogamous fertilization. Roths- 

 child (1953) reproduced a photograph of several pseudogamous 

 sea-urchin eggs, in which a swollen female pronucleus can be 

 clearly seen in the centre of the egg, though in this species, 

 Paracentrotus lividus (Lamarck), the nucleus of the unfertilized egg 

 is often eccentrically placed in the cytoplasm. 



To sum up, the movement of the male pronucleus may be caused 

 by the growth of the sperm aster, at any rate in Class 4 eggs ; the 

 movement of the female pronucleus is influenced by, but not en- 

 tirely dependent on, the male pronucleus. Further experiments 

 on the morphology and mechanics of the reaction are clearly needed 

 before much will be learnt from chemical and biochemical studies. 



Austin (1951a) has given an excellent account of the formation, 

 growth and conjugation of the pronuclei in the rat egg. The most 

 striking features of the process on the male side are the 'dissolu- 

 tion' of the sperm head 10-60 minutes after entering the egg, and 

 the appearance of numerous male nucleoli which swell and ulti- 

 mately coalesce to form the male pronucleus. On the female side, 

 fertilization catalyses the completion of maturation and the 

 development of female nucleoli which also swell and coalesce, to 

 form the female pronucleus. As mentioned earlier, asters are far 

 less prominent than in the eggs of the frog or sea-urchin. 



Changes in the shape and volume of the egg. One of the inost con- 

 sistent features of fertilization is that eggs change their shape at 



