THE MORPHOLOGY OF FERTILIZATION 15 



180 degrees so that the anterior end points outwards, in the direc- 

 tion from which the spermatozoon entered. According to E. B. 

 Wilson (1928, p. 423) the rotation of the sperm head is 'a very wide- 

 spread if not universal phenomenon'. This is an exaggeration as 

 it definitely does not occur in rat or mouse eggs. Rotation of the 

 sperm head occurs rather quickly, in 2-3 minutes, in the egg of 

 Lytechinus variegatus (Lamarck). Nothing is known about the 

 mechanism underlying the phenomenon. 



The subsequent movements of the male and female pronuclei 

 present the following problem: in the unfertilized egg the female 

 pronucleus may be situated almost anywhere in the cytoplasm; it 

 may be in the centre of the egg, or at the periphery, as in Fig. 4. 

 Usually, the spermatozoon may enter the egg at any point on the 

 surface and after passing through the cortex, becomes the male pro- 

 nucleus with its surrounding aster, a prominent structure in many, 

 but not all, eggs, formed in the cytoplasm under the influence of the 

 sperm head. Ultimately, the male and female pronuclei meet at 

 about the centre of the egg. How do they get there ? The first stage 

 in trying to answer this question is to examine the morphology of 

 the reaction. One of the most detailed descriptions of the move- 

 ments of the pronuclei in normal, uncompressed eggs is that of E. L. 

 Chambers (1939). Chambers says that, in spherical eggs, the male 

 pronucleus moves at a uniform speed * towards the centre of the 

 egg, along a straight line at right angles to the egg surface. Fig. 5. 

 In the older literature, summarised by E. B. Wilson (1928) and also 

 by Chambers, the male pronucleus was said to travel to the centre 

 of the egg along a curved path which could be resolved into two 

 components, a penetration path at right angles to the egg surface ; 

 and a copulation path, towards the female pronucleus. Chambers 

 thinks, probably rightly, that this curved path is abnormal, at any 

 rate in Class 4 fertilization, and due to the compression of the egg, 

 or to the egg being non-spherical. It seems likely that the movement 

 of the male pronucleus to the centre of the egg is caused by the 

 growth of the sperm aster round it. If so, when the rays of the grow- 

 ing sperm aster come up against the inside of the egg surface, their 

 elongation simply pushes the male pronucleus towards the centre 

 of the egg. According to E. B. Wilson (1902), this explanation is 



* According to Allen (1954), the velocity of the male pronucleus is far from 

 uniform; but his experiments were done on eggs which had been sucked into 

 narrow capillaries and which were therefore deformed in shape. 



