FERTILISATION 5I 



formed, known, because of its shape, as the 'spindle'. This is the first 

 cleavage spindle of the egg, and provides the mechanism for the first divi- 

 sion of the chromosomes. It will be seen that the egg centrosome nor- 

 mally takes no part in it, though there are certain exceptions to this rule. 



Meanwhile, changes have been taking place in the male pronucleus. 

 The originally compact sperm head swells and takes on a normal nuclear 

 appearance; probably this swelling, which must involve the imbibition 

 of water from the cytoplasm, is largely responsible for the formation of 

 the sperm aster, which is much more highly developed than such struc- 

 tures are in the later cleavages. The two pronuclei, as has been said, 

 move together, often to some rather definite position in the egg. They 

 rarely fuse entirely before the nuclear membranes break down and the 

 chromosomes arrange themselves on the metaphase plate of the first 

 cleavage spindle. It is, in fact, only at the first cleavage that the essential 

 union of the two haploid nuclei is fmally consummated, and fertihsation 

 can fmally be said to be complete. 



The two aspects of fertihsation, which we have distinguished as activa- 

 tion and the nuclear events, are not in fact completely separate from one 

 another, but have certain interactions. An interesting example of this has 

 been described by Allen (1954). He sucked an echinoderm egg into a nar- 

 row tube, so that it became considerably elongated. If, in such an egg in 

 which the nucleus is located at one end, the fertihsing sperm is introduced 

 at the other end, only this latter end becomes activated ; that is, it is only 

 at this end that the cortical granules break down and the fertilisation mem- 

 branes form. From a variety of experiments of this kind, the conclusion 

 could be drawn that the nucleus tends to inhibit the breakdown of the 

 cortical granules in its neighbourhood. At the same time, the events in 

 the cortex have a reciprocal influence on the nucleus. If the germinal 

 vesicle lies in a region in which the cortical granules remain intact, it 

 seems unable to migrate towards the sperm nucleus, and does not divide, 

 although its nuclear membrane disappears at the same time as that of the 

 sperm nucleus. 



3 . Artificial parthenogenesis 



The activation effect of the sperm can be separated in experiment from 

 its action in bringing the gamete nuclei together. Thus Hertwig, in 1916, 

 showed that sperm are still effective as activating agents after they have 

 been given a dose of x-rays which entirely puts out of action their nuclear 

 component. The sperm nucleus can also be rendered inviable by other 

 means ; for instance by ultra-violet or by certain chemicals such as trypa- 

 flavines. Eggs fertilised by such sperm can develop normally, but they will 



