THE MORPHOLOGY OF FERTILIZATION I3 



in mammals, that 'It seems clear that, in all the species in- 

 vestigated, the whole sperm enters the egg and that the cytoplasmic 

 components of the mid-piece mingle with the egg cytoplasm and 

 thus contribute something to the embryo.' Before any progress 

 can be made in resolving this question, we shall have to try and 

 find out what this 'something' is, and what its importance is to 

 the embryo. 



To turn from the echinoderms and examine the early phases of 

 fertilization in a different phylum is both interesting and instruc- 

 tive. In the unfertilized egg of Spirocodon saltatrix (Tilesius), 

 the egg nucleus lies at the base of a slight depression on the 

 egg surface, this depression appearing after the extrusion of the 

 second polar body. J. C. Dan says (19506) that the sperma- 

 tozoon invariably enters the egg in the immediate neighbourhood 

 of the female nucleus; also that other spermatozoa accumulate 

 round this part of the egg. These observations raise the possibility 

 that sperm chemotaxis, for which there is very little reliable evi- 

 dence in the animal kingdom, may occur in the coelenterates. 

 Immediately after the sperm head has penetrated into the cortex, 

 a tubular structure develops round its tail. The growth and 

 degeneration of this structure are shown in Fig. 4, at various times 

 after the beginning of fertilization. There are no membranes 

 round the egg, before or after fertilization. The sperm tail passes 

 completely into the egg cytoplasm in about 15 minutes and during 

 this time it is in continual but slight movement. 



The earliest phases of fertilization have now been observed in 

 mammalian eggs; Shettles (1954), for example, has reported the 

 successful fertilization of human eggs in vitro. Earlier claims of 

 success in this field have been sympathetically but firmly reviewed 

 by Austin (19516) and Smith (1951). 



Hyaline Layer. Soon after the elevation of the fertilization 

 membrane, the so-called hyaline layer or Hyaloplasm appears on 

 the surface of sea-urchin eggs. This is a thin, extracellular and 

 gelatinous layer which dissolves in calcium-free sea water. As 

 this layer can be removed without affecting the viability of sea- 

 urchin eggs for a considerable time, it used to be thought that this 

 structure was an extraneous membrane whose main function was 

 to hold the blastomeres together after cleavage. The suggestion 

 has, however, recently been made that the hyaline layer may play 

 some part in preventing more than one spermatozoon entering an 



