"^^ PRINCIPLES OF EMBRYOLOGY 



isms which have been evolved to assist the male and female in bringing 

 their gametes mto proximity; for details of copulatory and other devices 

 reference must be made to works on the general biology of sex We may 

 take up the tale of fertiHsation from the point when eggs and sperm are in 

 each other s presence. The first reactions between them are, in many cases 

 chemical. It was shown by LiUie in 1913 that the water in which eggs of 

 sea-urchms had been lying is able to induce a reaction m sperm of the same 

 species which are caused to agglutinate together into clumps. This is due 

 to a substance given off by the eggs, to which the name 'fertihzin' was 

 given. It has smce turned out that this substance is none other than the 

 jeUy (or some constituent of it) by which the eggs are surrounded It reacts 

 with a substance m the sperm, which has been named 'anti-fertihzin' Very 

 little IS known about the nature of these substances, except that they are 

 both protems; the reaction between them is probably similar to that 

 between antibodies and antigens. Such substances have only been defm- 

 itely proved m some of the marine invertebrates, but they may perhaps 

 be present m all ammals, smce the agglutination reaction by which they 

 are recogmsed is really due to an excessive performance of their real task- 

 a fertilizm which reacted with sperm, but did not go so far as to immobihse 

 It by agglutmation would easily escape detection. 



It has also been claimed that sea-urchin eggs secrete a substance which 

 attracts sperm, so that the latter move by chemotaxis towards the eggs 

 There seems, however, to be no conclusive evidence of this, and opinion 

 seems to be crystalhsing against it. On the other hand, there do appear 

 to be sperm secretions which affect the egg, assisting the sperm in the task 

 of penetratmg the egg surface or the jeUy which surrounds it. (hi mam- 

 mals, enzymes having this function occur in the secretions of glands which 

 contribute to the semen, and it is not clear that the spermatozoa them- 

 selves produce anything of the kind.) 



All active substances produced by either eggs or sperm and acting on 

 gametes of the opposite sort are sometimes coUectively known as Gam- 

 ones, the egg secretions being Gynogamones and the sperm secretions 

 Androgamones. In this terminology, fertilizin becomes Gynogamone II 

 and anti-fertihzin Androgamone II. The subject has recently been re- 

 viewed by Tyler (1948, 1949), Runnstrom (1949) and Rodischild (1951^, 

 b), and further details may be found in their papers. 



The actual process of fertihsation starts when the sperm first touches 

 the egg surface. As mentioned above, the ensuing processes fall into two 

 phases. These are: 



(i) The activation of the egg. 

 (ii) The union ofthe two haploid nuclei. 



