12 



THE INITIATION OF DEVELOPMENT; 



reached its surface, it penetrates into the egg. In many species, 

 the egg forms a small surface projection in the region towards 

 which the sperm is making its way, and the latter is taken 

 up by this so-called fertilisation cone, seen in sea urchin eggs 

 by Fol as early as 1876 (Fig. 2). Therefore, the egg is by no 

 means entirely passive during fusion. According to Chambers 

 (1930), the egg of the starfish even emits fine filaments of 





Fig. 2. Fertilisation of the egg of a starfish, (a) the egg in its en- 

 velopes, surrounded by spermatozoa; ib-e) a sperm pierces the 

 egg envelope and penetrates into the "fertilisation cone". The initial 

 stages of the formation of a perivitelline cavity between egg and 

 vitelline membrane. After Fol and Wilson. 



protoplasm, one of which establishes contact with the sperm, 

 and, contracting rapidly, draws the latter inwards. The activity 

 of the outer layer of the protoplasm may also be responsible 

 for the fact that fragments of unfertilised eggs can only be 

 fertilised if at least part of this "ectoplasm" is intact. 



The penetration of the sperm causes the egg to "awake" at 

 once. Within a few seconds, it reacts with an instantaneous 

 change of properties. Usually the formation of a fertilisation 

 membrane is the first visible effect. The unfertilised egg is 

 surrounded by a vitelline membrane, which is sometimes fairly 

 thick, sometimes however barely visible. Immediately after the 

 penetration of the sperm, this membrane begins to lift from 

 the egg surface (Fig. 2d-e, 3), because substances expelled by 



