10 THE INITIATION OF DEVELOPMENT: 



present in sea water play an important role (Dalcq, Pasteels, 

 Heilbrunn). These ions, possibly in combination with potassium 

 ions, appear to provide the specific stimulus which causes the 

 membrane of the germinal vesicle to disappear, and which 

 thereby initiates maturation. 



In normal development, the polar bodies are always formed 

 at one pole of the egg, the so-called animal pole (p. 39). At 

 the beginning of maturation, the germinal vesicle, or the 

 maturation spindle arising from it, moves towards this pole; 

 and the spindle there orients itself at right angles to the egg 

 surface (Fig. la). Apparently this part of the surface attracts 

 the spindle. In several cases it has been possible to weaken this 

 attraction by a special treatment of the egg, e.g. by means of an 

 excess of calcium ions in the medium in starfishes and in some 

 molluscs, or by means of a rise of temperature in sea urchins. 

 Dalcq (1924) has called this depolarisation. As a result the 

 maturation spindle does not come close enough to the egg surface, 

 and consequently abnormally large polar bodies are formed. 

 In the case of an even stronger depolarisation the. maturation 

 spindle does not even orient itself at right angles to the surface, 

 and the polar bodies fail to appear altogether. 



The maturation of the egg has still another consequence. 

 Once the nuclear membrane has dissolved, the nucleoplasm 

 contained in the germinal vesicle mixes with the cytoplasm. 

 This causes a change in several of the physical and physiolo- 

 gical properties of the cytoplasm (e.g., its viscosity, and 

 permeability). Moreover, it appears that many eggs cannot be 

 fertilised until this has happened. No sperms can penetrate into 

 immature oocytes, or, if they can, they remain inactive in the 

 the egg cytoplasm for the time being, and do not "awake" until 

 maturation has begun. Costello (1940), working with fragment- 

 ed oocytes of the polychaete worm Nereis, showed that only 

 those parts that contained the germinal vesicle could be success- 

 fully fertilised. In other animals, e.g. starfishes and sea urchins, 

 it is equally impossible to fertilise non-nucleated egg fragments, 

 if the germinal vesicle was intact at the moment of fragmenta- 

 tion. However, if maturation had already set in at that time, 

 non-nucleated fragments can be successfully fertilised, and will 



