THE FERTILISATION OF THE EGG 21 



role in activation. In several species, the egg can be activated 

 only in the presence of these ions in sufficient quantity 

 (Pasteels, 1938; Moser, 1939). Heilbrunn (1937) holds the view 

 that the egg cortex consists of a protein-calcium compound, 

 which can be broken down by the action of various stimuli. 

 The liberated calcium is then taken up by the internal cyto- 

 plasm of the egg. This causes an increase in the viscosity of 

 this region, while the cortex simultaneously liquefies. These 

 processes are considered to be the real cause of activation. 



Membrane formation and monastral cycle, the two processes 

 especially characteristic of activated eggs, are not linked to- 

 gether inseparably. In the starfish, the formation of a monaster 

 unaccompanied by a fertilisation membrane can be caused by 

 treatment of the egg with a mixture of certain chlorides 

 (Dalcq), (Fig. 6). On the other hand, in certain cases membrane 

 formation may take place without a monastral cycle. Nor is 

 the fertilisation membrane indispensable for further develop- 

 ment of the egg. It is possible to prevent membrane formation 

 in eggs, either after fertilisation, or during parthenogenetic 

 development. This does not always upset development. 



Activation causes a marked change in the physiological 

 properties of the egg. A great increase in the permeability of the 

 egg surface is quite common. This is probably due to the cortical 

 reaction. In the sea urchin egg, many chemical reactions take 

 place in the first few minutes after activation. Large molecules 

 are first broken down into smaller ones, and, somewhat later, 

 this is followed by resynthesis. Finally, activation often results 

 in infertilisability of the egg. This, however, is only true in the 

 case of optimal activation ; if the activating agent has operated 

 for too short or too long a period, so that activation is in- 

 complete, the egg retains its fertilisability. 



In normal fertilisation, the egg is activated by the penetrat- 

 ing sperm, as can be seen from the occurrence of the cortical 

 reaction. Apparently the sperm introduces a "regulating" factor 

 as well, preventing the occurrence of a monastral cycle, and 

 allowing the nuclei of egg and sperm to join in the formation 

 of a bipolar mitosis, which leads to cleavage of the egg. We 

 have seen above that the amphiaster usually arises in connect- 



