THE FERTILISATION OF THE EGG 9 



ing chromosomes a new nucleus is formed, the so-called female 

 pronucleus (Fig. Id). 



x^ 



\ 



Fig. 1. Maturation divisions in the egg of a starfish, (o) first 



maturation division; (b) extrusion of the first polar body; (c) 



second maturation division; (d) mature egg, (below: pronucleus, 



at top: first and second polar body). After Buchner. 



This maturation of the egg may occur at different times. 

 Sometimes both maturation divisions take place within the 

 ovary or in the genital ducts of the maternal animal. In other 

 cases, maturation begins in the ovary, but comes to a stop at a 

 certain stage, and is not completed until after fertilisation. 

 Finally, in many species the female lays unripe oocytes, and 

 the whole process of maturation takes place outside the mater- 

 nal body. In a number of instances of this last type, the nature 

 of the stimulus which causes the onset of maturation has been 

 investigated. Sometimes the maturation divisions do not begin 

 until after the sperm has penetrated into the egg. In other 

 cases, maturation occurs a few minutes after the oocytes have 

 been laid in sea water. It has been found that the calcium ions 



