12 Vertebrate Embryology 



known as " reduction division/' When the 

 first globule is formed, it is by the division of 

 the egg-cell into two cells, a large one and a 

 very small one, the small one being the polar 

 globule. In like manner the second globule 

 is formed by the very unequal division of the 

 larger of the first two cells, the larger cell of 

 the latter division being the true female element 

 which is capable of being fertilized. 



The essential element of the nucleus, that 

 part which is especially concerned in heredity, 

 or the transmission of parental characteristics, 

 seems to be the chromosome. The number of 

 chromosomes in the nucleus of any given 

 species is normally constant. 



It has been found that the number of 

 chromosomes in the egg after maturation is 

 just half what it was before that process, and 

 the amount of chromatin is reduced to one 

 quarter of the original quantity. 



O. Hertwig thinks that the reduction divi- 

 sions taking place in maturation are for the 

 purpose of increasing the relative amount of 

 cytoplasm, rather than for reducing the quan- 

 tity of nuclear material. The cell that is left 

 after the extrusion of the polar bodies, although 

 containing only one fourth of the original 



