704 



A MANUAL OF VETERINARY PHYSIOLOGY 



chromosomes in the ovum and spermatozoon were reduced in 

 each case to one half. On the union of the nuclei, the full 

 number peculiar to the species is created, each nucleus bringing 

 in half, and as the result of this a segmentation nucleus is formed. 

 From this segmentation nucleus the future animal is formed. 

 The segmentation nucleus now divides, the chromosomes it 

 contains split, so that each division contains the number of 



Fig. 245. — Section of a Rabbit's Ovum at the Close of Segmentation. 

 II., III., IV., Stages in the Formation of the Blastodermic Vesicle 

 (E. von Beneden). 



Z.R., Zona radiata ; Ex.L., external layer of cells ; I.M., inner mass of cells ; 

 I.L.M., Inner lenticular mass of cells ; s.c, segmentation cavity. 



chromosomes normal to the species, the chromosomes of each 

 parent being represented in each division of the cell, and in 

 each subsequent subdivision. It is believed that every cell from 

 which the embryo originates obtains maternal and paternal 

 chromosomes. 



It will be observed that the spermatozoon and ovum are the 

 complement of each other. The ovum contains an abundance 



