41 PRINCIPLES OF PHYSIOLOGY 



to this event, however, both the sperma- 

 tozoid and the ovum undergo remarkable 

 changes, which have been studied in certain of 

 the simpler forms of animals, but while for 

 obvious reasons these phenomena cannot be 

 followed in a human being, there is every reason 

 to suppose they are of the same nature. These 

 phenomena consists essentially of various forms 

 of cell division, by which, in the case of the 

 spermatozoa, these bodies are increased four- 

 fold, while in that of the ovum, by a process 

 of splitting and separating of the chromosomes 

 in the nucleus or germinal vesicle, half of the 

 chromosomes are extruded and are practically 

 lost. Thus, suppose the number of chromo- 

 somes before these changes to be twenty, ten 

 are thrown out and ten are retained. Fecunda- 

 tion then occurs by the blending of the head 

 of the spermatozoid (containing chromosomes 

 from the male parent) with the germinal 

 vesicle of the ovum (containing chromosomes 

 from the female). It would appear that the 

 number of chromosomes in the fecundated 

 ovum is now doubled, that is to say, in the 

 case we have supposed, the number is again 



