208 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



body in any given species the nucleus contains a fixed and 

 definite number of bodies, the chromosomes, so called because 

 they are readily colored by the various microscopical stains. 

 At each division of a cell these chromosomes are divided so that 

 each daughter-cell contains exactly as 

 many as did the mother-cell. Each egg 

 before it leaves the ovary contains these 

 chromosomes, and the number in each 

 egg corresponds exactly with the number 

 in any other part of the body of the 

 mother. In the formation of the sper- 

 matozoa, however, there is a peculiar 

 cell division, — a so-called reduction di- 

 vision, — the results of which are that 

 each resulting spermatozoon contains just 

 half the number of chromosomes normal 

 to the species. 



The spermatozoa also differ from the 

 eggs in their appearance. The egg is 

 passive, and it contains the nourishment 

 and material from which the young is to 

 be developed. The spermatozoon, on the 

 other hand, must be active; for it must 

 seek out and unite with the egg in order 

 that the latter may develop. To this end 

 it is made as small as possible. Deuto- 

 plasm is entirely absent, and the extra- 

 nuclear protoplasm is reduced to the 

 smallest amount. The chromosomes are 

 compacted into a small body, the so- 

 called head, while the protoplasm is 

 largely developed into a ' tail,' consisting 

 of an axial filament and a lateral mem- 

 brane, by means of which the spermatozoon is able to swim. 



Impregnation consists of the union of the egg and the 

 spermatozoon, and there is abundant e\idence to show that a 

 single spermatozoon is sufficient to impregnate a single egg. 

 This impregnation may take place either outside or inside of 



Fig. 213. A, human 

 spermatozoon front and 

 side view, after Retzius ; 

 B, diagram of vertebrate 

 spermatozoon, modified 

 from Bohm and Davidoff. 

 af^ axial filament ; e, end 

 piece of Retzius ; /;, head ; 

 m^ middle piece ; nih^ un- 

 dulating membrane; ?«_/", 

 marginal filament ; sfy sec- 

 ondary filament; /, tail. 



