292 



LECTURES ON BIOLOGY 



ancestors. Hence it follows of necessity that at a certain time, 

 before the germ-cells unite in the copulative act, there must take 

 place a reduction of the chromosomes to half their normal num- 

 ber. Investigation shows, in fact, that the mature spermatozoa 

 and ova contain only half as many chromosomes as the body- 

 cells, and that only by copulation the normal number is once 

 more restored. If, for instance, an animal species has in its- 

 body- cells four chromosomes, their mature sex-cells contain only 

 two, and if the human body-cells contain sixteen, we can say 

 a priori that the mature spermatozoa and ova have only eight 

 chromosomes. 



M 



Sch 



\ 



FIG. 71. CHANGE OF A SPERMATID IN A SCUTTLE-FISH (Octopus Defilippi) 

 INTO A SPERMATOZOON. 



(1) Spermatid. P, protoplasm ; N, nucleus ; C, centrosome ; S, sphere. (2) The 

 sphere has wandered to one nucleus-pole, the centrosome to the other. (3-8) Gradual 

 transformation. The sphere forms the pointed .'end Sp, the nucleus the 

 head K, the centrosome partly the neck M, and, together with part of the proto- 

 plasm the tail-thread Sch. the rest of the protoplasm is rejected and perishes. 



How is the reduction of the chromosomes in the germ-cells 

 affected ? 



In the testes of numerous animals may sometimes be distin- 

 guished several sharply denned segments in which the gradual 

 formation of the spermatozoa takes place. Usually in the pos- 

 terior section of the testicle lies the so-called germ-zone, 

 in which by division of the original germ-cells new cells are 



