24 INTRODUCTORY I 



while the single chromosomes, thus reduced in number, are 

 longitudinally split in the ordinary manner in the second 

 (homoeotypic) division. But however that may be, the number 

 is reduced and each secondary spermocyte (produced in the 

 first division), and again each spermatid (formed from the 

 division of these), contains in its nucleus only one-half of 

 the somatic number of chromosomes. Each spermatid is then 

 metamorphosed into a spermatozoon. This process shows 

 a remarkable similarity in the many forms in which it has 

 been studied : we may take a flagellate vertebrate spermatozoon 



-that of a Salamander as fairly typical (Fig. 6). The centro- 

 some of the spermatid divides into two, and these leave the 

 sphere of attraction. The two centrosomes place themselves 

 radially, while the sphere moves round to the opposite (or 

 anterior as it will be) end of the cell. Here it becomes altered 

 to form the acrosome or perforatorium. Of the two centro- 

 somes the inner or anterior places itself close to the nucleus, 

 in the hinder wall of which it becomes embedded. Here it 

 enlarges and elongates to form the so-called middle-piece or 

 neck. The posterior centrosome, from which the axial fila- 

 ment of the tail has in the meantime grown out, becomes trans- 

 formed into a ring. The ring is eventually broken in two, 

 one half remaining attached to the anterior centrosome, the 

 other travelling a little way down one (the ventral) side of the 

 tail. On the other (dorsal) side of the tail, the fin grows out 

 along the axial filament. The nucleus elongates, and becomes 

 finally dense and very chromatic. 



In the female, on the other hand, the growth in the second 

 period is very much greater, since it is at this time that the 

 yolk granules are deposited in the cytoplasm. The nucleus, 

 which has previously passed through the prophases of the first 

 maturation division, makes certain contributions to the cyto- 

 plasm ; that is to say, substances, which may be solid or 

 liquid, chromatic or achromatic, and are generally spoken of 

 as ' yolk-nucleus ', pass from the nucleus into the cytoplasm 

 and are there concerned in the metabolism of yolk-secretion. 



When growth has come to an end, the enlarged nucleus or 

 germinal vesicle breaks down and its contents are cast into the 



