122 THE GERM-CELLS 



2. Formation of the Spermatozoon 



Owino- to the extreme minuteness of the spermatozoon, the 

 changes involved in the differentiation of its various parts have 

 always been, and in some respects still remain, among the most 

 vexed of cytological questions. The earlier observations of Kolliker, 

 Schweigger-Seidel, and La Valette St. George, already mentioned, 

 established the fact that the spermatozoon is a cell ; but it required 

 a long series of subsequent researches by many observers, foremost 

 among them La Valette St. George himself, to make known the 

 general course of spermatogenesis. This is, briefly, as follows : 

 From the primordial germ-cells arise cells known as speinnatogonia} 

 which at a certain period pause in their divisions and undergo a con- 

 siderable growth. Each spermatogonium is thus converted into a 

 spermatocyte, which by two rapidly succeeding divisions gives rise to 

 four spermatozoa, as follows.- The primary spermatocyte first 

 divides to form two daughter-cells known as spermatocytes of the 

 second order or sperm mother-cells. Each of these divides again — 

 as a rule, without pausing, and without the reconstruction of the 

 daughter-nuclei — to form two spermatids or sperm-cells. Each of 

 the four spermatids is then directly transformed into a single sperma- 

 tozoon, its nucleus becoming very small and compact, its cytoplasm 

 giving rise to the tail and to certain other structures. The number 

 of chromosomes entering into the nucleus of each spermatid and 

 spermatozoon is always one-half that characteristic of the tissue-cells, 

 and this reduction in number is in many cases effected during the 

 two divisions of the primary spermatocyte. In some cases, however 

 {e.g. in the salamander), the reduced number appears during the divi- 

 sion of the spermatogonia and may even appear in the very early 

 germ-cells (cf. p. 194). The reduction of the chromosomes, which is the 

 most interesting and significant feature of the process, will be con- 

 sidered in the following chapter, and we are here only concerned with 

 the transformation of the spermatid into the spermatozoon. All 

 observers are now agreed that the nucleus of the spermatid is directly 

 transformed into that of the spermatozoon, the chromatin becoming 

 extremely compact and losing, as a rule, all trace of its reticular 

 structure. It is generally agreed, further, that the envelope of the 

 tail-substance is derived from the cytoplasm of the spermatid. 

 Beyond this point opinion is still far from unanimous, though it is 

 probable that the other structures — viz. the axial filament, the 



1 The terminology, now almost universally adopted, is due to I.a Valette St. George. Cf. 

 Fig. 90. 



- See Fig. 91. 



