GROWTH AND DIFFERENTIATION OF THE GERM- CELLS l6l 



tozoon in fertilization. Obviously the most important of the 

 questions, thus suggested, is the source of the sperm-nucleus and 

 centrosome, though the relation of the other parts to the spermatid- 

 cytoplasm involves some interesting problems. 



Owing 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 spcrviatogonia} 

 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 most, if not in all, cases effected 

 during the two divisions of the primary spermatocyte. The reduction 

 of the chromosomes, which is the most interesting and significant 

 feature of the process, will be considered in the following chapter, 

 and we are here only concerned with the transformation of the sper- 

 matid 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 further certain that in some cases at least 

 the spermatid-centrosome passes into, or gives rise to, a part of the 

 middle-piece, and that from it the axial filament grows out into the 

 tail. The remaining structures arise, as a rule, from the cytoplasm, 

 and both the acrosome and the envelope of the a.xial filament often 

 show a direct relation to the remains of- the achromatic figure ("ar- 



1 The terminology, now almost miiversally adopted, is due to La Valette St. George. Cf. 

 Fig. 1 1 8. - See Fig. 119. 



M 



