GERM CELLS AND THEIR FORMATION 125 



centrosomes or centrioles, and a large spherical nucleus with 

 a dense chromatin network. Internally we know that the 

 nuclei are unlike those of the somatic cells on account of the 



rt 



presence of only - chromosomes. Without any further divi- 

 2 



sion each is converted into the special form of the spermatozoon 

 typical of the species. While the details of this metamorphosis 

 vary considerably in different groups, the essentials of the 

 process are everywhere the same. The spermatid (Figs. 70, A, F; 

 71, A) contains, in addition to the typical cell organs just men- 

 tioned, a modified region of the cytoplasm which is sometimes 

 a centrosphere or idiosome, sometimes of rather doubtful charac- 

 ter and origin, which for convenience may be termed the sper- 

 matosphere. Close by lie the remains of the last division spindle. 

 The spermatids are further characterized by the presence of a 

 collection of chromidial structures termed the mitochondria 

 (Fig. 71). 



The details of the metamorphosis of these structures into the 

 parts of the spermatozoon are subject to wide variation; the 

 following account is based upon the history of the mammalian 

 spermatid (Fig. 71). The first step in the process is the migra- 

 tion of the centrosomes to the surface of the cell, and at the 

 same time the migration of the nucleus to the opposite side of 

 the cell. In most cases it is difficult to determine the relation 

 of the axis thus marked out, but in many instances this is per- 

 pendicular to the basement membrane of the germinal epithe- 

 lium, thus expressing a polarity which coincides with that of 

 the ovum; the nucleus lies toward the membrane, i.e. f the 

 attached surface of the cell, the centrosomes toward the free 

 surface. As the centrosomes and nucleus are taking these new 

 positions the spermatosphere moves up to the nucleus and 

 around it to the side opposite the centrosomes, quite to the 

 surface of the spermatid. The two centrosomes now separate, 

 one approaching the nucleus, the other remaining peripherally. 



Following these changes in the relative positions of the parts 

 come the real modifications of structure. The nucleus becomes 

 elongated or ovoid, and the chromatin condenses, first into a 



