vn] THE SPERMATOZOON; NUCLEUS 93 



The nucleus of the spermatozoon is formed directly from 

 that of the spermatid, but usually goes through a character- 

 istic series of changes. In many animals the nucleus of the 

 young spermatid consists of a vesicle with the chromosomes 

 all arranged immediately within the nuclear membrane. In 

 these nuclei little or no reticular structure is visible, but 

 the chromatin becomes spread out over the inner surface of 

 the membrane so that in section the nucleus has the appear- 

 ance of a darkly-staining ring, often at first with irregular 

 lumps of chromatin projecting into the cavity. Gradually 

 the nucleus contracts, and in animals which have sperma- 

 tozoa with elongated heads it becomes (in optical section) at 

 first oval and then a long narrow ellipse. Finally the clear 

 inner space disappears and the nucleus in the head of the 

 mature spermatozoon appears as a dense evenly stained 

 mass of chromatin. Although the nucleus with an outer 

 ring of chromatin and clear inner space is very characteristic 

 of the spermatid, it is by no means universal, and animals 

 of various groups have reticular spermatid nuclei with the 

 chromatin scattered on the network, usually in rather coarse 

 masses. The later development of these nuclei, however, 

 differs very little from that of the type described above. 



Apart from the nucleus, the only other structure of im- 

 portance in the head of the spermatozoon is the acrosome, 

 and it will be convenient to leave the discussion of its origin 

 to a rather later stage. 



While the spermatid nucleus is undergoing the changes 

 just described, the whole cell is elongating, and important 

 changes, which may have begun still earlier, are taking 

 place in connection with the centrosome. During the sper- 

 matocyte divisions, which usually follow each other rapidly, 

 the centrosomes (centrioles) have remained conspicuous 

 objects, generally lying near the edge of the cell, and in the 



