SPERMIOGENESIS OF THE PRIBILOF FUR SEAL 475 



The first period: from the second division of the spermatocytes up 

 to the appearance of the caudal tube: figures 1 to 15 



In describing the changes which the spermatid undergoes, 

 there are at least four principal structures to be noticed, the 

 cytoplasm, the nucleus, the idiosome, and the centrosomes. 

 Other important structures appear later and will be mentioned 

 as they occur. 



Immediately after the second division of the spermatocytes, 

 the small spermatids are found along the inner surface of the 

 epithelial hning of the tubule (fig. 1). As a rule each is poly- 

 hedral, due to the pressure of the adjacent cells. Those sper- 

 matids lying nearest the lumen of the tubule appear less influ- 

 enced by this factor and are often quite rounded. The cytoplasm 

 is clear and transparent, or but slightly granular, and is bounded 

 by a sharply defined cell membrane. The spherical nucleus is 

 at first central in position, but as development proceeds, the 

 whole spermatid becomes distally elongated toward the lumen of 

 the tubule, and the nucleus shifts in position toward the proximal 

 end of the cell. It is comparatively large, resembling both in 

 size and form that of the Von Ebner cells which precede them. 

 So marked is this resemblance that it is often difficult to distin- 

 guish between these two stages. The chromatin is scattered, 

 as a rule, throughout the whole area of the nucleus in the form 

 of large irregular clumps which are connected by slender bands 

 of colorless linin (figs. 1 to 5). The nuclear membrane is quite 

 clear and distinct and is generally incrusted on its inner surface 

 with a layer of chromatin of varying thickness. In the closing 

 portion of this period the chromatin begins to change toward 

 the appearance which it has in the adult sperm. The large 

 irregular clumps become resolved into finer granules which are 

 more evenly distributed over the linin network, the nucleus as 

 a w^hole losing its mottled appearance and taking on a more 

 homogeneous aspect (figs. 11 to 14). 



As the end of the period approaches the nucleus also elongates 

 toward the wall of the tubule and shifts bodily in that direction, 

 finally coming to lie at the extreme proximal end of the cell 



THE AMERICAN JOURNAL OP ANATOMY, VOL. 14, NO. 4 



