SPERMIOGENESTS OF THE PRIBILOF FUR SEAL 477 



likewise flattening against it. This fusion becomes so intimate 

 that it is often impossible to distinguish the acrosome in the 

 later stages of the spermatid. To what the striking variations 

 in size of the acrosome, such as are shown in figures 5 to 9, may 

 be due I am unable to decide. 



In the early stages of the contact of the hyaline body with 

 the nucleus the wall of the latter frequently shows a marked 

 degree of flattening, as though yielding to an external pressure, 

 and in many cases the line of contact becomes actually concave 

 (figs. 5 to 7). This condition is, however, but transitory and 

 the convex outhne is soon resumed. A marked change also occurs 

 in the substance of the hyaline body now extending back over 

 the nuclear wall. Its substance becomes more dense anteriorly, 

 and stains more readily with eosin. This process continues back- 

 ward toward the nuclear membrane until the whole head cap 

 becomes transformed into this denser substance. Figure 9 shows 

 a midway stage in which the more anterior part of the head 

 cap has become dense, while the part next to the nuclear mem- 

 brane is still hyaline. Figures 11 to 15 show the head cap en- 

 tirely composed of the denser substance. During this differ- 

 entiation the process of flattening and overgrowth is continuing, 

 so that by the end of the first period the head cap has extended 

 well down over the anterior half of the nucleus. It is manifest 

 from the above that the head cap reaches nearly its adult form 

 in the first period of spermiogenesis, and its subsequent develop- 

 ment consists mainly in a process of further differentiation, no 

 new elements being added to the form which it has already 

 assumed. 



The first appearance of the centrosomes in the young spermatid 

 is at the periphery of the cell, in the form of two small, rounded 

 granules, one a little larger than the other, lying close to the 

 cell membrane (fig. 1, c). Actual proof of the centrosomal nature 

 of these granules in the fur seal is lacking, as I have not traced 

 their earlier history. From subsequent events, however, and 

 from analogy with other forms in which their history is fully 

 known, I deem it safe to consider them as the centrosomes. In 

 the earliest stage in which I have been able to detect them there 



