68 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



" pseudopodia " in other regions being withdrawn, and the cell outline 

 again assuming a more regular contour (Figs. 95, 96). This often 

 results in peculiar shapes, the cells sometimes becoming S-shaped or 

 C-shaped or even nearly complete rings. However, when the sperma- 

 tids lie in a region of the testis where there is no considerable crowding; 

 as, for instance, next to the lumen occupied by spermatozoa, the process of 

 lengthening results in the formation of cells more regular in outline (Figs. 

 95, 96). Here the elongation has proceeded in an approximately straight 

 line. This results in long cells containing the nucleus at one end, which 

 by anticipation we may call anterior. From the posterior side of the 

 nucleus, extending backward the entire length of the cell, is the conspic- 

 uous deeply stained axial filament (Fig. 95). No case has been observed 

 in which this fibre penetrating the cell wall extended beyond the boun- 

 daries of the cell. I cannot say positively that this uever occurs, but it 

 is certain that in many cases observed both at this and later stages, 

 the filament may be seen to be wholly included within the cell. In 

 Figures 95 and 96 the end lies free in the cytoplasm at a short distance 

 from the cell membrane. 



The cell continues to elongate throughout all the succeeding stages of 

 transformation (see Figs. 95-121) until, in the fully developed spermato- 

 zoon, it has reached a length of several hundred micra, and measures 

 only 1.5 micra in diameter. In Figure 121 (Plate 7), representing the 

 anterior portion of one of the mature spermatozoa, the actual length of 

 head and acrosome is 260 micra, and it is probable that the tail of such 

 a spermatozoon measures several times as much. 



As they arise from the second spermatocytes, the large spermatids 

 always occur in groups of 8-16-24, etc. (the products of one or more 

 pairs of first spermatocytes). This grouping is usually preserved through- 

 out the whole period of transformation, so that at any stage a group 

 usually consists of eight elements or a multiple of eight. The cells com- 

 prising one of these groups generally have their nuclei at about the same 

 level (Figs. 95, 96), their elongating tails extending backward parallel 

 to one another and in close apposition. Occasionally, when the region 

 of the testis in which they lie is much crowded, the spermatids of a 

 single group elongate in different, sometimes in opposite, directions, so 

 that in later stages they no longer exhibit the characteristic grouping. 

 This, however, is rather rare, and even the mature spermatozoa usually 

 show the groupings into eight or multiples of eight. 



When the axial filament has attained a considerable length (Fig. 91), 

 the archoplasm is still apparently increasing in amount, and is all aggre- 



