22C Papers from the Marine Biological Laboratory at Tortiigas. 



the bundle of axial fibers. It remains to be pointed out that this direction 

 of growth is determined at the time of the appearance of the first centro- 

 somal structure (the centriole) in the very young spermatoblast (fig. 8) and 

 is parallel with an imaginary line passing through the centriole and the 

 center of the nucleus. This line is the chief axis of the cell and it establishes 

 not only the anterior and posterior poles of the adult spermatozoon, but 

 also the plane of its bilateral symmetry. The centrosome is known to lie 

 in the proximal half of the cell and upon its disappearance the centrioles 

 come to be pretty evenly distributed on the periphery of the same half 

 (figs. 26 and 27). If, iujassembling, the centrioles move toward a central 

 point, as there is every reason to believe, then the center of the plate formed 

 by them will lie in the chief axis of the cell. After the division of the cen- 

 trioles the distal halves move across the cell in a direction vertical to the 

 plate and therefore parallel to the chief axis. 



That the chief axis of the cell establishes a bilateral symmetry is shown 

 by comparing figure i with figure 2 and figure 3 with figure 4. The first 

 and third of these figures represent two different stages of the apyrene 

 spermatosome, while figures 2 and 4 represent stages almost corresponding 

 to them, but viewed at right angles to them. In the first case, a plane 

 passing vertically through the longitudinal axis of the bundle of axial 

 fibers divides either of the cells into like halves, while in the second case a 

 similar plane divides either of the cells into unequal and dissimilar portions. 

 The same bilaterality is established by the chief axis in the spermatoblast, 

 indeed in very early stages (figs. 12, 13, and 14). Thus it appears that the 

 polarity of the apyrene spermatozoon is determined at a very early stage 

 in its development, at the time when the chief axis is established. The 

 distal side of the spermatoblast marks the anterior end of the spermatozoon 

 and the proximal side the posterior end. The latter may therefore be 

 spoken of as the base of the spermatoblast or spermatosome. 



While the dissolution of the nuclear membrane and the disappearance 

 of the centrosome are initiated simultaneously, the phenomena immediately 

 concerned with these two processes and also those subsequent to both do 

 not keep step in different cells. It has already been pointed out that while 

 in one cell (figs. 15 and 16) the centrosome was still perfectly visible after 

 some of the karyomerites had escaped into the cytoplasm, in another 

 (figs. 17 and 18) no traces of the centrosome could be found even before this 

 had occurred. Similarly, in one cell (figs. 19 and 20) all of the centrioles 

 have not yet reached the periphery and the larger karyomerites are frag- 

 menting, while in another (figs. 21 to 23) the centrioles already show their 

 secondary rays, although we are dealing with what is clearly an earlier stage 

 as regards the chromatin. But in a third cell (figs. 24 and 25), and here 

 the greatest variation occurs, we have a condition in which the centrioles 

 have reached the periphery and their secondary rays have not as yet 

 appeared, while the formation of the ultimate karyomerites has already 

 taken place. 



