BIGELOW: NUCLEAR CYCLE OF GONIONEMUS MURBACIIII. 331 



the migration of tlie proximal ccntrosome, come to lie on either side of 

 the axial filament. While the nucleus decreases in size these steadily 

 increase in bulk until they occupy nearly the whole space between its 

 posterior margin and the cell boundary, as is seen in the cells represented 

 in Figures 99, 101. Such a cell seen in edge view in a direction per- 

 pendicular to that of the last mentioned figures, shows the appearance 

 represented in Figure 98, where the two archoplasmic masses lie one 

 above the other, the axial filament clearly passing between the two. 

 With the assumption of the conical form by the nucleus, the growth of 

 these archoplasmic bodies continues until finally they become closely 

 apposed or even fused to each other, thus forming a more or less rectan- 

 gular mass in contact with the base of the nucleus. But this mass still 

 plainly consists of two deep-staining centres (Fig. 101) ; these, however, 

 are now no longer morphologically distinct, as is shown by the fact that 

 in crushed specimens, or in preparations macerated by the Hertwig 

 method, they adhere to each other and do not separate as they do in 

 earlier stages after similar treatment. 



During the progress of metamorphosis the cytoplasmic cell body, as 



well as its various organs, undergoes a series of changes. In the early 



spermatid, at the time of its final separation from its sister cell, the cell 



body is comparatively large, its diameter being 3.5 /x, and the cytoplasm 



is dense, finely reticulate, and throughout of uniform structure, enclosing 



no vacuoles or metaplasmic masses. This condition persists during the 



growth period of the nucleus and the migration of the proximal centro- 



some ; but at the time when the change in the form of the nucleus 



commences, small vacuoles appear in the neighborhood of the nuclear 



membrane ; these become more or less confluent, and soon form from 



one to three large ones. After this has taken place, a polar view of the 



cell shows the appearance represented in Figure 100, where the circular 



nucleus (seen of course in optical section) lies in a clear area, which is 



bounded by a shell of finely reticulate cytoplasm. The commencement 



of the same process is shown in side view in Figure 101, where, at the 



left of the nucleus there is a single large vacuole, while on the opposite 



side the cytoplasm still remains unmodified. This change progresses until 



finally the nucleus comes to lie free in a clear space, surrounded by only 



a thin shell of cytoplasm, a condition very similar to that figured by 



Downing (:05) in the case of the spermatid of Hydra fusca ; but in 



Gonionemus the long axis of the nucleus continues to coincide with the 



axis of the cell, and never assumes the various aberrant forms described 



for Hydra. In later stages the cytoplasmic shell cannot be detected. 



