No. 3.] DEVELOPMENT OF MARINE SPONGES. 29I 
found. Whether or not they follow one another in precisely 
the sequence I have indicated, it is plain that the nuclear 
division, though it perhaps cannot be ranked as a karyokinetic 
one, is something more complex than a simple constriction of 
nuclear matter into two parts. 
Gemmules increase in size not only by means of ordinary 
growth, but by fusion with one another. I think this is evident 
from the following facts. It is extremely common for small 
gemmules to occur in groups. In such groups, Pl. XIV, Fig. 8, 
and Pl. XV, Fig. 17, the separate follicles are often so closely 
pressed together as to be indistinguishable one from the other. 
Instances are met with not infrequently, where the shape of the 
gemmule gives strong indication that it has been formed by the 
fusion of separate parts. This is true of the gemmule +x in 
Fig. 17, and still more so of x in Fig. 16. In Fig. 16 the dual 
origin of the gemmule is further indicated by the fact that one 
half the gemmule has nearly all its nuclei in one phase, while 
the other half has its nuclei in a different phase. Fusion is, I 
think, confined to the smaller gemmules such as those just 
referred to. I have not met evidence of it in the case of larger 
gemmules such as that shown in Fig. 19. 
The gemmule, increasing in size in these two ways, grows 
steadily larger. An idea of the amount of increase may be got 
from a series of figures representing gemmules of successively 
larger size from quite small ones up to the mature gemmule. 
Such a series is given in Pl. XV, Figs. 17, 20, 20, 19, 18. It is 
remarkable that while the small and large gemmules are both 
very abundant, medium sized ones such as that shown in Fig. 
Ig are hard to find. As the gemmule increases in size, it 
undergoes certain other changes as well. The fine yolk con- 
tained in the cells becomes more abundant, and the cells in 
consequence take a somewhat deeper stain. The cells become 
gradually much more tightly packed together than they were in 
the younger gemmules, and the cell outlines grow less distinct. 
The nuclei grow smaller. In the mature gemmule, Pl. XV, Fig. 
18, the cells are so full of yolk and so tightly packed that it is 
very difficult to make out the cell outlines. They appear as 
cracks in a uniformly granular and deeply staining substance. 
