No. 3-] DEVELOPMENT OF MARINE SPONGES. 291 



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, PI. XIV, Fig. 8, 

 and PI. 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, Pigs. 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. 

 19 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, PI. 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. 



