Now| DEVELOPMENT OF MARINE SPONGES. 289 
to a difference in the stage of division. In young gemmules 
such as these, and in considerably larger ones as well, the cell 
outlines are exceedingly plain. 
Young gemmules like those just described are formed from 
groups of mesoderm cells, such as are shown in Pl. XIV, Fig. 8, 
and Pl. XV, Figs. 13, 14, 15 (mes. gr.). The cells composing the 
mesoderm group are essentially like the gemmule cells. Like 
the latter they have plump bodies filled with the same finely 
granular yolk, in consequence of which they stain well, and 
have conspicuous nuclei. Such groups of mesoderm cells 
occur in abundance. They have no definite shape and may 
contain few cells or many, and the component cells may lie 
together very loosely or be packed pretty closely. They are 
formed by the migration towards a common point of certain 
mesoderm cells in which a considerable amount of yolk has 
been deposited. Such cells are found in abundance lying 
singly, or in twos and threes through the mesoderm. In Pl. XV, 
Fig 13, there are several (g. m.c.). They do not form a class 
by themselves, but are merely ordinary mesoderm cells contain- 
ing a maximum amount of yolk, and are connected by transi- 
tional stages, containing less and less of yolk, with the delicate 
spindle-shaped mesoderm cells, the body of which contains no 
yolk and scarcely stains at all. The congregation of such cells 
to form groups may be inferred from such preparations as 
those shown in Pl. XV, Figs. 13 and 14. 
In the transformation of such masses of mesoderm cells as 
are shown in Fig. 1§ (mes. gr.) into gemmules, the outer cells 
must flatten and become the follicle. But I have not succeeded 
in getting preparations actually showing this. I do not believe 
the gemmule, when first formed, is of any particular size, for 
groups of mesoderm cells are met with, differing greatly in this 
respect. The great number of very small gemmules such as 
g', Fig. 8, and g’, Fig. 9, make it evident that very frequently 
gemmules are formed from masses consisting of but a few 
mesoderm cells, for instance mes. gr. in Fig. 8. On the other 
hand, it seems likely that a mass of cells so rounded as the 
larger group in Fig. 15 was about to form a single gemmule, 
which would have been of considerable size. 
