No. 3-] 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 PI. XIV, Fig. 8, 

 and PL XV, Figs. 13, 14, i 5 {mcs.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 PI. XV, 

 Fig 13, there are several (g. ni. 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. 15 (iiics.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 nics. gr. in Fig. 8. On the other 

 hand, it seems likely that a mass of cells so rounded as the 

 larger group in Fig. i 5 was abo-ut to form a single gemmule, 

 which would have been of considerable size. 



