No. 3.] DEVELOPMENT OF MARINE SPONGES. 315 
shown lying near one another in the pillars of tissue separating 
the subdermal cavities. Other long straight spicules are scat- 
tered freely about in the deeper parts of the body. The bow- 
shaped spicules present in the swimming larva are found in 
small numbers distributed irregularly through the mesoderm 
of the attached sponge, Fig. 44. The embryonic shovels which 
in the swimming larva are united in rosettes, Pl. XVI, Fig. 30, 
and Pl. XVII, Fig. 34, are always found free in the attached 
larva. In the young sponges there are not many of these 
spicules, and the few to be seen are usually found in the 
dermal membrane, Pl. XVII, Figs. 44, 50. 
Summary of the Leading Facts in the Gemmule Development 
of ve sperel. la. 
1. Gemmules appear in any part of the sponge mesoderm, 
and when present in large numbers, cause degeneration in the 
' sponge tissue. 
2. A number of mesoderm cells well supplied with yolk 
collect together and the mass so formed rounds itself off into a 
gemmule, the outer cells becoming the follicle. 
3. The gemmule grows not only by cell division, but by the 
fusion with it of other small gemmules. It becomes a large 
mass of closely packed cells, full of fine yolk. 
4. The gemmule, when mature, breaks up into irregular 
masses of cells, and these separate into the constituent individ- 
ual cells. 
5. The outer cells become ectoderm. Those at the posterior 
.pole flatten, and develop neither flagella nor pigment. The 
other ectoderm cells become columnar, and develop both 
flagella and pigment. 
6. The inner mass of cells forms an intercellular network. 
It is a parenchyma in which there is no distinction between an 
ectoderm and a mesoderm. The parenchyma cells at the 
posterior pole become closely compressed. 
7. In the swimming larva there is a bundle of long straight 
spicules in the posterior end. Bow-shaped spicules and embry- 
onic shovels (in rosettes) are scattered through the parenchyma. 
