THE DEVELOPMENT OF ALCYONIUM DIGIT ATTJM. 71 
stages, and seems to be simply an earlier secreted part of it. At 
the time of the early formation of the mesenteries the lamella 
of the body wall is -first thickened by further secretion of meso- 
gloea which is deposited on its outer side and separates it from 
the ectoderm. This process continues during later stages, and 
it is found that the most recently secreted part of the meso- 
glcea, i. e. that part lying nearest the ectoderm (PI. 5, fig. 39, 
R.Mes.), always stains more feebly than the rest (E.Mes.). 
The mesogloea, after thickening the supporting lamella of the 
attached base of the polyp (PI. 5, fig. 40, Mg.), streams 
between the round cells of the multilayered ectoderm (Ect.) 
and unites with the original disc of adhesive mucus ( M. P. ),. 
thereby strengthening the attachment of the polyp to the 
substratum. The mesogloea is thickest in the body wall, much 
thinner in the bases of the tentacles, and very little thicker 
than the original supporting lamella in the distal ends of the 
tentacles, the pinnules, the oral disc, and stomodseum. No 
cells have been found embedded in the mesogloea which could 
be shown responsible for its secretion. On the other hand, 
streams of newly secreted mesogloea (PI. 5, fig. 35, S. Mg.), 
which would seem in this stage to be a very viscous fluid, 
are found running outwards, firstly from the very slightly 
thickened supporting lamella (PI. 5, fig. 35, Mg.), and later 
from the gradually thickening mesogloea of the body wall 
(PI. 5, fig. 36, Mg.) towards the ectoderm. They encroach 
on and finally surround the round cells at its base (PJ. 5, 
fig. 36, Sc. and Ne.), isolating them singly or in small groups 
in the mesogloea (S. cell and G-r. cell). These mesogloeal cells 
arise as interstitial cells (PI. 5, fig. 37, Sc.), and give rise 
either to spicules or nematocysts. The formation of the latter 
has not as yet been followed, but it was supposed during 
these investigations that the small rounded cells which stain 
very blue with picro-nigrosin aud are only about half the size 
of the young scleroblasts when engulfed are young nemato- 
cysts. Perfectly formed nematocysts may also be thus sur- 
rounded, and occasionally a scleroblast secretes a young 
spicule while still lying at the base of the ectoderm, i. e. 
