288 W. WOODLAND. 
free from one another for the greater part of their course. 
At the base of the epithelium there are a few spherical 
interstitial cells of different sizes.’ In the course of my 
observations of the spicules I came across a number of ex- 
ceedingly small cells—some even smaller than the nuclei of 
other cells—which were situated in the peripheral layers of the 
mesoglea. These cells, which I have represented in Pl. 17, 
fig. 20, were slightly granulated, and many contained a distinct 
though faintly-stained nucleus; in others, however, it was 
impossible to distinguish one. It is probable that these are 
the interstitial cells of Hickson, more especially since, owing 
to variations in size, it would be possible to trace cells inter- 
mediate in size and appearance between these and the sclero- 
blasts they im all probability give rise to. Hickson says 
that “it is difficult to determine with certainty the origin of 
the cells that give rise to the spicules, but, for many reasons, I 
am inclined to agree with von Koch’s results on Gorgonia and 
Clavularia, and attribute them entirely to the ectoderm. 
Among the interstitial cells of this tissue one frequently finds 
large spherical cells which lie beneath their neighbours, and 
cells very similar to these may be seen isolated in the sub- 
jacent mesogloea.” These scleroblastic mother-cells are 
represented in Pl. 17, fig. 19, and Pl. 16, fig. 1. 
In addition to the epidermal cells, the interstitial cells, and 
the scleroblasts which they probably give rise to, and two 
or three other kinds of cells, which I shall mention, there 
exist, according to Hickson (and I can confirm his statements), 
in the peripheral portion of the colony, the endodermal 
canals, the “solid cords of endoderm,” and “isolated 
cells connected with one another and with the 
endoderm by fine anastomosing fibrils.” ‘The cords 
[see fig. 18] are, in some, fairly compact, resembling a canal 
in all respects except the presence of a lumen, but in others 
the cells are only loosely connected with one another, become 
elongated or star-shaped, giving off fine fibrils at their angles. 
There may be only a single row of oval or cubical cells, or in 
some cases the row may be drawn ont into a chain of elongated 
