DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
759 
appear to be surrounded by delicate membranes and become in some cases incorporated 
into the substance of the supporting lamella. They are not to be confounded with 
the rounded nucleated cells which are sometimes also found in the granular mass and 
may likewise become incorporated into the lamella. Besides the latter, cellular elements 
derived from the entoderm may in some cases enter into the composition of the lamella. 
Now and then an entoderm cell (see figs. 125 and 132) may flatten down, become 
incapable of development and become incorporated with the mass of the lamella. 
After the secretion of the granular matter is completed the cells re-assume their 
high columnar form with their inner ends often resting upon the lamella as on a 
basement membrane (figs. 127-130). The latter appears as a very distinct, narrow, 
structureless membrane sharply separated from the ectoderm and entoderm. Outside 
of it is usually a narrow, clear space, but the granular matter has entirely disappeared. 
This condition of the ectoderm is maintained until the sixtieth or seventieth hour, 
when the ectoderm totally changes its character. The lamella remains unchanged 
up to the latest stage of the colony observed, without increasing in thickness or 
undergoing visible change of structure. 
Review, 
The material of the lamella is derived from the cells of the ectoderm by a peculiar 
form of cuticular secretion, which consists in the separation of rounded granular masses 
from the inner ends of the cells. The formation of these bodies is a process entirely 
different from cell-division since the nuclei do not divide, and they remain quite 
unchanged during the process. The granular bodies in most cases disintegrate, but 
sometimes appear to discharge their contents as in ordinary secretion. Possibly the 
ectoderm cells may also in some cases discharge the contents of their swollen basal 
ends without the separation of a- part of the cell, but this must, I believe, be 
exceptional. The mode of secretion described is a very anomalous one, and appears to 
stand midway between the disintegration and discharge of an entire cell during 
secretion, as in the formation of “ goblet-cells ” in mucous glands, and the more usual 
forms of secretion in which the product exudes from the cell without the destruction 
of the latter. 
The formation of the supporting lamella in other forms has not been worked out 
with sufficient care to afford any basis for comparison, Kowalevsky concluded that 
the lamella in Actinia is secreted by the entoderm, since it penetrates into the septa, 
which are entirely entodermic. This is, however, an unwarrantable conclusion; for, 
as will be shown later, the lamella of the radial septa has an entirely different origin 
from that of the peduncular septum, and both differ in origin from that of the body- 
wall. It is evident that the supporting lamella, though probably containing cellular 
elements derived both from the ectoderm and entoderm, is not in any sense a special 
mesodermic layer, but has only the significance of a structureless cuticular membrane 
separating and supporting the two fundamental layers of the body. 
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