DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
757 
§ 7. Changes in the ectoderm and formation of the supporting lamella. 
In the larva of eight hours, as already described, there is a delicate sinuous 
membrane lying between the ectoderm and entoderm, upon which the cells of the 
former are planted as ivpon a basement membrane. This is perhaps the first beginning 
of the characteristic supporting lamella, but it is far less well defined and less 
conspicuous than in la,ter stages, and the great bulk of the lamella is formed somewhat 
later by a peculiar transformation of the inner ends of the ectoderm cells. It is 
difficult to determine the origin of this preliminary membrane, but appearances 
indicate that it is secreted by the expanded bases of the ectoderm cells. The 
membrane varies much in appearance and is sometimes quite invisible even in much 
later stages. It is often apparent in one part of a section and quite invisible in other 
parts, while the true lamella, once formed, is remarkably constant and distinct. 
The ectoderm cells of this stage have a high columnar form, which, though ultimately 
lost, is retained throughout the succeeding stages until a late period. At intervals, 
however, the cells rapidly proliferate (fig. 131, twenty-eight hours), and the columnar 
form may at these times be temporarily lost, the cells assuming various rounded forms and 
becoming in many cases entirely separated from the underlying entoderm cells. The 
division of the cells takes place both in horizontal and vertical planes, so that the 
ectoderm gradually becomes several layers deep. At the close of a period of pro¬ 
liferation most of the cells resume the high columnar form, some of them extending 
through the entire thickness of the ectoderm, others extending inwards from the 
surface and terminating by attenuated extremities without reaching the entoderm. 
Others, again, are placed with their broader end—which contains the nucleus—lying 
near the bottom of the ectoderm, and others still are of a fusiform shape with the 
thickest part containing the nucleus, near the middle of the ectoderm. The structure 
of the ectoderm at this stage is very like that of Heliopora (Moseley, Phil. Trans., 
Vol. 166, 1876). 
Besides the columnar cells there are others of a rounded form with centrally placed 
nuclei, which lie in the deeper parts of the ectoderm or in the narrow clear space which 
often separates the layers ; they often lie directly on the outer ends of the entoderm 
cells. These never return to the columnar form and persist throughout the entire 
development. They give rise to elements of the so-called mesoderm, some of them 
becoming the matrices for the development of the spicules, and others remaining as 
peculiar rounded cells which are possibly nerve-cells. 
In the larva of about twenty-two hours (figs. 125, 132), the basal ends of the columnar 
ectoderm cells undergo a peculiar change of form and structure. They separate 
completely from the entoderm, become smoothly rounded, the character of the 
granulation changes, and they stain less readily than before. At the same time a 
large quantity of a finely granular substance makes its appearance in the space 
between the ectoderm and entoderm (figs. 125, 133). This space is sometimes very 
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