DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 749 
cells of the central mass, and in many parts of the sections are separated from the 
latter by an irregular, scarcely defined membrane (this is represented as too distinct 
in the figures). In some sections the membrane does not appear. This membrane 
appears to be the first rudiment of the supporting lamella (“ Stiitzlamella”) which is 
so characteristic a structure among the polyps and hydroids, but the main body of 
the lamella is formed somewhat later, as described in the next section. The ectoderm 
cells now stain very differently from those of the central mass, and the peripheral zone 
has disappeared. The substance of the cells appears scarcely coloured, while the 
nuclei are deeply stained. The central cells, on the other hand, stain deeply, so as to 
be very sharply differentiated from the ectoderm in colour as well as in form, The 
deutoplasm spheres have the same distribution as in earlier stages, being very 
abundant and clearly defined in the more centrally placed cells (fig. 124) and becoming 
indistinct and scanty, or quite disappearing in the outer cells. There is still not the 
least trace of a digestive cavity. 
I am unable to say what the significance of the peripheral zone may be. A very 
similar zone is described by HorrmMann in the ectoderm cells of Tetrastemma at an 
early stage and by Rast in the ectoderm of Unio. It is possibly concerned in the 
formation of the cilia, but this seems improbable, since it disappears in Renilla long 
before the cilia are formed. 
In this stage the sections of the embryo sometimes appear exactly as if taken from 
an epibolic gastrula, for the ectoderm cells may be very different both in size and form 
on opposite sides of the embryo. This appearance is however entirely deceptive, and 
is produced simply by the tardy multiplication of the cells over one half of the 
embryo. 
§ 5. Formation of the entoderm and appearance of the digestive cavity. 
The embryo has thus far remained quite solid with no trace of a digestive cavity. 
For some hours longer this condition continues, the only change consisting in the 
multiplication of the cells of both layers. About the twentieth hour, however, or at 
the time when the cilia make their first appearance, peculiar changes become evident 
in the central cells which are the forerunner of the formation of the proper entoderm 
and the stomach cavity. The deutoplasm spheres disappear completely from the 
central cells, which then have a coarsely and irregularly granular appearance with very 
distinct membranes and deeply stained nuclei. The central mass is still solid, 
however, and the cells are all of the same irregularly rounded form. In a few hours 
a very perceptible difference can be seen between the outer and the inner cells of the 
central mass. Those which lie just beneath the ectoderm (figs. 125 and 126, en.) become 
much clearer, their substance stains very little, and many of them assume a slightly 
columnar form. Their nuclei are very distinct, of a slightly oval form, and very 
deeply stained. The cells are in some parts of the larva arranged in a single layer, 
MDCCCLXXXIII. 5 D 
