748 DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
lends support to the view suggested at p. 744, to account for the failure of the earlier 
cleavage furrows to reach the centre of the egg. 
A comparison of figs. 118 and 97 shows that the central mass in the later stage is 
somewhat greater than in the earlier; and an examination of a number of sections 
indicates that this difference is a constant one. Hence it seems probable that more 
than one delamination cleavage may take place, that the central mass may from time 
to time receive accessions from the outer layer through the occurrence of horizontal 
cleavages. I have not been able to demonstrate this, though some of my sections give 
indications of such a process. In some cases the ectoderm cells appear elongated, and 
as if about to divide in the horizontal plane. It is certain, as will subsequently appear, 
that such cleavages occur in the ectoderm until a late period, though in later stages, 
when the supporting lamella is formed, the cells thus produced remain, of course, 
ectodermic. There seems to be no reason why such cleavages occurring at an early 
stage should not produce entoderm cells, and such, I am inclined to think, is actually 
the case. 
After the stage shown in fig. 118, however, the cleavages take place for a considerable 
period mainly in vertical planes, so that the columnar form of the ectoderm cells 
becomes more and more marked. 
The structure of the embryo may be far less regular than is indicated by fig. 118, 
since the cells often multiply more rapidly over one half of the embryo, and the 
division of the central cells is often irregular. 
Sections through the Renilla embryo of about four and a half hours are represented 
in figs. 119 and 120. The embryo has the same general characters as in the last stage 
figured, but the cells have largely increased in number. The ectoderm cells have a 
definitely columnar form, and consist of a granular substance which is not, apparently, 
enclosed in cell membranes. They are separated by narrow, clear spaces which contain, 
apparently, a small quantity of intercellular substance. The central cells, on the 
other hand, are surrounded by definite membranes, which appear in the sections as 
narrow, dark lines. 
There is still no indication of a definite membrane separating the ectoderm from the 
central mass. The cells of the two layers are to some extent dovetailed together, 
and have nearly the same structure. Here and there in the ectoderm are rounded or 
pyriform cells which appear to be in course of division. The clear peripheral zone still 
appears distinctly at the outer ends of the ectoderm cells. In some specimens it 
bears a fringe of fine filaments which appear like cilia, but are in reality the remnants 
of the spermatozoa with which the embryo remains covered for a considerable period. 
In the embryo of eight hours (figs. 122-124) the ectoderm layer is sharply 
differentiated from the central mass, but the latter has undergone very slight change 
except in the further division of its cells. The ectoderm cells have now a high 
columnar form, though here and there rounded cells may be observed (fig. 122), At 
their inner ends, where they are usually somewhat expanded, they abut against the 
