750 DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
but in other parts seem to be placed two or more deep. It is difficult in this, as in 
subsequent stages, to say whether this appearance of several layers in the entoderm 
may not be due to the sections being always more or less oblique in different parts of 
the section ; but I believe, after examination, that they do form several layers in some 
parts of the embryo at this stage. This layer of clear cells is the permanent or true 
entoderm. 
The central cells, on the other hand, remain rounded and very granular, and stain 
more deeply than the entoderm cells. They become ultimately disorganised and are 
absorbed as food by the true entoderm cells ; hence they may hereafter conveniently 
be termed the yolk-cells, 
The yolk-cells form at first a solid mass which is directly continuous with the 
entoderm cells surrounding it. Soon, however (fig. 125), the yolk-cells become more 
loosely connected, and considerable cavities appear in the central mass in which the 
yolk-cells often lie quite disconnected from the other cells or united in groups of two 
or more cells. It is difficult to gain a clear idea of the changes which bring about 
this condition. Apparently the entire larva increases somewhat in size while the 
membranes of the yolk-cells become partially disorganised. In parts of the yolk-mass 
the cell-contents with their nuclei seem actually to drop out of the cell-membranes, 
which remain as a delicate network (fig. 126) in which the form of the cells is still 
perfectly preserved. Possibly this occurs only as a result of rough handling after the 
sections are made. Still I believe it may be in part a normal occurrence and that 
some at least of the free naked cells in the yolk-cavities may have been thus liberated. 
Others of the free cells have at first delicate cell-membranes, but these afterwards 
disappear. 
The yolk-cells are rounded, but vary greatly in form and size. Most of them are 
still distinctly nucleated, but the nuclei are less sharply defined and have less regular 
outlines than those of the entoderm cells. The cell-substance contains no deutoplasm 
spheres, and consists of a granular substance which stains irregularly and in some places 
not at all. Besides the yolk-cells there is a considerable quantity of granular substance 
in the form of small balls or masses lying in the yolk-cavities, and here and there may 
be seen a deeply stained free nucleus surrounded by a small quantity of granular 
matter. It is probable that the granular matter is derived from the breaking down 
of the yolk-cells, but it is difficult to say how far these appearances are the result of 
normal phenomena of disintegration, and how far due simply to mechanical injuries 
produced by manipulation. The general features of a section of this stage (twenty- 
two and a half hours) are well shown in fig. 125. 
Still later the yolk-mass becomes completely disorganised, breaking up into a kind of 
débris in which several distinct elements can be recognised (fig. 127). There are : 
firstly, rounded cells with distinct nuclei and membranes, which are simply free yolk- 
cells ; second, similar but usually smaller cells which have no membrane ; third, free 
nuclei which are usually associated with a small quantity of granular matter ; fourth, 
