ON DEVELOPMENT OF ATTEPHIRA COMPRESSA. 
411 
small, but sometimes so large as to fill up nearly two thirds of 
a yolk-segment. Within the yolk-segments are seen a fair 
number of nuclei, placed not at the centre, but rather to one 
side of them. Each nucleus is furnished with a dark-staining 
nucleolus, and a layer of protoplasm prolonged into a reticu- 
lum. I have not traced the origin of these nuclear bodies. 
It is, however, probable that they are derived from the seg- 
mentation nuclei. In one or two of my sections I have 
observed them in eggs segmented to 128 parts. Balfour 
figures and describes the nuclear bodies in the “ yolk-seg- 
ments” of Agelena, which in all particulars correspond with 
what I have seen in those of Atyephira. Of the origin of the 
nuclear bodies in the “ yolk-segments,” he says ( 4 tt*) : “The 
nuclei of the yolk-cells are probably derived from the nuclei 
of the segmentation rosettes, and it is probable that they take 
their origin at the time when the superficial layer of proto- 
plasm separates from the yolk-columns below to form the blas- 
toderm.” 
The depressed area appears white by reflected light. 
Starting from this area, the lenticular cells are gradually 
formed all over the rest of the surface, by the separation of the 
superficial protoplasmic layer from the yolk- segments below. 
While this is going on, the cells of the area become a little 
thicker (fig. 43) and the depression smaller, and the egg again 
assumes the elliptical form. The “ yolk-segments ” are now 
of nearly equal size. 
Gastrula, &c. — The white area, or “the Keimscheibe,” is 
depressed a little, and the egg appears bean-shaped from the 
side (figs. 42 and 44). The cells near one point (nearer to one 
pole of the egg) of this depression multiply faster than those 
in other parts of the surface. These cells gradually sink down 
into the yolk and eventually form a cup-shaped cavity whose 
mouth is bounded by about twenty cells (figs. 45, 46, and 51, 
y. m.). This cavity, or the gastrula, is at first very shallow, 
but it soon grows inward and forward, so that it becomes com- 
paratively deep (fig. 52, g. m.). While this is going on, an 
elevation of the cells takes place on the middle part of the 
