EMBRYOLOGY. 101 
In Stage 8 (PL VI, fig. 2) the posterior or abdominal region of the 
embryo has now completely changed its position, having folded back 
dorsally about the ovarian yolk. Plate VI, figure 2, shows a sagittal 
(slightly oblique) section of an embryo at this stage. There are ap- 
parently only 9 abdominal segments. Both the stornodseum and the 
proctodeum are plainly in evidence now, and the mesenteron is in 
course of formation. The latter is formed above and rests upon the 
ovarian yolk. This yolk now has a granular appearance, and the yolk 
cells within it appear to be breaking down. It is still divided off into 
subspherical masses. The polar organ is smaller than in the pre- 
ceding stage and the pear-shaped area in the center is filled with st 
yellowish substance as before. The ovaries are represented in this 
section by a circular mass of cells above the ovarian yolk. The pri- 
mary yolk is grouped and divided off by protoplasmic threads, very 
much as in the preceding stage, but is not quite so abundant now. 
Plate V, figure 3, shows a surface view of the embryo, straightened 
out to its full length. It will be seen that the appendages are now 
much more elongate, the thoracic appendages showing traces of 
segmentation. All the appendages are now directed posteriorly 
and he flat upon the body. 
This is the stage in which the majority of the embryos pass the 
winter. It is very doubtful if any of the stages earlier than the 
seventh are able to survive the winter. Instances have come under 
our observation in which embryos in the sixth stage have been killed 
by very low temperatures. When heavy freezes do not occur until 
sometime in December, a -very large percentage of the eggs hatches ; 
on the other hand, however, when heavy freezes begin in November, 
large numbers of the eggs are killed in the early stages, since large 
numbers of the eggs are deposited in this month. An early autumn, 
therefore, followed by a severe winter, would limit to a great extent the 
number of stem mothers of the following spring. 
Stage 9 (PL VII, figs. 1-4) represents the stages of growth occur- 
ring in the latter part of February and the first of March. When 
the embryo is ready to come to the surface of the egg (PL VII, 
fig. 1), it moves forward in the yolk until the cephalic lobes come 
into contact with the polar organ. It will be observed that there 
is quite a gap between figures 1 and 2, and at present we have no 
material from which this missing link can be supplied. Figure 2 
shows the dorsal organ already formed. As we have no intermediate 
stages we can not state definitely whether this is the true dorsal 
organ or the dorsal and polar organ combined. It is probably the 
latter, as we do not find any traces of the polar organ at any other 
point in the embryo. It is very probable that the surplus cells of 
the serosa, at the time the embryo comes to the surface of the egg, 
collect at and group themselves about the polar organ, as there 
