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 stomodaeum 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 a 

 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 lie 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 (Pi. 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 (PI. 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 tins 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 



