CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 229 
cells of the lower hemisphere, which would lead to a stage of twenty-four 
cells. Such a stage was figured in my preliminary paper (94, Plate I. 
Figs. 1 and 2; here reproduced in Plate IX. Figs. 51 and 52), and it 
was there demonstrated that the hemisphere in which division is earliest, 
as the egg passes from the 16-cell stage, becomes later the ventral or 
ectodermal hemisphere of the embryo. 
Accordingly the serios of observations illustrated by Figures 19-2 
goes to prove that the four larger cells of the 8-cell stage, which are 
more remote from the polar globules, form the ventral or ectodermal 
half of the embryo, whereas the four smaller cells, on which the polar 
globulos rest, become the dorsal or endodermal half of the embryo. 
The same thing is shown by Figures 27-34 (Platos V. and VI.), a series 
of drawings of an egg viewed from its anterior end. In Figures 27-29 are 
seen successive phases of the 2-cell stage. Figure 30 shows the 4-cell 
stage, and Figures 31 and 32 two phases of the 8-cell stage. At the 
8-cell stage in this series, as well as in the series previously examined, 
the four cells nearest the polar globules are smaller than the other four; 
they will form, as we shall see, the dorsal hemisphere. ‘There has been 
no shoving of cells across the median plane, but shoving has occurred 
among the cells of the right and left halves of the embryo separately, as 
was seen also at this stage in the series previously examined, (Soo Plate 
IV. Fig. 23.) According to the rule already stated, we should find in 
contact with each other the diagonally opposite pairs of cells which are 
to form respectively the posterior dorsal and anterior ventral portions of 
the embryo; while the other two pairs of cells should be completely 
soparated. If this is true in the ease before us (Figs. 31 and 32), we 
are looking at the anterior end of the embryo, for the pair of ventral cells 
nearest the observer is seen to be in contact with the most remote pair 
of dorsal cells. 
Figure 33 (Plate VI.) shows the 16-cell stage, and Figure 34 the 
24-cell stage in process of formation. In this egg also the cells of 
the hemisphere most remote from the polar globules wore first to divide 
in passing from the 16-cell stage. Those of the other hemisphere divided 
in this case about twelve minutes later. Therefore by this series also the 
hemisphere more remote from the polar globules is shown to be the ventral 
or ectodermal. That one is looking in this series at the anterior end of 
the embryo, as already suggested, and not at the posterior end, is shown 
by a comparison of Figure 34 (Plate VL) with Figure 51 (Plate Ls 
both of which represent the 24-cell stage. The posterior end of the em- 
bryo is seen in Figure 51 to be marked by a noticeably small pair of 
