CASTLE: EMBBYOLOGI OF CIONA ENTESTINALIS. 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. .'.1 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 series of observations illustrated by Figures 19-2G 

 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 

 globules rest, become the dorsal or endodermal half of the embryo. 



The same thing is shown by Figures 27-34 (Plates V. and VI.), a series 

 of drawings of an egg viewed from its anterior end. In Figures 27-29 arc 

 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. (See 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 

 separated. If this is true in the case 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 were 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 shoicn to be the ventral 

 or ectodermal. That one is looking in this series at the anterior end of 

 the embryo, as already sucr^osted, and not at the posterior end, is shown 

 by a comparison of Figure 34 (Plate VI.) with Figure 51 (Plate IX.). 

 both of winch 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 



