230 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
cells, the like of which does not appear in Figure 34, but may be sup- 
posed to lie hidden from view at the more remote end of the embryo. 
Moreover, the cells 493, 492, 591, 7592 of Figure 51, which are situated 
at the anterior end of the embryo, correspond well in size with the four 
cells nearest the observer in Figure 34. "Therefore the rule previously 
stated for the orientation of the 8-cell stage is exemplified in this series 
also. 
In Figures 45-50 (Plate VIIL) is shown another series of drawings 
illustrating what has been said regarding the clear protoplasmio region (a) 
which throughout cleavage marks the posterior end of the embryo. In 
this series one looks down obliquely on the dorsal surface of the em- 
bryo from its posterior end. The polar globules are not visible, for the 
reason that they do not come into profile at the margin of the egg, 
a circumstance which is necessary for an exact determination of their 
position. 
In Figure 45, a 2-cell stage, the clear region appears in each blastomere 
at æ. During each successive cell division it bulges out as represented 
in F'igure 46, and again in Figure 47, just as if it were the most plastic 
portion of the egg and responded most readily to the internal tension 
which accompanies cell division. Such indeed is probably the case, for 
this region is free from yolk granules, consisting of protoplasm only, as 
has been already pointed out. 
In Figure 48, the 8-cell stage is seen to be completely formed. 
Applying our rule for the orientation of the egg at this stage, we decide 
that the pair of cells oceupying the centre of the figure and nearest to 
the observer is to form the posterior dorsal portion of the embryo; 
for (1) it belongs to the sot of four smaller cells formed by the first 
equatorial plane of cleavage, and (2) it is in contact with the diagonally 
opposite pair of cells of the other hemisphere. The sequel justifies our 
conclusion. Figure 49 represents the 16-cell stage, and Figure 50 
the 24-cell stage.! In Figure 50 it is seen that the small posterior cells 
of the ectodermal hemisphere, unmistakably identical with 0% and D®8 
of Figure 51 (Plate IX.), have appeared just where the clear portions 
forming prominences at the time of cell division have all the time been. 
These portions have become a. part of the small cells in question, which 
contain less yolk than any other cells of the egg at this stage, and 
subsequently cleave less rapidly than any other cells of the ventral 
1 Tt will be observed that between the stages represented in Figures 49 and 50 
there has been a slight rotation of the egg, so that the latter figure exhibits an 
exactly dorsal view instead of an obliquely dorsal one. 
