230 bulletin: museum of compaeative zoology. 



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 A 6 - 1 , .l 6,2 , 1P A , i? 6,2 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 VIII.) is shown auother series of drawings 

 illustrating what has been said regarding the clear protoplasmic region (x) 

 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 x. During each successive cell division it bulges out as represented 

 in Figure 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 occupying 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 set 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. 1 In Figure 50 it is seen that the small posterior cells 

 of the ectodermal hemisphere, unmistakably identical with C 6 - 3 and Z) 68 

 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 It 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. 



