No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 8 1 



32-celled stage as mesoderm, while the vitellophags I regarded 

 as representing the entire endoderm. I now believe that this 

 view was incorrect, one of the so-called mesoderm cells con- 

 taining endodermal material as well as mesodermal. This cell 

 is evidently situated near the ventral mid-line, the remaining 

 cells of the circle being purely mesodermal. In the stage at 

 present under discussion the endoderm {l.eji) and the vitello- 

 phags {vi) become finally separated from the mesoderm (ine) 

 by the oblique separation off of the liver endoderm i-cell, and the 

 differentiation of the germ layers is thoroughly established. 

 Fig. 16 is a side view of an ovum of this stage in which the 

 division has been completed, and it is seen that the staining 

 properties of the cells remain very nearly the same as in earlier 

 stages, though some of the ectodermal cells in the mid-ventral 

 line just anterior to the mesodermal circles are no longer readily 

 to be distinguished by their staining powers alone from the 

 mesoderm cells. 



Beyond this stage I have not been able to follow the divi- 

 sions stage by stage, and the next stage figured is somewhat 

 more advanced than the last, and probably represents the 

 results of two cleavages. Interesting changes which will 

 result in the differentiation of the embryo are now beginning. 

 In Figs. 17 and 18 are represented two views, a ventral and a 

 dorsal, of the same egg. In the ventral view one sees a notice- 

 able increase of the area occupied by the mesoderm {vie), or 

 at least by darkly-staining cells which appear to be mesoder- 

 mal, though it is possible that a few of them may be ecto- 

 dermal cells which have stained more deeply than usual. For 

 the most part, however, these cells are mesodermal, and in the 

 mid-line there projects backwards from them the liver endo- 

 derm {l.en), which is now composed of several cells. The 

 number of vitellophag cells still remains at eight, and in front 

 of the mesoderm are seen the ectoderm cells, which show 

 the anastomosing processes so well-marked in the 32-celled 

 stage (Fig. 13). In comparison with the stage represented in 

 Fig. 16 the presence of these processes is very marked, but it 

 seems probable that that figure represents an ovum preparing 

 to divide, and that in the resting stage the syncytial condition 



