No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 79 



forms the cell-boundary and from which other delicate processes 

 extend into the yolk, probably uniting with processes of other 

 cells. The ectoderm cells {Ec) have much less protoplasm 

 around their nuclei, and the protoplasmic processes are, as a 

 rule, longer and stouter than those of the mes-endoderm 

 while a layer of protoplasm marking the cell-boundaries is not 

 so distinct, though indications of it may be seen. The vitello- 

 phag cells, on the other hand (vi), seem to consist simply of 

 nuclei imbedded in the peripheral protoplasm, and in sections 

 I could distinguish no processes in connection with them. The 

 segmentation, though appearing from the surface to be total, is 

 in reality not at all so, the greater portion of the yolk taking no 

 part in it, and indeed being apparently destitute of protoplasm. 



In the next stage (Fig. 14) the vitellophag cells iyi) have 

 increased to eight, the mes-endoderm {MEn) is composed of 

 two circles, each consisting of twelve cells, and the number of 

 ectoderm cells has also been doubled, so that a 64-celled stage 

 is reached. This stage does not call for any more detailed 

 description, except that it may be pointed out that the various 

 cells still retain the staining peculiarities which distinguished 

 them in the preceding stage. 



The 64-celled stage is the last one in which the division in- 

 volves every cell, that is to say, in which the increase of cells 

 follows a geometrical progression. In the next stage the 

 vitellophag cells do not take part in the division, and, in fact, 

 do not divide again for some time. The next stage, conse- 

 quently, is composed of 120 cells, a view of the posterior pole 

 of an Qgg passing into it being given in Fig. 15. It will be 

 there seen that the cells of the two mes-endoderm circles are 

 dividing in such a manner that each circle will eventually be 

 formed of twenty-four cells. It will be observed, however, 

 that one of the cells of the posterior circle is dividing in a 

 somewhat different plane from all the others, so that one of 

 the daughter cells {l.en) will encroach upon the area occupied 

 by the vitellophags. The significance of this cell is a little 

 doubtful, since I have not been able to follow the fate of its 

 products. I believe, however, that it is destined to give rise 

 to the endoderm which will form the so-called liver lobes, and 



