No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 103 



The scattering of the mesendodermal cells and the more per- 

 fect differentiation of the ectodermal teloblasts continues in 

 later stages, producing the appearance shown in Fig. 58. Here 

 the entire area of the blastoderm presents a dark appearance 

 due to the presence under the ectoderm cells of lower layer 

 cells, the mesendodermal region being but little darker than 

 the remaining portions. The scattering of the mesendodermal 

 cells is complete ; and careful focusing, as well as sections, 

 show the presence of vitellophag cells in the yolk. No definite 

 differentiation of these cells from the remaining mesendoder- 

 mal elements is, however, to be distinguished, and, taking the 

 three forms of cleavage here described into comparison, it will 

 be seen that we have in them three gradations of differentia- 

 tion of the mesendodermal elements. In Jacj-a there is first 

 of all a distinct differentiation of the vitellophags from the 

 rest of the mes-endoderm and later a differentiation of the 

 liver-endoderm for the mesoderm, so that three distinct portions 

 of mes-endoderm are visible. In Asellus the differentiation of 

 vitellophags from the remaining mes-endoderm is at certain 

 stages distinct, though later becoming inconspicuous, and a 

 recognizable differentiation of liver-endoderm is questionable. 

 Finally, in Armadillidiuin and Porcellio no differentiation into 

 the three portions can be made out, and it is necessary to speak 

 simply of the mes-endoderm. There can be no doubt, however, 

 that this mes-endoderm is equivalent to the mesoderm, liver- 

 endoderm, and vitellophags of Jaera, giving rise to cells which 

 play the parts taken by these various elements in the develop- 

 ment of the embryo. The significance of this variation in the 

 differentiation of the mesendodermal elements will be consid- 

 ered more in detail in a later portion of this paper. 



In the stages represented in Fig. 58 the further differentia- 

 tion of the ectodermal teloblasts is also shown, there being 

 thirteen recognizable in the preparation figured. One cell of 

 the row is hardly on a level with the others, and one is tempted 

 to consider it the central teloblast, in which case there is a 

 marked disparity in the number of cells present on either side 

 of it, there being but five on one side and seven on the other. 

 The number five is significant in view of what has been de- 



