No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 99 



42) a section through an ovum in the four-celled stage. The 

 section has passed through two of the cells, and shows clearly 

 their position in the yolk exactly as in Jacra or Asellus, or 

 any other form which segments in the typical centrolecithal 

 manner. 



In the eight-celled stage there seems to be considerable 

 irregularity in the arrangement of the cells in different ova, 

 but in some an arrangement comparable to that oijaera was 

 found. In Fig. 43 is represented an ovum of Arniadillidhim 

 which is just passing into the i6-celled stage, all the nuclei 

 but one being in the spindle stage. This one, which has not 

 yet started to divide, lies near the posterior pole of the ^gg and 

 corresponds to the D cell of Jacra and Asellus so far as its 

 position is concerned. In other eggs both of Ponellio and 

 Arviadillidiwii this same arrangement was found, and it seems 

 to be normal for some eggs, though in others it could not be 

 made out. The i6-celled stage derived from an ^gg of this 

 arrangement shows as vcvjaera two cells at the posterior pole 

 (Fig. 44, D and X), surrounded by a circle of apparently four 

 cells in Porcellio (Fig. 44) and five in Annadillidiinn (Fig. 45). 

 Whether this difference be due to the slightly different stages 

 represented, the Porcellio ovum represented having just com- 

 pleted division, while that of Armadillidiinn is just passing 

 into the 32-celled stage, I cannot say, but just as the number 

 of cells composing the circle differ in Jacra and Asellus, so too 

 they may do here without any appreciable alteration of the 

 similarity of the general features of development. In fact I 

 may state that in one ^^^ of Porcellio I saw six cells in a cir- 

 cle, and I believe that even in different eggs from the same 

 individual there may be considerable differences of arrange- 

 ment of the spherules at this stage, although the two polar 

 cells are probably always distinguishable. Sections of ova of 

 Porcellio in this stage (Fig. 47) show that the cells have 

 reached the surface of the yolk and are imbedded in the periph- 

 eral protoplasm, so that the ^^^ is a syncytial blastula whose 

 cavity is completely filled with yolk, there being certainly no 

 nuclei, and apparently no protoplasm, scattered among the yolk 

 granules. 



