No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 85 



at the time of immigration, but I believe that this is entirely 

 due to a subsequent multiplication, and that all the vitello- 

 phags trace back their origin to the cell D of the eight-celled 

 stage. 



By the time the ectoderm has grown back over the mes-endo- 

 derm plug the embryo has the appearance represented in Fig. 

 22, in which the area occupied by the mesoderm is indicated 

 by the dark shading. The ectoderm cells covering this region 

 are manifestly arranged in rows as they are also towards the 

 sides, while anteriorly they become more scattered, though this 

 is not well shown in the figure. The penultimate row of cells 

 seems to be the teloblast row, though I could not make out a 

 perfect correspondence between them and the rows of cells ex- 

 tending forward from them. It will be noticed, however, that 

 the multiplication of those nearer the middle line has taken 

 place somewhat more rapidly or to a greater extent than that 

 of those situated towards the sides, and the posterior edge of 

 the ectoderm plate has now become convex backwards instead 

 of concave as it was originally. In front of the mesoderm and 

 to the sides a series of rows of cells passing forwards and out- 

 wards can be seen, and at the extremity of these is a patch of 

 larger cells which will later give rise to the eyes and which are 

 clearly seen in the side view (Fig. 23, E). These rows of cells 

 leading towards the eyes were not always, however, as clearly 

 marked as in the embryo from which this drawing was made. 

 One other fact in connection with the preparation requires 

 mention, and that is that the cells of the mesendodermal 

 plug are much less closely aggregated than in earlier stages 

 and seem to have a tendency to scatter themselves, so that the 

 shaded area of the figure has a greater extent than that origi- 

 nally occupied by the plug. 



The division of the teloblasts progresses in later stages, and a 

 layer of ectoderm, whose cells are arranged in regular longitu- 

 dinal and transverse rows, extends backwards over the yolk. 

 In Figs. 24 and 25 is represented an embryo in which the 

 blastoderm has extended about two-thirds of the way around 

 the yolk, Fig. 24 representing the anterior half of the embryo, 

 and Fig. 25 the posterior half. Looking at the anterior half 



