Euphausiidæ. 1 9 



one) whereas those constituting the entodermal layer take 

 their orgin only from the other segment. The successive 

 divisions of the former segment take place at a much 

 more rapid rate than that of the latter, and the ectodermal 

 cells accordingly become much more numerous than the 

 entodermal cells, which at first occupy the one pole of the 

 egg opposite the ectodermal cells, but subsequently are 

 overgrown by the latter, so that they at last occupy the 

 centre of the egg. This may be rather distinctly seen in 

 fresh and still pellucid eggs, on slightly shifting the focus 

 of the objective, so as to get a dioptrical view of the 

 ovum. In somewhat advanced stages, such a view will 

 always show (see PI. 1, figs. 8, 9) a cortical layer of densely 

 crowded cells, and a central mass consisting only of a re- 

 stricted number of much larger cells. The former are the 

 ectodermal cells, the latter the entodermal cells. In pre- 

 pared eggs it is likewise easily determined, that the cells at 

 one of the poles of the egg are considerably smaller than 

 on the other, and in some cases a single large cell is seen 

 occupying the latter pole (see PI. 2, fig. 5, cf. also PI. 1, fig. 

 6); in other cases this cell is replaced by 2 cells lying 

 side by side, and likewise larger than the others (see PI. 2, 

 fig. 6). These cells are unquestionably entodermal in char- 

 acter. In somewhat more advanced stages (fig. 7), 2 juxta- 

 posed cells are likewise seen occupying this pole; but now 

 these cells appear rather small, as compared with the other 

 cells. This, however, is merely due to the fact that they 

 are partly overgrown by the adjacent ectodermal cells. At 

 last the latter meet from all sides (see figs. 10, 11), whereby 

 the entodermal cells are completely received within the 

 ovum. At the place where the ectodermal cells meet, a 

 slight depression is formed, and this is the Mastoporus or 



