No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 'JJ 



or in which traces of the spindle were still visible, I found at 

 the posterior pole the arrangement which is represented in 

 Fig. II. The two cells vi are the products of the division of 

 D of the preceding stage, and represent the vitellophags ; they 

 are surrounded by a circle of seven cells, six of which result 

 from the division of C, c', and d' of the eight-celled stage, while 

 the seventh is derived from one of the cells of the anterior pole, 

 probably A. In a number of eggs, however, I found the num- 

 ber of cells forming the ring to be six only (Fig. 12), a number 

 which agrees with what is found in the next stage. It seems 

 that shortly after the completion of the division there is a 

 migration or rearrangement of certain cells, one of those which 

 at first formed the ring leaving it and migrating towards the 

 anterior pole of the egg. My reason for supposing this to be 

 the case is that I have found the seven-celled ring in ova in 

 which the division was not quite completed, while the six-celled 

 ring was only observed in fully divided eggs, and furthermore 

 it will be seen that the next stage of development can be de- 

 rived directly only from a six-celled ring. The cell which 

 leaves the ring is not that {A') which came into it during the 

 division from the anterior-pole, but so far as I can discover it 

 is one of the cells produced by the division of c {i.e., c^ of 

 Fig. 1 1). From Fig. 12 it might be supposed that it was C of 

 Fig. 1 1 that had migrated out of the ring, but this appearance 

 is, I believe, due to an alteration of the position of C after c^ 

 has moved away. From the observation of a number of eggs, 

 and noting the position of the cell which persists in the ring 

 w^ith relation to the two endoderm cells, I have come to the 

 conclusion that c"^ is the one which disappears. 



What the significance of this migration may be I cannot even 

 suggest. Why a cell belonging to the anterior hemisphere 

 should enter the posterior one, and vice versa, I cannot under- 

 stand. The result of the process is, however, a differentiation 

 of the germ-layers at this early stage. At or in the near vicin- 

 ity of the posterior pole are the two vitellophag cells ; the six 

 cells surrounding them will give rise to the mesoderm and the 

 liver endoderm ; while the eight remaining cells occupying the 

 anterior hemisphere of the ovum will form the ectoderm. 



