No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 75 



is strictly comparable to that of Nereis. That this is so is evi- 

 denced by the relation of the antero-posterior axis of the future 

 embryo to the first segmentation plane, which is the same as in 

 Nereis, a fact which can, however, be more readily perceived 

 when the next segmentation has been completed. 



In Fig. 8 is shown a preparation in which the division into 

 eight cells is not quite completed, the cells being still united in 

 pairs by a short band of protoplasm. It will be seen that A 

 and B have divided in such a way that their spindles must have 

 been parallel, one of the daughter-cells of each lying immedi- 

 ately below the other, and being thus concealed from view. 

 This division is practically an equatorial one. The spindle of 

 cell Cy however, assumed a position at right angles to those of 

 A and B, and thus underwent what may be considered a third 

 meridional division, while the spindle of D was inclined at an 

 angle of 45° to the other three. As the result we get the ar- 

 rangement which is represented in Fig. 9, taken from an ^^^ 

 which was rotated slightly so as to bring all four cells resulting 

 from the division of A and B into view. It will be seen from 

 this that the cells have arranged themselves around the long: 

 axis of the &gg, an axis which represents the longitudinal axis 

 of the future embryo, and which stands at right angles to the 

 axis indicated by the polar globules. Around one of the new 

 poles of the ^%Zi which the later development shows to corre- 

 spond to the anterior extremity of the embryo, four cells. A, a', 

 B, b\ are arranged at almost equal distances, and near the pos- 

 terior pole is a second circle of three cells, C, c\ and d\ also 

 almost equally spaced, while the eighth cell, D, occupies an 

 almost polar position, lying, however, a little to one side of the 

 actual pole. This cell is destined to give rise to the vitello- 

 phags, by which term its descendants may hereafter be de- 

 noted. 



We have seen that the first division-plane was parallel to the 

 shorter axis of the ^g%, at the extremity of which were the 

 polar globules, while the second plane was parallel to the larger 

 axis, and we now see that the longitudinal axis of the future 

 embryo corresponds with that of the second division-plane, an 

 arrangement which agrees with what occurs in Nereis, Crepiditla, 



