No. I.J EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 89 



however, that in this species also variations occur in the seg- 

 mentation, certain cells in different eggs dividing in different 

 directions. I have not, owing to technical difhciilties, at- 

 tempted to follow out these variations, but have confined my 

 attention to that variety which resembled what has been de- 

 scribed ior Jaera and which seems to be the most typical. 



The center of the yolk is occupied in the unsegmented &gg 

 by a mass of protoplasm containing the segmentation nucleus 

 and prolonged at the surface into a number of branching pro- 

 cesses, w^hich undoubtedly are continued through the yolk to 

 form a network in the meshes of which are situated the yolk 

 granules. I was not able to detect this network in mature 

 ova, but a peripheral zone of protoplasm, somewhat thinner 

 relatively than that of Jaera, was readily observable and is 

 probably, as in that form, in organic continuity with the cen- 

 tral mass. The first cleavage divides this latter into two por- 

 tions (Fig. 28, A, C), its plane being in this case apparently also 

 at right angles to the long axis of the future embryo. The 

 second division gives rise to four cells (Fig. 29, A, B, C, D), 

 two of which lie in a plane at right angles to that of the other 

 two, while the third division produces eight cells, four of which 

 {A, a', B, b') are arranged in a circle near the anterior ex- 

 tremity of the embryonic axis, three others {C, c', d') in a 

 second circle near the posterior extremity of this axis, while 

 the eighth [D) lies slightly to one side of the extremity of the 

 axis, the whole arrangement being identical with that of Jaera 

 in the same stage {cf. Fig. 9). In the i6-celled stage the 

 arrangement is slightly different from what obtains in Jaera. 

 All the cells divide as in that form, and there are produced two 

 circles, each of four cells, in the anterior half of the Qgg, while 

 at the equator are two other cells, which arise from two of the 

 cells forming the circle of three in the eight-celled stage. 

 Behind these is a circle of four) cells which correspond in 

 position to the mesodermal circle ^f Jaera, but have not the 

 same fate, while near the posterior pole are two cells {D and 

 X) corresponding in position and history, but not exactly in 

 function with the endoderm cells of Jaera. I have not ob- 

 served any shifting of cells such as I have described for Jaera^ 



