No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 1 13 



within this is the yolk scattered in the meshes of a protoplasmic 

 network extending from the peripheral protoplasm to a more or 

 less centrally situated mass containing the nucleus, this central 

 mass and the nucleus alone undergoing cleavage, at least in the 

 early stages. As cleavage proceeds the central masses formed 

 by it separate from one another, remaining connected, however, 

 by strands of the network. It is difficult, therefore, to under- 

 stand how any extrinsic forces can determine the position as- 

 sumed by the central masses, and since they are not in contact 

 with one another, but are suspended, as it were, in a proto- 

 plasmic reticulum, like knots in a fish-net, it is also difficult to 

 understand how Berthold's ('86) law of minimal contact areas 

 can determine their position. I see no escape from the con- 

 clusion that the cleavage form of Jaera is determined entirely by 

 intrinsic conditions, and though we cannot exclude the action of 

 extrinsic forces in holoblastic ova, yet the presumption is allow- 

 able that even in these the intrinsic forces are important. In 

 discussing the cause of the direction of cell division, it must be 

 remembered that the forces which primarily determine karyo- 

 kinesis reside in the cell, since recent observations have 

 strengthened the view which regards the archoplasm and the 

 centrosomes as of the greatest importance in this respect. The 

 generalization of O. Hertwig that the spindle tends to form in 

 the direction of least pressure is one to which there are excep- 

 tions, and in centrolecithal ova, such as those of the Isopods, it 

 cannot come into the question. We are left, then, no choice 

 but to refer the vis essentialis which determines the direction of 

 the karyokinetic spindle, and therefore the cleavage form of 

 Jaera, to the constitutio7ial peculiarity of the ovnm. 



I do not, however, in the least intend to imply that external 

 conditions, such as pressure, etc., do not influence the direction 

 of the spindle in holoblastic ova ; indeed, the evidence we have 

 indicates that they do ; my remarks refer simply to the ova of 

 Jaera, and those of similarly developing forms. It may be 

 pointed out, however, that the formation of the second polar 

 globules, if not of the first also, is apparently determined by in- 

 trinsic forces, and the relation of the first segmentation plane 

 to the point of emergence of the polar globules is also probably 



