ON THE DEVELOPMENT OF PARASITIC COPEPODS. 7 1 



chelestid the egg at each end of the string is hemispherical in 

 shape, due to the fact that it is pressed on only one side (Fig. 

 36). In the proximal egg the ventral side is rounded and in the 

 distal egg the dorsal side is rounded. The first protoplasmic 

 cell (ah) is cut off at one edge of the hemisphere. The second 

 cleavage results in the formation of three protoplasmic cells (a, 

 b, c) whose centers form the apices of a triangle on the spherical 

 surface at its edge (Fig. 36, A and B}. We should assume that 

 this arrangement is nearer the ancestral type, which was prob- 

 ably a sphere, and that the first three protoplasmic cells being 

 in the equatorial plane (Fig. 27) is due to the pressure. Fig. 

 36, A and B, shows a similar arrangement of cells to the same 

 stage in the cleavage of Lepas as figured by Biglow, save that in 

 the dichelestid the yolk is much greater in amount and one side 

 of the egg is flat. In both cases d (the yolk cell) extends under 

 a, b, and c but in the dichelestid the yolk cell is so large as to 

 push c over b (in the distal egg). 



This altered arrangement of the protoplasmic cells does not 

 seem to affect the normal development of the embryo. The 

 ectoderm grows over the yolk in the usual manner, except that 

 it is stretched more on the rounded side of the egg (Fig. 36, C). 

 The four entoderm cells are thicker, and in the distal egg of 

 more volume, than normally and after the entoderm forms a 

 syncytium the nuclei have not exactly their normal arrangement, 

 but when the ensuing nauplius escapes from the egg membrane 

 everything is apparently restored to its normal relation, save that 

 a nauplius developing from a distal egg is larger. 



This is contrary to the idea of Schimkewitz, who attributes 

 many abnormalities in parasitic copepod embryos to slight differ- 

 ences in pressure in the egg string ; but the eggs he studied had 

 less yolk than those considered in this paper. Differences in 

 pressure in the dichelestid egg result principally in differences in 

 form of the yolk mass. This yolk mass does not, save to very 

 small extent, enter as such into the composition of cells, but is 

 dissolved and used as food by the cells. The protoplasmic cells 

 always being on the surface of the yolk, their relation to the 

 food supply remains unchanged. 



Experiments on the effect of unequal compression on cleavage 



