76 MCMURRICH. [Vol. XI. 



and Umbrella. It has been shown, however, by the observations 

 of Miss Clapp ('91) that too much importance has been attached 

 to the question of the axial relations of the ovum and the 

 embryo, and further consideration of this point may be omitted. 

 But not only is the future longitudinal axis marked out in the 

 eight-celled stage, but the dorso-ventral axis is also clearly indi- 

 cated by the position of the vitellophag cell, that side of the 

 &gg upon which it lies being the future ventral surface of the 

 embryo. Unfortunately I have not been able to ascertain what 

 relation this surface bears to the point of extrusion of the 

 polar globules, since these structures cannot be discovered in 

 the preserved ova upon which I worked ; it would be interest- 

 ing to know if in this particular, also, Jaera resembles the 

 forms mentioned above. 



The relations of the cells to the yolk and the peripheral pro- 

 toplasm in the eight-celled stage are shown in the section rep- 

 resented in Fig. lo. The peripheral protoplasm (//) is still to 

 be seen as a distinct layer covering the surface of the yolk 

 mass, and at a short distance below it, imbedded in the yolk, 

 are masses of protoplasm surrounding the nuclei. During the 

 segmentation these structures have passed peripherad, and in 

 the eight-celled stage the protoplasmic masses surrounding 

 them have almost reached the surface, their connection with 

 the peripheral protoplasm by means of slender processes being 

 very distinct. Other processes radiate into the yolk, and 

 though actual anastomoses of the processes from different 

 masses were not observed, the presumption is very strong that 

 they exist. The central portion of the yolk presents a very 

 different appearance from that which encloses the cells, having 

 broken up into small masses, from which apparently all proto- 

 plasmic matter has been withdrawn, there being \w Jaera, as in 

 many other Crustacea {e.g., Moina), a central mass of yolk en- 

 tirely destitute of protoplasm. 



In the next stage a division of all the cells again takes place, 

 and a sixteen-celled stage is the result. The greatest interest 

 attaches to the cells situated at the posterior pole of the egg, 

 and to these alone our attention need be directed. In a num- 

 ber of eggs in which the segmentation had just been completed. 



