DEVEIiOPMENT OP THE CTENOPHOEA. 99 



tails regarding the later stages of this process. After the cleav- 

 age has advanced to the formation of four equal spheres lyino- in 

 one plane, the next cleavage intersects the four spheres in an 

 oblique direction, and divides each of them into a larger sphere 

 and a smaller one which lies obliquely upon it. The eight 

 spheres, now no longer lying in the same plane, present a further 

 difference in the fact that the main mass of the granular ectoplasm 

 has passed over to the four small superjacent spheres, clothino- 

 them with a thick layer, and rendering their colour darker than 

 that of the others — a condition which I had not noticed in BeroU, 

 where, as Chun admits, it is by no means obvious. 



As a general rule the eight segment-spheres thus formed 

 divide almost simultaneously, each into two of dissimilar size. 

 In this process nearly the whole of the ectoplasm is carried over 

 into the eight smaller superjacent cells, while the eight large 

 cells on Avhich these lie consist chiefly of the endoplasm, being 

 overlain only by a thin, scarcely visible layer of ectoplasm. In 

 the later stages the endoplasm collects more and more about the 

 nucleus of the large clear spheres, while their vacuoles flow 

 together into a continuous cavity traversed by irregularly radia- 

 ting and branching filaments from the central endoplasm. 



After the cleavage has thus advanced so far that eight small 

 cells lie on the eight larger ones, the first begin very suddenly 

 and with great activity to divide, while the others remain quies- 

 cent. In Lampetia alone, which presents in its development 

 many peculiarities, do we find the division of the large cells 

 going on at the same rate with that of the small. 



By the energetic division of the small cells the mass formed 

 by the subjacent eight large cells becomes surrounded by a ring- 

 like mantle, which gradualJy extends towards the poles. In the 

 meantime division has been resumed in the subjacent cells, which 

 from eight have become sixteen ; and by the centrifugal separa- 

 tion of these cells from one another in the centre of the mass 

 there is here formed a large vacuolar cavity which extends 

 through the entire axis of the embryo. The enveloping cell- 

 layer continuing to advance towards the poles, one of these 

 becomes soon closed in, v/hile the opposite pole remains as yet 

 uncovered. About this time division is once more repeated in 

 the sixteen large cells, without, however, presenting the regularity 

 which had hitherto characterized this process. 



In the stage at which the embryo has now arrived, with its two 



