vol.3.] Robertson. — Embryonic Fission in Grisia. 137 



It is the latter only which takes part in cleavage and from 

 which the blastomeres are formed. At the first cleavage the 

 plane of division does not pass entirely through the egg, even 

 of that part out of which the embryo is formed, and as a 

 consequence the first two blastomeres, while being connected 

 at one pole, fall asunder at the other. The undivided 

 portion, called the middle piece (mittelstiick) , remains intact 

 through the two, four, and eight-cell stages, while the 

 blastomeres are widely separated at the animal pole. In the 

 meantime the granular zone disintegrates more or less, its 

 granules become larger, and nuclei appear between the free ends 

 of the blastomeres. It is in the sixteen-cell stage that the 

 resemblance between the embryos of Plumatella and Grisia is 

 closest. At this time the middle piece disappears and the 

 blastomeres being set free completely separate from each other. 

 They continue to increase in number, although not regularly, 

 while in the spaces between them are numbers of small cells. 

 With further increase in the number of blastomeres, the small 

 cells gradually decrease in number until, in the twenty-four cell 

 stage the blastomeres having united into a ball, the small inter- 

 polated cells disappear almost entirely. From this point develop- 

 ment proceeds in the regular manner. A comparison of the 

 series of figures I to V in Fig. 104, PI. IV, of Braem's paper, 

 with Figs. 22, 23, and 24 of this paper will show the similarity 

 of the cleavage in the two cases. The resemblance consists not 

 only in the separation of the blastomeres but in the appearance 

 between them, as if shoving them apart, of numerous small cells 

 resembling those similarly situated in the embryo of Crisia. 

 The function of these cells in both cases is probably identical, 

 i.e., they serve as nourishment for the embryo. As in Grisia 

 the interpolated cells gradually disappear and the blastomeres 

 unite at about the twenty or twenty-four cell stage iuto a solid 

 ball. 



The Hull Stage. — From the twenty-cell stage onward the 

 embryo of Grisia forms, as has been said, a more or less com- 

 pact ball. PI. XIV, Fig. 24, represents an embryo measuring 

 43 h- in diameter and containing from sixty to seventy blas- 

 tomeres which have united into a ball, although still surrounded 



