FREE-SWIMMING SPOROSACS OF THE HYDROID GENUS DICORYNE. 279 



the latest stage examined. Their ultimate fate cannot be ascertained in our material, 

 but it is probable that they will later form the thin, cuticular covering of the young 

 polyp. The nucleus of each blastomere lies near the peripheral margin, is vesicular, 

 and usually exhibits two or three deeply staining nucleoli. Ten of the nuclei of this 

 blastula are in karyokinetic division, and the axes of the spindles are all placed 

 tangentially (see fig. ll); the planes of division of the blastomeres are therefore 

 radial, and by such divisions the cells increase rapidly in number, with the result that 

 they become less in size, for five blastulae in this later stage, with about 400 blasto- 

 meres, are no larger than the blastula first described. 



The next phase of development, represented in our material by four specimens, 

 shows the cleavage-cavity containing numerous cells — the endoderm — with here 

 and there small spaces between them (fig. 12). A few nuclei in karyokinesis 

 are visible in the ectoderm ; the spindles of some of these lie tangentially, but 

 others are at right angles to the periphery, so that the divisions of these cells 

 result in the formation of inner (endodermic) and outer (ectodermic) elements. 

 The appearances presented by sections of these specimens suggest that the 

 endoderm has been produced at several or many places (multipolar immigra- 

 tion), though here and there are blastomeres which have apparently not yet taken 

 part in this process. Some of the endoderm cells are of considerable size, but by 

 division of the central cells, and the accession to the endodermic mass of other 

 cells from the wall of the blastula, the cleavage-cavity becomes tightly packed with 

 cells, in a few of which karyokinetic divisions are present. The surrounding ectoderm 

 is composed for the most part of cells of almost uniform size, and the nuclei are 

 situated fairly regularly near the periphery. 



In the latest phase (fig. 13), represented by a single complete and two damaged 

 specimens, there is a central cavity— the ccelenteron, — which has been formed by 

 the breaking down of some of the endoderm cells, the debris of which lies in the 

 cavity. Cell-outlines are scarcely distinguishable in the ectoderm, except at the 

 extreme periphery of the cells, and are apparent only in a few places in the endoderm. 

 The nuclei of ectoderm and endoderm are similar in size and structure. There is no 

 indication of the mesoglceal lamella at this stage. 



The general development of the egg most closely resembles that of Bougainvillea 

 superciliaris as described by Gerd (1892), a point of some interest, since on other 

 grounds, e.g. resemblances in the trophosomes, Dicoryne and Bougainvillea are 

 placed in the family Bougainvillidse. 



The egg is usually completely detached from the sporosac. We have three 

 examples of D. conybearei in which the sporosac has ruptured and the egg, having 

 escaped from the now wrinkled body of the sporosac, has, however, remained 

 attached near the point of union of the spadix and the tentacle. Here it has 

 undergone normal cleavage : in one case a blastula and in the others morula stages 

 had been reached. 



