STRUCTURE OP THE LARVA OP SPONGILLA LAOUSTEIS. 427 



successive division of any kind of nuclei into two, then into 

 four, and so on, but by the breaking up or fragmentation of 

 one nucleus, the fragments becoming the nuclei of the cell 

 group. 



The discussion so far has had but a negative tendency, my 

 aim being to show that the cell groups have not originated 

 either by immigration from the flagellated layer or by division 

 of the cells with granular nuclei. It remains, therefore, to 

 bring forward some positive proof of their origin by the third 

 possible method, — that is, directly from the cells with vesicular 

 nuclei by fragmentation of the nucleus. 



It has been pointed out that there is a considerable amount 

 of variation among the vesicular nuclei, and it is quite pos- 

 sible that a certain amount of it is connected with the produc- 

 tion of the nuclei of the cell groups. There are also in type 

 B a certain number of cells which possess an irregular and 

 blotchy nucleus, as well as certain cells which apparently do not 

 possess a nucleus at all. In the cells just mentioned there are, 

 however, a number of bodies quite distinct from the yolk 

 bodies, and usually exhibiting a certain amount of structure. 



It will be my purpose at present to show how these frag- 

 mented nuclei give rise to those of the cell groups, the changes 

 involved in their production being represented in fig. 40. The 

 cell marked a presents a rather unusual condition of the vesi- 

 cular nucleus. The granules, instead of being small and 

 numerous, are large and few, while the network of linin fibres, 

 instead of containing small meshes, encloses large ones, and 

 consists of a few threads instead of several. The cell h repre- 

 sents the next stage, in which the central corpuscle has lost the 

 sharp outline it had in the previous stage, seen in cell a, and 

 the granules are situated nearer the centre. The cell c illus- 

 trates the next stage, in which there is no sign of a central 

 corpuscle, but the nucleus contains a great number of small 

 granules of about the same size as those found in the cells a 

 and b. The cell d exemplifies the next stage, in which the 

 granules have grown so as to give the nucleus a blotchy 

 appearance, and in some cases the chromatin can be seen 



VOL, 42, PART 4. NEW SERIES. G Cf 



