STRUCTURE OF THE LARVA OF SPONOILLA LAOUSTRIS. 381 



The nuclei of these cells are the largest in the whole larva, 

 measuring sometimes as much as 7/x in diameter, hut their 

 size, like that of the cell itself, depends on the state of deve- 

 lopment of the individual. The nuclear membrane, like that 

 of the granular nuclei, is thinner than that of the nuclei 

 of the flagellated cells, but, as a rule, is easily made out. The 

 centre of the nucleus is occupied by the central corpuscle — 

 the so called nucleolus, — which, again, like the cell and its 

 nucleus, varies considerably in size. The space between the 

 central corpuscle and the nuclear membrane is occupied by 

 a variable number of granules situated at the nodes of the 

 nuclear reticulum. When the number of granules is small 

 they are comparatively large in size, and the threads of the 

 nuclear reticulum are few and coarse. On the other hand, 

 when the number of granules is large they are small in size, 

 and the threads of the reticulum are correspondingly close and 

 fine. The granules in these nuclei vary also as regards position. 

 In some nuclei the granules, whether exceedingly numerous 

 and small, or few and comparatively large, are concentrated 

 in the neighbourhood of the nuclear membrane, leaving a clear 

 zone round the central corpuscle ; in others the granules are 

 evenly distributed throughout the whole space, and consequently 

 the clear zone is absent (fig. 41 a, and fig. 40a). 



These differences in the structure of the vesicular nuclei 

 seem to suggest that the class of cells here described consists 

 of several different kinds of cells. The term " cells with vesi- 

 cular nuclei" may, in fact, be regarded simply as a convenient 

 one to hide our ignorance of the division of labour that has 

 already come into existence among these cells. The fact that 

 megascleres are being developed in some of these cells may be 

 taken as evidence in favour of this view (figs. 36 a, b, and c). 



To sura up, these cells are the largest in the whole larva, 

 occupying the interior of the inner mass, and containing a 

 number of enclosures in the form of " nutritive vacuoles," 

 "yolk bodies,^' and " refringent granules." They possess a 

 nucleus which contains a large central corpuscle, sometimes 

 surrounded by a clear zone. The nuclear membrane is thin, 



