142 J. J. LISTEB. 



position suggests that they have recently been set free by the 

 breaking up of such a compound nucleolus (fig. 21). Young 

 specimens always have large nucleoli, and it appears probable 

 that the nucleoli become smaller as development advances. 



The reticulum varies much in the closeness of its meshes. 

 In some cases (fig. 16) it forms an open network, while in others 

 its communications are so intricate and minute as to give the 

 appearance of an almost homogeneous substance. This is 

 especially the case in the shrunken nuclei of the specimens 

 kept for some months in dishes. 



Separation of portions of the Nucleus. — Very commonly in 

 the chambers which lie towards the centre of the shell from that 

 which the nucleus occupies — in the chambers, that is to say, 

 through which the nucleus has passed as it has moved on with 

 the growth of the individual — there are present fragments 

 which take a pale reddish stain with picro-carmine, and hence 

 are readily seen as they lie in the yellow stained protoplasm 

 (fig. 24). The contour of these fragments is generally in- 

 distinct, a,nd in sections it is seen that they are more or less 

 homogeneous granular bodies, while at their periphery they 

 gradually acquire the reticular structure of the surrounding 

 protoplasm. In some cases sections of the fragments show them 

 to contain globular bodies, staining bright red with picro- 

 carmine, which appear to be precisely similar to the nucleoli 

 of the nucleus (fig. 25). In most cases, however, no such bodies 

 are to be seen. These fragments lie as has been said in the 

 track of the nucleus, and never extend far beyond it. In a few 

 cases a prominence is seen in the profile of the nucleus as 

 though a portion were being separated off (fig. 24). 



From the appearance of these bodies, their position in the 

 track of the nucleus, and from the fact that they may contain 

 bodies like nucleoli, it seems probable that they are indeed 

 fragments of the nucleus which have been given off into the 

 protoplasm as it has moved on from chamber to chamber. It is 

 to be observed that though these fragments sometimes contain 

 nucleoli, the reticular structure is not seen in them. 



Not unfrequently after treatment with picro-carmine, certain 



