No. 2.] COMPARATIVE CVTOLOGICAL STUDIES. 495 



nuclei which contain nucleoli, from one to three of the latter 

 occur, and one or all of these are frequently found in close con- 

 tact with the nuclear membrane (Figs. 315a, 318, 320), while in 

 the largest nuclei observed only a single nucleolus is present, and 

 this one is relatively large and is always at or near the center 

 of the nucleus, never at its periphery (Figs. 319, 322, 323). 

 In connection with the problem of the origin of this nucleolus 

 we recall those small granules contained in the cytoplasm, 

 which I have ('96) termed nutritive particles. These particles 

 {Nut. Gl.) stain with eosin quite as intensely as the nucleolus, 

 and in the smallest cells are either wholly absent or present in 

 only small number ; but in the larger cells they are usually 

 much more abundant, or when not more numerous they are of 

 greater size, and are often quite densely grouped around the 

 nucleus. It would seem probable that the nucleolar sub- 

 stance is derived from these supposed nutritive particles. Thus 

 when the nucleoli first appear they are most frequently in con- 

 tact with the nuclear membrane ; and this shows that they are 

 formed at the periphery of the nucleus, and only later come to 

 occupy a central position within it. And since the nutritive 

 particles are usually very numerous in the immediate vicinity 

 of the nucleus, we may conclude that the nucleoli are formed 

 from substance of these nutritive particles, which has been 

 taken up by the nucleus. In the smallest nuclei alone do more 

 than one nucleolus appear, so that the nutritive substance 

 would seem to be taken into the nucleus from several points 

 on its periphery, and then subsequently these several assimi- 

 lated portions of nutritive substance may fuse together and so 

 produce a single large nucleolus. Accordingly, the substance 

 of the nucleolus would in this way appear to have an extra- 

 nuclear origin. That these nutritive particles are being succes- 

 sively absorbed by the nucleus is shown by the fact that the 

 increase in the size of the nucleus and of the nucleolus go hand 

 in hand. On the other side, these nutritive bodies in the cyto- 

 plasm cannot be considered to be of nucleolar origin, since 

 they usually make their first appearance in the cell body before 

 a nucleolus arises in the nucleus ; and if they did have a nucle- 

 olar origin, i.e., if they were excreted portions of the nucleolus. 



