SURVEY OF CELL-LIFE. 175 



formed, and around that the germinal vesicle, which is the 

 nucleus of the ovum-cell, Eizelle. 1 The youngest germinal 

 vesicle there represented by Wagner, appears to be hollow . 

 This is not generally the case, however, in the formation of 

 cell-nuclei. Plate III, fig. 1, e, appears to be a cell-nuclens 

 of a cartilage-cell in the act of forming. A small round 

 corpuscle is there seen, surrounded by some minutely gra- 

 nulous substance, whilst the rest of the cytoblastema i> 

 homogeneous. This granulous substance is gradually lost 

 around the object; at a subsequent period it begins to 

 be sharply defined, and then exhibits the form of a cell- 

 nucleus, which continues to grow for a certain period. (See 

 pi. Ill, fig. 1, a, b.) Such a nucleus usually appears solid 

 in the first instance, and many nuclei remain in this con- 

 dition ; in others, on the contrary, the portion of the sub- 

 stance situated nearest to the external surface continuallv 



m 



becomes darker, and not unfrequently at last forms a dis- 

 tinctly perceptible membrane, so that the nucleus is hollow 

 in such instances. The formative process of the nucleus 

 may, accordingly, be conceived to be as follows : A nucle- 

 olus is first formed ; around this a stratum of substance 

 is deposited, which is usually minutely granulous, but not as 

 yet sharply defined on the outside. As new molecules are 

 constantly being deposited in this stratum between those 

 already present, and as this takes place within a precise dis- 

 tance of the nucleolus only, the stratum becomes defined 

 externally, and a cell-nucleus having a more or less sharp con- 

 tour is formed. The nucleus grows by a continuous depo- 

 sition of new molecules between those already existing, that 

 is, by intussusception. If this go on equably throughout the 

 entire thickness of the stratum, the nucleus may remain solid ; 

 but if it go on more vigorously in the external part, the latter 

 will become more dense, and may become hardened into 

 a membrane, and such are the hollow nuclei. The circum- 

 stance of the layer generally becoming more dense on its 

 exterior, may be explained by the fact that the nutritive fluid 

 is conveyed to it from the outside, and is therefore more con- 

 centrated in that situation. Now if the deposition of the new 



1 See the Supplement. 



