174 SURVEY OF CELL-LIFE. 



lages of the tadpole, or larger corpuscles may be subsequently 

 formed in the interior of hollow nuclei, for instance, the 

 innumerable corpuscles in the germinal vesicle of the fish, 

 and fat-globules in the nucleus of the fat-cells in the cranial 

 cavity of fishes. 



The nucleus, in most instances, contains one or two, more 

 rarely three or four small dark corpuscles, the nucleoli. Their 

 size varies from that of a spot which is scarcely discernible to 

 that of "Wagner's spot [macula germinativa) in the germinal vesi- 

 cle. Nucleoli cannot be distinctly recognized in all cell-nuclei. 

 They may be distinguished from the larger corpuscles, which are 

 sometimes developed in certain hollow nuclei, from the circum- 

 stance of their being formed at a much earlier period ; they 

 exist, indeed, before the cell-nucleus. They are placed eccen- 

 trically in the round nuclei, and in the hollow ones are dis- 

 tinctly seen to lie upon the internal surface of the wall. It is 

 very difficult to ascertain their nature ; it may also vary very 

 much in different cells. They sometimes appear to be capable 

 of considerable enlargement, as in the nuclei of the fat-cells in 

 the cranial cavity of the fish, and in such instances often have 

 the appearance of fat. According to Schleiden, hollow nucleoli 

 also frequently occur in plants. 



Most cell-nuclei agree in the peculiarity of not being dis- 

 solved, or rendered transparent by acetic acid, at least not 

 rapidly so, whilst the cell-membrane of animal cells is in 

 most cases very sensitive to its action. Some cells, (such 

 as those of the yelk-cavity of the e^g } plate II, fig. 3,) 

 which have no perceptible nucleus of the ordinary form, ex- 

 hibit a globule having the appearance of a fat-globule, which 

 grows as the cell expands, though not in the same proportion, 

 and was probably formed previous to the cell. Whether such 

 a globule have the signification of a nucleus or not, must re- 

 main an undecided question. 



The formation of the cell-nucleus. In plants, according to 

 Schleiden, the nucleolus is first formed, and the nucleus around 

 it. The same appears to be the case in animals. According 

 to the observations of R. Wagner on the development of ova 

 in the ovary of Agrion virgo, 1 the germinal spot is first 



1 See Wagner, Beitrage zur Geschichte der Zeugung und Entwickelung; Erster 

 Beitrag., tab. II, fig. 1. 



