114 POSITION OF THE NUCLEUS OF CELLS. 



and in Kolliker's opinion,* is the constant form of the nucleus in the 

 early stages of the cell's life. It has heen long known that the nucleus 

 has a different composition to that of the cell, many agents which act 

 upon the one having no effect upon the other. Kollikert is of opinion 

 that the membrane of the vesicular nucleus is composed of pyin, the 

 clear contents of albumen, and the nucleolus of fat. His opinion that 

 the membrane is composed of pyin, is derived chiefly from the fact of the 

 nucleus being insoluble in acetic acid, a property which is possessed by 

 no other nitrogenous compound except chondrine ;| and this substance is 

 soluble in the gastric fluid, while the nuclei are not, neither is pyin. The 

 presence of albumen in the contents of the vesicular nuclei he thinks is 

 proved by the contents of the germinal vesicle (which he considers to cor- 

 respond to the so-called nucleus of other cells) being rendered granular by 

 ether. The fatty nature of the nucleolus is indicated by its aspect, and by 

 the presence of fat in parts composed chiefly of cells. 



Schleiden^ described the nucleus in the cells of plants as being inva- 

 riably situated within the substance of the cell-wall, which at that point 

 divides into two laminae, between which the nucleus is placed. In animals 

 also the nucleus is commonly situated at the wall of the cell, sometimes 

 apparently imbedded in its substance, but according to Schwann,|| most 

 frequently attached to its inner surface, and never invested internally 

 by a layer of the cell-wall as it is in plants, according to Schleiden. 

 Henle H states that sometimes, as in pigment-cells and the cells of the crys- 

 talline lens, the nucleus is situated outside the cell-wall, which at that part 

 presents a shallow depression to receive it ; but Dr. Sharpey** is inclined to 

 doubt the exterior position of the nucleus in these cases. Occasionally 

 the nucleus is situated towards the centre of the cell,ff as is well shewn 

 in the cells of cylinder-epithelium. JJ In such cases, however, the nucleus 

 does not usually appear to lie free in the cavity of the cell, and to admit 

 of being altered in position as the cell rolls over, but it seems to be quite 

 fixed, and probably adheres to the internal surface of the cell- wall which, 

 in cylindrical cells, closely surrounds it in one plane, and in flat cells is 

 in contact with it at opposite sides. It was stated by Hewson, however, 

 that when the nucleated blood-corpuscles of fish or reptiles are swollen with 

 water, and watched when rolling over, the nucleus may be distinctly seen to 



* Entwickelungs-geschichte der Cephalopoden, p. 142. f Op. cit. p. 144-5. 



J And according to Kolliker, fibrine, but in this he is manifestly wrong. 



Muller's Archiv. 1838, p. 148. || Microscopische Untersuchungen, p. 210. 



IT Allg. Anat. p. 1 92. ** Quain's Anatomy, Fifth edition, p. xliii. 



+f In the case of vegetable cells, M. Mohl believes that the nucleus is invariably central 

 at the earlier periods of the cell's life, and that its parietal position, when it occurs, is only a 

 secondary state. (Botanische Zeitung, 1846 ; and Taylor's Scientific Memoirs, vol. xviii. 

 1846.) 



JJ See Muller's Physiology, Second edition, p. 418, Fig. 36, B and c. 



Works, edited for the Sydenham Society by Mr. Gulliver, p. 222. 



