THEORY OF DEVELOPMENT FROM CELLS. 113 



DEVELOPMENT OF THE ANIMAL TISSUES.* 



IT is proposed in the following pages to offer some account of the pre- 

 sent condition of the theory of cell-development, especially in relation to 

 the following points : 



1. The nature and composition of the several parts of which a cell is 

 constructed. 



2. The order in which these several parts are developed in the forma- 

 tion of cells. 



3. The manner in which the multiplication of cells is effected. 



4. The transformations undergone by cells in the development of tissues. 

 1. Very little requires to be said concerning the composition of the cell 



itself. Its membrane or wall appears, by almost universal assent, to be 

 formed of a protein-compound, most probably of albumen, except in a few 

 cases in which it seems to be composed of a substance more allied to 

 fibrine.t Although the cell- wall is rendered transparent and indistinct by 

 acetic acid, yet it is not dissolved by this reagent, as is usually supposed to 

 be the case, for on the addition of an alkali, such as a solution of potash or 

 ammonia, its form and other external characters are in many cases restored.]; 

 With respect to the contents of cells, it is perhaps sufficient to state that further 

 investigations continue to shew how various these may be, the varieties 

 being as numerous as the functions which the cells discharge, and often 

 differing in the same cell at different periods of its life. These contents, 

 although occasionally composed of a clear fluid of various consistency and 

 colour, are usually more or less granular, the granules consisting of 

 different colouring matters, of fat particles, and of a fine molecular sub- 

 stance, whose nature is still obscure. 



There is much discrepancy in the accounts given by different writers 

 concerning the composition and general characters of the nucleus. This 

 discrepancy is probably in great measure due to the fact, that after their 

 formation the nuclei undergo various alterations in aspect if not in compo- 

 sition, and in some measure also to the fact of there probably being some 

 original differences in the nuclei of different cells. Sometimes the 

 nucleus occurs as a more or less solid body of a granular aspect, while at 

 other times it appears as a pale vesicle with a distinct cell-wall and fluid 

 contents. And between these two conditions varieties are occasionally 

 found which would seem to prove that the one is only a modification of the 

 other, and that these several varieties represent so many transitional stages 

 between the two. The pale vesicular form is by far the most general one, 



* Miiller's Physiology, p. 1641. 



"h Kolliker, Entwickelungs-geschichte der Cephalopoden, p. 154. 

 Donders, Hollandische Beitragezu den anat.und physiol. Wissenschaften, 1846. 



I 



