

SECTION II. 



ON CELLS AS THE BASIS OF ALL TISSUES OF THE ANIMAL BODY, 



The young cells contained within the cartilage-cells (see 

 plate I, fig. 8, ff) may be regarded as the elementary form 

 of the tissues previously considered, and may be described as 

 round cells having a characteristic nucleus, firmly attached to 

 the internal surface of the wall. As the above were proved to 

 correspond with the vegetable cells, it follows, that it is only 

 necessary to trace back the elementary structure of the rest 

 of the tissues to the same formation,' in order to show their 

 analogy also with the cells of plants. In some tissues this 

 proof is easy, and immediately afforded ; in others, however, it 

 is obtained with much difficulty, and it would frequently be 

 altogether impossible to demonstrate the cellular nature of 

 some, if the connexion between the different steps in this 

 investigation were lost sight of. The difficulty arises from the 

 following circumstances: 1st. The minuteness of the cells ; in 

 consequence of which it is not only necessary to use a power 

 magnifying from 400 to 500 diameters, but it is also frequently, 

 indeed generally found impossible to press out their contents. 

 2dly. The delicate nature of the cell-membrane. When this has 

 a certain density, its external as well as internal outline may 

 be recognized, and the distinction between it and the cell-con- 

 tents may thus be placed beyond a doubt. But if the cell-mem- 

 brane be very delicate, the two outlines meet together in one line, 

 and this may readily be regarded as the boundary line of a 

 globule, not enclosed by a special enveloping membrane. 3dly. 

 The similar power of refraction possessed by the cell-wall and 

 cell-contents, in consequence of which the internal outline 

 of the former cannot be observed. 4thly. The granulous nature of | 

 the cell-membrane, which when the contents are also granulous, 

 cannot be distinguished from them. Lastlv, the variety of 



