156 



STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



252. Now we shall find that one method, by which the requisite firm- 

 ness and solidity are given to the animal fabric, consists in the depo- 



Fig. 33. 



Portion of shell-membrane, showing the origin of cells in the midst of horny intercellular substance; a, 

 nuclei; 6, incipient cells ; c, the same further advanced, but separated by intercellular substance ; d, the, 

 cells become polygonal by mutual pressure. 



sition of earthy substances in the interior of such cells, by a peculiar 

 secreting action of their own. Thus in Shell, we find them completely 

 filled up with carbonate of lime ; and in the enamel of Teeth with phos- 

 phate of lime. When this is the case, there is a tendency to an apparent 

 coalescence of the cells, by the obliteration of their partitions ; or rather, 

 perhaps, by the removal of the whole intercellular substance from between 

 them, the actual cell-walls being so very thin, that they are not distin- 

 guishable. The incipient stages of this coalescence, as seen in another 

 portion of the same membrane as that represented in the last figure, are 

 shown in Fig. 34. At #, the nucleated cells are very distinct ; and are 

 separated by a large quantity of intercellular substance. At 5, they 

 approach each other more closely, the amount of intercellular substance 

 being less ; the widest intervals are seen at the angles of the cells. At 

 c, the approximation is much closer ; and the cell-walls are scarcely dis- 

 tinguishable at the points where they come into immediate contact. 

 Proceeding further, we observe that the partitions are much less com- 

 plete ; so that the originally distinct cellular character of the membrane 



Fig. 34. 



Portion of shell-membrane, showing the gradual coalescence of distinct cells ; at a, the cells separated by 

 intercellular substance; at b, the partitions are thinner; and at c, they almost disappear. 



is chiefly indicated by the bright nuclei, which are regularly dispersed 

 through it, and by the triangular dark spots, which show the remain's of 



