130 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



closely resemble the nuclei of cells in general, from which it is unques- 

 Fig. 15. tionable that the new crops of cells may arise ( 212). 

 The only difference is, that in the latter case, the 

 groups of new cells are for a time contained within the 

 parent-cell (Fig. 18) ; whilst in the former they are 

 developed on the free surface of the basement-mem- 

 brane. In Fig. 15 is shown a portion of the same 

 component ceils of me mbrane as that represented in Fig. 14 ; but having 



primary memorane, .. ,.-, 



with adherent epithe- been rendered transparent by acetic acid, its real nature 

 as a layer of flattened nucleated cells is more obvious ; 

 the nucleus or germinal spot of the central cell has given origin to a 

 cluster of oval epithelial cells, of which five still adhere to it. 



209. Hence we are probably to regard this primary or basement-mem- 

 brane as a transitional, rather than as a permanent structure ; and to 

 look upon it as furnishing the germs of all the cells, which are developed 

 upon its surface ; as well as serving for the medium, through which they 

 are supplied with nutriment. It must be continually undergoing disin- 

 tegration, therefore, on its free surface : and must be as continually 

 renewed, at the side in relation with the blood-vessels. 



4. Of Simple Isolated Cells, employed in the Organic Functions. 



210. 'The active functions of the Animal body are performed, to a 

 much greater extent than was until lately believed, by the agency of 

 simple isolated cells ; of which every one grows and lives quite inde- 

 pendently of the rest, just as if it were one of the simplest Cellular 

 Plants ( 30) ; but of which all are dependent upon the general nutritive 

 fluid for the materials of their development, imbibing it from the cur- 

 rents that circulate in their neighbourhood. It rnay be said, indeed, 

 that all the Vegetative functions of the body, all the processes of 

 Nutrition and Reproduction, all those operations, in short, which 

 are common to Plants and Animals, are performed in the Animal 

 and Vegetable structures by the very same means, the agency of cells ; 

 and this is true, not only of the healthy actions, but of various morbid 

 operations, in which the unusual development of cells, possessing peculiar 

 endowments, performs a most conspicuous part. Hence it will be neces- 

 sary to enter somewhat at large into the history of* cell-development in 

 the Animal body : and the various modifications under which this process 

 may take place. In fact, a k'nowledge of the Physiology of Cells may 

 be regarded as the foundation of all 'accurate acquaintance with that 

 department of the Science, which relates to the Nutritive and Repro- 

 ductive processes ; and it has a considerable bearing, as we shall see 

 hereafter, upon the history of the purely Animal functions. 



211. The history of the Animal cell, in its simplest form, is essen- 

 tially that of the Vegetable cell of the lowest kind : excepting in so far 

 as it is dependent for its nutriment upon organic compounds previously 

 elaborated, instead of generating these for itself. It lives for itself 

 and by itself ; and is dependent upon nothing but a due supply of nutri- 

 ment, and of the appropriate stimuli, for the continuance of its growth 

 and for the due performance of all its functions, until its term of life be 

 expired. In whatever method it originates (and we shall presently see 



