100 



PROF. AGASSIZ S 



becoming loose, form the blood corpuscles, and 

 are circulated through the system. Here are cells 

 which line (Plate V, fig. A) the inner surface of 

 the cavities of the body, forming the so-called 

 epithelial membranes upon the different organs, 

 (Fig. B } The irregularity of the outlines of some 

 of the forms, with their nuclei and nucleoli, 

 still preserved, may be seen in these different dia- 

 grams, (Fig. C). In some, the cell membrane is 

 contracted, and assumes therefore a simple thread- 

 like shape. There are others (Fig. D) which form 

 a sort of pavement, and preserve their regular nu- 

 cleoli and nuclei. There are some of these cells 

 which have thus been elongated, upon whose 

 broader flat end there are vibrating Cilia formed, 

 which preserve, nevertheless, the nucleus and nu- 

 cleolus within. That these may present different 

 sizes in different layers, you see here (Plate VI, 

 fig. A) in the skin of the Frog, where the external 

 one constitutes the epidermis, and are successively 

 cast from the surface of the skin ; and the lower 

 cells grow successively, and form a new layer, 

 which will be again cast, and so on. How these 

 cells may combine to form a new tissue, is here 

 represented, where the walls of the cells are trans- 

 formed into regular threads, with swellings from 

 distance to distance. Here is another portion 

 (Plate VI, fig. C) which we may consider as an in- 

 tercellular space, with blood corpuscles circulating 

 in it, or becoming a fleshy mass by the fixation of 

 the nuclei, in which the walls (Plate V, fig F) 

 of the cells having united, will form the fibrous 

 portion of the flesh, and in which nuclei remain 

 for a time distinct ; and here they are still more de- 

 veloped, (Plate VI, figs. H, I, J.) The flesh of the 

 young animals is not yet completely fibrous; the 

 elements of the cells constituting those masses 

 are still to be distinguished. 



Now if the cells themselves become loose and 

 move between the spaces and other cells, then we 

 have blood currents without walls, at first. But if 

 there be fluid between the spaces of cells, (Plate 

 VI, fig. C) they will form tubes, and in these tubes 

 blood corpuscles will be circulated. The nervous 

 substance consists still of similar elements, as we 

 perceive here, (PlateVI,fig. D) where the nuclei are 

 separated from the fibrous part of the nerve. Here 

 are other cells, which, from the regular form they 

 have in the beginning, (Plate V, fig. F) have grown 

 into branching ramifications. And these are filled 

 with colored matters. All those soots upon Fishes, 

 particularly the bright spots seen upon Trouts, for 

 instance, are only cells in which there are various 

 colored pigments, usually different sorts of oil of 

 various colors, and the forms of the cells differ 

 widely as you see here. Side by side, you may 

 have cells of different size and of different form. 

 Even in the bone, (Plate VI, fig. F) you may have 

 the same kind of cells, and also in the cartilages, 

 which finally make up the hard parts of the body. 

 There is, however, still a mystery in the manner in 

 which these parts are introduced and carried to 



[PLATE VI MODIFICATIONS or CELLS.] 



the special parts of the body in which they have to 

 remain. That it is the food which supplies the 

 body with every additional particle of substance 

 all must see, from the fact that, by eating, animals 

 as well as men grow and increase the bulk of their 

 various organs. But from ?o uniform food, there 

 are such diversified organs produced, and with 

 such special properties, that we cannot but wonder 

 at the process by which it is made possible ; for in= 

 stance, that at some point of the body we have 

 bones produced from the metamorphoses of food 

 of just those precise substances which are fit to 

 become the peculiar substance of that particular 

 part. How it is, for instance, that the brain is 

 nourished, and that always those parts of the blood 

 which can be transformed into brain substance, 

 are carried in greater proportion into the bead and 

 into the cavity of the skull, than those parts of the 

 blood which form and restore the fleshy massed. 

 It is a common experience, that with the use of 

 the arm, the fleshy mass is shortly increased. One 

 who has not been in the habit of practising the 

 muscles of his arm, if he begin to do so, will in a 

 very few hours feel pain. But after a few weeks, 

 he will notice a very considerable increase in the 

 substance of the flesh which forms the muscular 

 part of the arm. And this is brought about by the 

 accumulation of those particles of the food in dif- 

 ferent parts of the body, which are fit to nourish, 

 them. That every organ has such an assimilating 



