THE VITAL AND PHYSICAL FORCES. 
739 
histological change, nor do they multiply after the ordinary fashion. But here, again, 
we find that these cells are produced within others, whose whole endowment seems 
that of multiplication ; the sperm-cells being generated in vast numbers within folli- 
cles or parent-cells, which have themselves no power of producing spermatic filaments ; 
and each germ-cell, also, being a secondary product of the parent-cell of the ovule, — 
the other cells to which it gives origin (those which fill the embryo-sac in the vege- 
table ovule, and which form the vitellus of the animal unimpregnated ovum) being 
of very inferior character and transient duration. That a relation of reciprocity 
exists between the forces concerned in the growth, development, and maintenance of 
the individual organism, and those which are employed in the generative act, — so 
that an excessive expenditure of either diminishes the amount of vital force which is 
applicable to the other, — is an idea so familiar to physiologists, that the author need 
not here dwell upon it, further than to point out how completely it coincides with, 
and illustrates, the view for which he is contending. 
When we look, moreover, at the tissues which have been developed from the original 
cells by histological transformation, we find that in proportion as they lose the cel- 
lular character, they for the most part cease to perform any strictly vital operation ; 
as if the act of transformation had expended their vital power. We seem to see this 
in the development of tubes from cells, alike in the plant and in the animal, the tubes 
thenceforth serving merely for the conveyance of liquids ; and in the development of 
the simple fibrous tissues of animals, the endowments of these fibres being purely 
physical, and what vital force they may retain serving merely to enable them to resist 
chemical change. When we look at the cells concerned in the production of me- 
chanical movement, we find the same principle holding good in a most remarkable 
manner, these cells being apparently incapable of performing any other function. 
Thus the cells which constitute the fibrillse of striated muscular fibre exercise no 
power of chemical transformation, they undergo no histological change, and they 
appear to be entirely destitute of the power of multiplication; the expenditure of 
their vital force in the act of muscular contraction involves their death and disin- 
tegration ; and their renewal appears to be accomplished by a production of new ceils 
by the continued agency of the parent-cell (or sarcolemma), which, itself possessing 
no contractile power, seems to hold the same relation to the contractile cells of the 
fibrillse, that the parent-cells or follicles of glands hold to the true secreting cells 
occupying their interior. Again, the ciliary action, when the special endowment of 
a particular set of cells — as those lining the excretory ducts of the glands, respiratory 
organs, &c. of higher animals — appears to be in like manner incompatible with any 
other action, but to be the sole manifestation of the vital force of these cells. For 
the ciliated epithelium is never a secreting epithelium ; so that in tracing the one form 
into the other, there seems to be such a marked transition in function (the mode of 
production and the general conditions of development being essentially the same) as 
clearly indicates that the ciliary action and the secreting agency, although very 
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