The change from albumen to fibrin is, there fore, the first important step in the process of assimilation. It commences in the absorbent system ; for the chyle is usually found to con tain fibrin, even before it enters the mesenteric glands (as is indicated by its tendency, however feeble, to spontaneous coagulation); and utter it has passed through them, the quantity of fibrin is considerably increased, so that chyle drawn frorn the thoracic duct usually coagulates with tolerable firmness. This process of elabora tion continues in the blood : for the quantity of fibrin it contains is always kept up, in health,' to a certain standard, although there must be a continual withdrawal of it for the nutritive pro cesses, without a corresponding regular supply from the chyle ; and we find it, moreover, un dergoing a sudden and remarkable increase, under the influence of local agencies. The question naturally suggests itself, therefore— what is the cause of this change ? It has been usually attributed to some influence effected upon the albuminous fluid, by the living sur faces over vvhich it is passing; and the increase in the amount of fibrin in the chyle, which is specially noticed after its passage through the mesenteric glands, has been thought due to some peculiar action of the blood that may come into relation with it, through the thin walls of that aipillary plexus, which forms, with the convoluted lacteal tubuli, nearly the whole bulk of those bodies. The writer is inclined to at tribute it, however, to another agency—the vitalizing, influence of certain floating cells, which the chyle and the blood contain ; and the chief points of the evidence of this doctrine will now be set forth.
A comprehensive survey of the vital pro cesses, performed both by plants and animals, enables us to bring together a number of ex amples in which cells are developed in a tem porary manner, growing, arriving at maturity, and then disappearing ; apparently without having performed any particular function. In the albumen of the seed, for instance, this often takes place to a remarkable extent. In the yolk of the egg there is a similar transitory developement of cells, of which several genera tions succeed each other, without any perma nent structure being the result. In the germi nal vesicle, again (according to Dr. Barry.), several annuli of cells are seen to occupy its cavity, when it is prepared for fecundation ; and the oldest and largest of these contain another generation : yet all these disappear by liquefaction, as soon as the two permanent cells begin to be developed in the centre. Further, in the subsequent developement of all the cells which are descended from these, and form the " mulberry mass," the same process is re peated; a great number of temporary cells being produced, only to liquefy again as soon as the two permanent central cells make their appearance. It can scarcely be imagined by the vvell-judging physiologist, that all this cell life comes into existence without some decided purpose; and if we can assign to it an object, the fulfilment of which is consistent with the facts supplied by analogy elsewhere, this may be reasonably considered as having a fair claim to be received as a physiological induction.
In all these instances, and in many more which might be quoted, the crude alimentary materials are being prepared to undergo conver sion into permanent and regularly org,anised structures. The very first union of the inorganic elements into the simplest proximate principles is effected by the cell-life of plants. The change of these principles into the peculiar compounds which form the characteristic secretions of plants, is another result of their cell-life. And there seems equal ground for the belief, that the change of the proximate principles, sug-ar and gum (of which the latter appears to hold the same place in the vegetable economy that albumen does in the animal), into the peculiar glutinous sap, which is found wherever a forma ion of new tissue is taking place, is equally dependent upon the agency of cells. The process is probably commenced in the leaves ; but as the ordinary descending sap, which is the product of their elaboration, is not so re markable for its plasticity as the fluid drawn from certain rapidly growing parts, it seems probable that a local agency takes place in these, analogous to that which we shall be able to trace in certain conditions of the animal economy. Thus, the starchy fluid which is
contained in the ovule, previously to its fecun dation, is probably not in the state in which it can be immediately rendered subservient to the nutrition of the embryo ; and the developement of successive generations of cells, which exert upon it their vitalizing influence, may be rea sonably reg,arded as the means by which the requisite change is effected. Exaetly the same may he said of the albuminous matter con tained in the yolk of the egg, which is certainly not in a condition in which it can be imme diately applied to the purposes of nutrition ; and its conversion may be regarded as com- • mencing with the developement of transitory cells within its own substance, and as being completed by means of the cells forming the inner layer of the germinal membrane, by which it is subsequently taken up and intro duced into the current of blood flowing through the vascular area. A similar purpose is proba bly answered by the transitory cells developed within the germinal vesicle ; and by those which appear at a similar period in the evolu tion of the descendents of the " twin cells " produced in it.
Many other examples of a similar process might be adduced, but they would all lead to the same general conclusion, which harmonizes well with the important principle of general physiology,—that the higher the ade of struc ture ultimately to be attained by any part, and the more permanent its character is destined t be, the longer and more elaborate are the preli minary stages of its evolution. As an instanc of this law, whiCh bears a remarkable analo vvith the facts just recorded, we may advert t the production of a temporary respiratory a paratus in the higher plants and animals, cor responding with that which is permanent i the lower parts of the scale. There are proba bly cases, however, in which cells are ve rapidly called into existence, without that p paratory elaboration of their nutrient materials which we regard as due to the vital operation of a preceding generation. Thus the Bovist gi,ganteunt, a large fungus of the Puff-bal tribe, has been known to increase, in a singl night, from a mere point to the size of a hug gourd, estimated to contain 47,000,000;00 cellules. In such a case it is difficult to su pose that any but the most rapid mode o generating cells can have been in operation and the idea that these could not have bee developed by any such elaborate process a that just alluded to, is borne out by the fact o their extremely transitory character, the decay of such a structure being almost as rapid as its production. The same may be remarked of those fungous growths in the animal body which sprout forth most rapidly. Hence the apparent exception assists in proving the rule. We have thus a class of facts which indicates that the conversion of the chemical compound into the organizable principle—the aplastic into the plastic material—is effected in the parti cular situations where it is most wanted by the vital agency of transitory cell-life ; that is, by the production of cells which are not them selves destined to form an integral part of any permanent structure, but which, after attaining a certain maturity, reproduce themselves and disappear, successive generations thus following one another until the object is accomplished, after which they altogether vanish. We shall now consider another class of facts which seem to indicate that a change of this kind is being continually effected in the nutritious fluids of animals during their circulation through the body, by cells which are either carried about with them, or which are developed for the pur pose in particular situations, as in plants. The former is the more common occurrence ; since the conditions of animal life, usually involving a general movement of the body, require also a constant general reparation of its parts, and an adaptation of the circulating fluid therefore to the wants of the whole fabric.