Biological growth

“ All the activities of organisms which are at work in the transformation of species and the production of new ones in the struggle for existence may be reduced to their physiological functions—to growth, nutrition, adaptation, and heredity ”

“ Pathology, therefore, is really nothing else than modified physiology. Physiological development is the evolution of organs and the growth of organisms under the impression of natural external conditions; pathological development is the evolution of organs and the growth of organisms under the impression of unnatural external conditions. Adaptation of structure to function—of organisms to surrounding media—is the designed conservative purpose of both kinds of growth. ”

“ When the organism is ready to reproduce its kind the germ-cells awaken to activity, usually undergoing a period of multiplication to form more germ-cells before finally passing through a process of what is known at maturation, which makes them ready for fertilization. The maturation process proper, which consists typically of two rapidly succeeding divisions, is preceded by a marked growth in size of the individual cells. ”

“ Indeed, as will be seen at the proper time, the mysteries of daily occurrence, such as growth, budding, and reproduction, are as great as any which beset the passage from form to form, from species to species. No one who has thought of it sufficiently can deny that the ordinary facts of generation, as the necessity of the union of two cells of nearly-spent vitality to produce a third cell, or a brood of cells, endowed with primeval vigor, present difficulties more unsurpassable than the origin of the germs of living things from inorganic matter ”

“ As life developed from simpler forms, each individual animal or plant became more complex, or carried a little further the process of growth. But the method of reproduction of new individuals remained fundamentally the same. Each individual began, like its ancestors, as a single-cell being. By the process of nutrition these single cells in each case would grow, divide, and produce various tissues and organs, but [1264] always repeating the general story of the development of the race. ”

“ The great leading use of this argument is in inducing us to regard the development of the individual animal as the precise equivalent of the series of changes by which the species was developed in the course of geological time. These two kinds of development are distinguished by appropriate names. Ontogenesis is the embryonic development of the individual animal, and is, of course, a short process, depending on the production of a germ by a parent animal or parent pair, and the further growth of this germ in connection more or less with the parent or with provision made by it. ”

“ The individual development, or the ontogenesis of every single organism, from the egg to the complete form is nothing but a growth attended by a series of diverging and progressive changes. This applies equally to animals, plants, and protista. If, for example, we consider the ontogeny of any mammal, of man, of an ape, or of a pouched animal, or if we follow the individual development of any other vertebrate animal of another class, we everywhere find essentially the same phenomena. ”

“ In order the more thoroughly to investigate these matters it is necessary to consider the mode of growth, and specially the plan of evolution or development of each organ. This is the more needful owing to the common origin of things ultimately very different one from the other, and to the presence of organs which, in the adult state, are identical or nearly so in aspect, but which nevertheless are very unlike in the early stages of their existence. ”

“ Because of their extraordinary powers of reproduction they are capable of producing these changes very rapidly and can give rise in a short time to large amounts of the peculiar products of their growth. ”

“ Here, again, we notice that one fertilized ovum gives rise to not one, but a number of liver-flukes.
We must now pass on to consider the growth and development of organisms. Simple growth results from the multiplication of similar cells. As the child, for example, grows, the framework of the body and the several organs increase in size by continuous cell-multiplication. Development is differential growth; and this may be seen either in the organs or parts of an organism or in the cells themselves. As the child grows up into a man, there is a progressive change in his relative proportions. ”

“ In this way originates the first of the vital functions which becomes established in the simplest organism, i.e., nutrition. The environing containable fluids are, then, for the living body of very great simplicity, a veritable chyle entirely prepared by nature.
“Mutilation cannot operate without gradually increasing the consistence of the parts contained within the minute new organism and without extending its dimensions. Hence soon arose the second of the vital functions, growth or internal development.” ”

“ Thus, in all organised beings, there is a process of growth, mere increase in bulk as it were, and a process of evolution or metamorphosis, [Pg xxviii] in accordance with which certain parts assume a different form from the rest, in order the better to fit them for the performance of different offices. Should growth and development be uniform and regular, that is in accordance with what is habitual in any particular species, there is no monstrosity, but if either growth or development be in any way irregular, malformation results. ”

“ Moreover, it is well known that long before this no other term than evolution was used to designate the growth of a single organic individual from the primordial cell and egg to its fully developed form and vital function. ”

“ There seems to be some evidence that we may stunt the growth of a plant or animal by insufficient or unsuitable food, and that all the cells of the body may thereby be reduced in size, the sexual cells among the rest, and that these reduced cells give rise to small progeny in the next generation. ”

“ Q. What is the peculiarity of living matter as to the power of nutrition and growth? A. The non-living increases in size by external additions; but bioplasm selects appropriate material from its food, or pabulum, changes the chemical relations of this material, and appropriates it to its own structure in such a way that it grows from within.
14. Q. What is the peculiarity of bioplasm as to reproduction A. Bioplasm can generate or reproduce its own kind of living matter.
15. Q. What power has a living thing to preserve its own identity? ”

“ The fact that it is possible to propagate these cells of one animal for years in other animals of the same species, without any loss of their vegetative vitality, suggests that this continued growth is kept up by a growth-stimulating substance present in the proper species of animal ”

“ As growth, organization, and reproduction are the functions of physical life, as soon as [Pg 28] the primordial germ began to live, it began to grow, to fashion organs however simple, for its nourishment and increase, and for the reproduction, in some way, of living forms like itself. ”

“ They have lost that plasticity, that freedom of growth, which characterizes the simpler and more primitive forms of life; in their case the power of producing fresh organisms entire from a single fragment, once diffused equally over the whole body, is now confined to certain specialized cells which, in their developed form, we know as seeds or eggs. Yet, even among animals, at a low stage of development, this original power of reproducing the whole from a single part remains inherent in the organism ”

“ In practice, however, it can be successfully applied only to the lower and the higher forms of animal life. On the one hand, we could picture the growth of certain lowly organisms in the border land between the animal and vegetable worlds; on the other, we could portray the development of a child, or even the life changes of a human being from childhood to old age. ”

“ Physiology is also the first science in which we recognise the influence of habit—the tendency of something to happen again, merely because it has happened before. From physiology, too, we get our clearest notion of what is meant by development, or evolution. The growth of a plant or animal from the first germ is the typical specimen of a phenomenon which rules through the whole course of the history of man and society—increase of function, through expansion and differentiation of structure by internal forces. ”

“ We may take it, then, as an established fact, whose full extent and bearings are still awaiting in­ves­ti­ga­tion, that there exist substances both within and without the organism which have a marvellous power of accelerating growth, and of doing so in such a way as to affect not only the size but the form or proportions of the organism. ”

“ And, to begin, let us ask, "How shall we know when physiological development is following its natural course, and when it is not?" There are two striking characteristics which can always here be taken as guides, viz., slowness of organic change, and latency. The physiological growth of tissue is always gradual in its progress, both as regards change of shape, bulk, and variation in physical properties. These go on with an insidious progression that is, for the most part, quite imperceptible to ordinary methods of observation. Natura non facet saltem. ”

“ In the present case, it means that for a certain large group of animals there was established a system of reproduction by the sexual union of male and female, each contributing a cellular substance peculiar to itself, in the formation of a compound cellular substance in which the separate substances are united, and which is to be developed into the fœtus by a law of growth; and as a further design there is wrapped up in the compound germ of each distinct species of animal a typical plan of ultimate form and structure. ”

“ Modern embryology has completely negatived any such view as that of preformation, and as completely established that the evolution is not the unfolding of a miniature germ, but the growth and differentiation of primitively similar cell-elements. In different animals, as might be expected, the manner and course of development are different. ”

“ Every cell in such an aggregate leads a life, which, in a certain limited sense, may be said to be independent; and each discharges its own function in the [51] general economy. Each cell has a period of development, growth, and active life, and each ultimately perishes; the life of the organism not only not depending upon the life of its elemental factors, but actually being kept up by their constant destruction and as constant renewal." [10] Growth, health, and disease are cellular manifestations. ”

“ Now let me call attention to a similar process in the formation of the organism itself. Every organite, and every tissue, has a potential growth of indefinite extent, but its real growth is rigorously limited by the competition and antagonism of the others, each of which has its potential indefiniteness, and its real limits. Something, in the food assimilated, slightly alters the part which assimilates it. ”

“ Very energetic cell proliferation of course is necessary for the transformation of an ovum into the fully developed young organism. This growth, which ceases as soon as the definite organism has been formed and the various organs have been differentiated, we call embryonic growth. ”

“ Every individual organism from the simple plant known as red snow to the oak, from the zoophyte to man is developed from such a cell. This is unquestionable scientific knowledge. The phenomenal process of organic advancement is through growth of the cell by selective appropriation of material, self multiplication of the cell, chemical transformations of the pabulum of the cell, endowment of the muscular and nervous tissues produced by those transformations with vital and psychical properties. ”

“ Thus Part I. deals with organic matter and its activity or metabolism, the action and reaction between organisms and their environment, the correspondence between organisms and their circumstances, and similar general data. Part II. states the big inductions regarding growth, development, adaptation, heredity, variation, and so on. Part III. deals with the arguments suggestive of organic evolution and with the factors in the process. Part IV. is a detailed interpretation of the evolution of organic structure, and Part V. an analogous interpretation of the evolution of functions. ”

“ These are Growth and Development. The mark of growth is that an organism, by assimilation from the outside world, becomes larger than it was. But in development it becomes different from what it was. The history of an embryo in the womb presents a succession of phenomena which, when one comes to realize them, almost stagger thought; for, while remaining the same thing all through, it is continually becoming a different class of thing—first two cells, then one cell, then a fish, a quadruped, ultimately a human being. ”

“ We may say that variation and heredity are only phases of reproduction, and reproduction is a property of the living cell. We may say that this power of reproduction is dependent upon the power of assimilation and growth, for cell division is a result of cell growth. We may further say that growth and assimilation are chemical processes resulting from the oxidation of food, and that thus all of these processes are to be reduced to chemical forces. In this way we may seem to have a chemical foundation for life phenomena. ”

“ While the increase of the protoplasm is going on, we find that there is an advance in the structure, in the differentiation. Now you may recall what I have mentioned earlier in this lecture, the further fundamental fact that the loss in the rate of growth is greatest in the young, least in the old, and that as we go back from old age towards youth, and then into the embryonic period, we find an ever-increasing power of growth, but that it is during the embryonic period that the loss of the power of growth is greatest. ”