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CHAPTER XIII

FACTORS IN ONTOGENY AND EXPERIMENTAL

DEVELOPMENT

Many biologists find their greatest triumph in the doctrine that the living body is a "mere machine," but a machine is a collocation of matter and energy working for an end, not a spinning toy, and when the living machine is compared to the products of human art the legitimate deduction is that it is not merely a spinning eddy in a stream of dead matter and mechanical energy, but a little garden in the physical wilderness.

What the distinction (between vital and nonvital) may mean in ultimate analysis, I know no more than Aristotle or Huxley, nor do I believe that anyone will know until we find out.—BROOKS.

WHILE in the foregoing chapter there is outlined in some detail the general facts and processes and so-called "laws" of ontogenetic development, we purposely omitted any reference to what is known or guessed concerning the causes and control of this development. Only less wonderful than life itself is the unfolding and changing of a single tiny apparently homogeneous speck of life substance (a fertilized egg cell), into a great myriad-celled, extraordinarily heterogeneous, but perfectly organized fully developed plant or animal body. And only second in point of insistence to man's queries about the whence and whither of life itself are his demands to be informed concerning the causes and control of development. It is indeed strongly felt by most biologists that the study of development, that is, the study of the initiating and guiding factors of development, is more likely to reveal to us the basic factors and mechanism of evolution than any other kind of study. It is plain that evolution, its causes and method, are intimately bound up with the general primary phenomena of life, as

assimilation, growth, differentiation, adaptation, heredity, variation, etc., and it is also plain that these fundamental life. phenomena are to be most effectively studied in their relations to the development of individual organisms.

The most casual analysis of development shows that numerous and various influences play their parts in determining its course; it satisfies no one any longer to say that the course and character of an animal's development is determined by heredity. No influence or "force" of heredity can make up in any degree in the case of the development of a chick, for example, for the absence of a proper temperature. This purely external factor of heat is as indispensable to the development of the new chick creature as is the mysterious inherent capacity of the tiny protoplasmic mass to unfold or change so radically that it (and what it adds to itself) may become a peeping chicken. And temperature is but one of a number of other external factors that contribute to the creation of the new chicken, as indeed the inherent capacity of the protoplasm of a hen's egg cell to rearrange itself chickwise and no other wise during development is but one among a number of necessary intrinsic factors whose correlated influence or working is part of the developmental mechanism.

The influences or factors which determine the initiation, course, and outcome of development, then, may be roughly classified into intrinsic and extrinsic factors. And as in our search for rational mechanical explanations of vital phenomena we look on factors as causal, we may use the word "causes" in place of "factors" or "influences" if we like. The intrinsic causes we must believe to be dependent on or incident to the protoplasmic structure of the germ stuff and to be largely the guiding and determining factors in development, while the extrinsic causes are largely such as supply stimulus and energy for the development. Among intrinsic developmental factors are included assimilation, growth, division, differentiation, etc., all constituting what His calls the "law of growth"; under extrinsic factors may be listed heat, light, moisture, food, gravitation, osmosis, etc., composing, according to His, the conditions under which the "law of growth" operates.

In order to understand just what part each one of the various developmental factors or causes plays, there is necessary a most thorough analytical study of development, and an

attempt o determine in measurable or quantitative degree just what specific effects each factor produces. Obviously the most reliable way to effect this analysis and this determination of the specific cause and effect relations is to appeal to experiment. But biology has always been looked on as, and until recently has actually been, almost wholly a science of observation. It is now becoming, in part at least, a science of experiment as chemistry and physies have long been (these are now becoming more and more sciences of calculation, that is, exact sciences like mathematics), and this change and advance-for it is truly an advance when a science formerly relying for its facts on observation begins to base its foundations on the results of experiment-is due primarily to the modern interest and work in the problem of developmental causes. The search for a rational, causomechanical explanation of the complex and at first sight wholly baffling phenomena of development. has been a great stimulus to the bold questioning of many other vital phenomena heretofore looked on as to be explained only by the assumption of a mystic vital force or capacity wholly beyond and foreign to the physicochemical world of matter and force. Mechanism versus vitalism is one of the greatest present-day battles in biology, and nowhere is the struggle keener or are the mechanists more bold in their position than in the particular field of the processes and factors of development. To the mechanists the play of familiar physicochemical forces through the complex and unique structure of germ plasm and living tissues has for result all the extraordinary outcome of developmental course and outcome; to the vitalists this course and outcome are far too complex and purposeful to be explicable without the assumption of an extraphysicochemical force, with a capacity beyond any single or any combination of several physicochemical forces, which they call vitalism.

There is little need of discussing the great mechanism versus vitalism problem here: it is too difficult a subject, and one as yet too little illuminated by known facts, to introduce into any elementary discussion of evolution matters. But it may not be amiss to call the attention of even the most elementary student of evolution and general bionomics phenomena to the obvious fact, that the moment one indulges a penchant for assuming a mystic, extra-physicochemical force

to explain a particularly hard problem, one has simply removed his problem from the realm of scientific investigation. It is no longer a problem. It is explained—that is, it is explained for whoever accepts the vitalistic assumption.

less familiar with as a part

wing energy acting upon,

is the causomechanical

The varying behavior of things in the inorganic world, the functions and capacities of these things, depend on the varying physical and chemical make-up of these things acted upon by the various kinds of energy, such as heat, motion, electricity, and what not, which we are mo of the physicochemical world or better, through varying struc explanation of all the phenon the inorganic world. Should we not in any open-minded consideration of the phenomena in the organic world strongly incline to hold to this same explanation until it is definitely proved incompetent, untenable? Answering the question with a hearty "Yes," the mechanists look first of all in their study and analysis of the so-called vital phenomena to the matter of structure of the vital masses and to the play of energy through the masses, to discover, if possible, a tangible clew to the "mysteries" of the life process. In the study of development, then, we strive first to see and to understand the intimate structure of the germ plasm, this protoplasmic stuff with its wondrous endowment of potentiality.

In Chapter III we have already stated summarily what is known of the chemical and physical make-up of protoplasm. What is actually known, by chemical analysis and earnest microscopic peering, of this structural make-up is wholly insufficient to serve as a satisfactory basis of any causomechanical explanation of protoplasmic properties. Although some of the simpler capacities of protoplasm, as its motion, its taking up of outside substances (feeding), etc., have been to some degree explained by seeing in them direct physicochemical reactions to external stimuli or conditions, practically nothing has been really accomplished as yet toward a mechanical explanation of such more complex or unusual capacities as irritability, assimilation, and reproduction. This last function of protoplasm is in a way its most apparently hopelessly inexplicable property. And this is especially so when the reproduction is of the sort peculiar to the germinal protoplasm; that is, where the reproducing protoplasmic mass does

not simply divide and thus make two masses each capable of the growth and change necessary to make it like the parent mass, but where the parent mass (a fertilized egg cell, or a sexual egg or bud cell) can grow and develop into a highly complex many-celled new organism of type like that from which the parent germ plasm was derived. The special capacities, therefore, of germ plasm have furnished for centuries, and do to-day,

the great problem of biology (next to that provided by the existence of life itself).

If we cling to a belief that in some way, after all, the explanation of the general protoplasmic and special germ plasm capacities lies in an unusual combination of structure and play of familiar form of energy through the structure, we are at once forced to assume a structural make-up of protoplasm and germ plasm beyond the highest powers of our microscopes to detect. And this assumption actually is made. by most biologists. No agreement, however, exists among biologists as to this assumed structure. Biology does not have its atomic theory as chemistry does, to explain the ultramicroscopic make-up of the substances with which it has to deal, but has its atomic theories, a score of fairly well-marked theories as to the ultimate structure of germ plasm having been advanced in the last couple of centuries of biologic study.

FIG. 144.-Egg cell of a sea urchin, Toropneustes lividus, showing cytoplasm, nucleus, and nucleolus, and network or alveolar appearance of the protoplasm. (After Wilson.)

Almost all of these theories assume a micromeric structure of protoplasm; a few are antimicromeric. By micromeric is meant simply that the plasm which appears to us as a viscous colloidal substance, somewhat differentiated into denser and less dense parts, appearing as fibrils or grains or alveoles in a ground substance of different density, is assumed to be composed of myriads of minute, ultramicroscopic units of the general nature of combinations of chemical molecules. These unit combinations are given, in the theories of various authors,

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