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ample may suffice to illustrate this point. It has been known that when the acid contents of the stomach are liberated into the small intestine, the pancreas is stimulated to secrete pancreatic juice. Here is a typical illustration of the coördination, or working together, of different functions to achieve a particular result-the digestion of the food in the intestine. How is the coördination brought about? One might plausibly conjecture that it is effected through the agency of the nervous system, the stimulation of the walls of the intestine evoking a nervous reflex which excites the pancreas to secrete. Bayliss and Starling showed, however, that when all nervous connections between the intestine and pancreas had been cut, the pancreatic secretion nevertheless followed when an acid substance was present in the intestine. This is obviously inconsistent with the theory of a nervous reflex. The same investigators also showed that an extract from the mucous surface of the intestine would excite pancreatic secretion when injected into the blood. They drew the conclusion that the presence of acid in the intestine causes the production of a substance, which, diffusing into the blood and being carried to the pancreas, stimulates that organ to pour out its characteristic secretion. The hormone plays the part of a chemical messenger; and by virtue of its influence the pancreatic juice comes into the intestine just when it is most needed for the work of digestion.

The example of chemical correlation just cited is doubtless duplicated many times over in the integration of bodily activities. Through the agency of the endocrine glands and the nervous system, the manifold activities of the organs of the body are made to work together for the maintenance of the organic whole. These two systems of control have also a close interrelationship. They form what Berman, in his book The Glands Regulating Personality, has felicitously designated as the "interlocking directorate of the body." Much remains to be learned about the interrelations of these organs, but the field of inquiry is one which promises to throw much light upon problems not only of physiology, but of psychology as well.

REFERENCES

BAYLISS, W., General Physiology (4th ed.). London, Longmans, 1924. BURLINGAME, L. L., HEATH, H., MARTIN, E. G., AND PIERCE, G. J., General Biology. N. Y., Holt, 1922.

CALKINS, G. N., Biology. N. Y., Holt, 1917.

DENDY, A., Outlines of Evolutionary Biology (3rd ed.). N. Y., Appleton, 1924.

HEGNER, R. W., An Introduction to Zoology. N. Y., Macmillan, 1910. College Zoology. N. Y., Macmillan, 1912.

HERRICK, C. J., An Introduction to Neurology. Fhiladelphia, Saunders, 1916.

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The Neurological Foundations of Animal Eehavior. N. Y., Holt, 1924.

HERTWIG, R., A Manual of Zoology (transl. Kingsley; 3rd ed.). N. Y., Holt, 1912.

HOWELL, W. H., A Text-Book of Physiology. Philadelphia, Saunders, 1920.

HUXLEY, T. H., The Crayfish. N. Y., Appleton, 1880.

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KINGSLEY, J. S., Vertebrate Zoology. N. Y., Holt, 1899.
Comparative Anatomy. N. Y., Holt, 1912.
MCFARLAND, J., Biology, General and Medical (3rd ed.). Philadelphia,
Saunders, 1918.

NEWMAN, H. H., Vertebrate Zoölogy. N. Y., Macmillan, 1920.

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Outlines of General Zoology. N. Y., Macmillan, 1924. PARKER, G. H., The Elementary Nervous System. Philadelphia, Lippincott, 1919.

PARKER, T. J., AND HASWELL, W. A., Text-Book of Zoology, (3rd ed.).

London, Macmillan, 1922.

SEDGWICK, W. T., AND WILSON, E. B., General Biology. N. Y., Holt, 1895.

SHERRINGTON, C. S., The Integrative Action of the Nervous System. N. Y., Scribner, 1906.

SHULL, A. F., Principles of Animal Biology. N. Y., McGraw-Hill, 1920. WOODRUFF, L. L., Foundations of Biology. N. Y., Macmillan, 1922.

CHAPTER IX

EMBRYONIC DEVELOPMENT

Having given a brief account of some of the chief organ systems and functions of the animal body, we shall now consider the way in which the animal body comes to be formed. Each kind of animal, and plant also, has its own peculiar way of reaching its fully developed state. All higher animals begin their existence as a single cell called the ovum, or egg, which is produced in the ovary of the female. After being fertilized by the minute sperm cell of the male, the ovum proceeds to develop into an animal more or less like its parents. There is perhaps no phenomenon in organic nature more remarkable, or seemingly more miraculous, than the transformation of a single and apparently simple egg cell into the complex and wonderfully organized mechanism of the adult body. Development was formerly regarded by many biologists as due to the simple unfolding and growth of a preformed germ, the embryo being represented in miniature in the egg. Animals were conceived to develop much after the manner of the unfolding of a flower whose component parts are already present in the bud. Development was held to consist not in the formation of new parts, but in the growth of preexisting rudiments. "No part in the animal body was formed before another," the great physiologist Haller declared; "all were created at the same time."

This view led to certain curious logical consequences which some of the extreme preformationists, such as Haller and Bonnet, did not hesitate to accept. If the egg contains the adult form in miniature, it must therefore contain other eggs, which in turn contain still others, and so on until all possible progeny are provided for. This is the so-called box-within-box theory. Some of the expounders of this doctrine calculated that there must

have been 200,000 millions of germs in the ovary of Eve in order to account for her existing descendants. The discovery of the spermatozoön and the part it played in causing the egg to develop, gave rise to the question whether the preformed germ was contained in the sperm or the egg. Naturalists became divided into two rival camps, the ovists and the animalculists, the former contending that the embryo is contained in the egg, the latter that it is contained in the sperm. Some of the animalculists who supplemented their rather imperfect microscopes with a particularly lively imagination, actually figured the head, arms, and legs of a man in the head of a spermatozoön!

Sharply opposed to these preformation theories was the doctrine of epigenesis, championed in 1759 by Caspar Friedrich Wolff in his celebrated Theoria Generationis. Wolff was a diligent student of the development of the chick, and he maintained that at the beginning of development the germ consists merely of simple, unorganized matter which, in consequence of fertilization, gradually takes on a more complex and highly organized form. After the cell theory came to be established in 1838–39, it was soon recognized that the egg is a single cell, and later studies demonstrated that the spermatozoön is also a single cell. The older theories of preformation were definitely set aside, and a much clearer insight was gained into the true nature of embryonic development. It is a remarkable fact that it was only in a relatively recent period that organisms were clearly shown to develop at all. Even after the beginning of the nineteenth century it was still possible for the preformationists to maintain that development is merely an appearance due to our imperfect means of observing the initial stages in the growth of the embryo.

.-m

A human

FIG. 80spermatozoön: h, head; m, middle piece; t, tail. (After Retzius.)

Commonly embryonic development is said to begin with the fertilization of the egg by the sperm cell. Before describing this process, however, it is desirable to give an account of the germ cells and the changes which they undergo preparatory

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to their union. The eggs, or ova, are formed in most animals within special organs called ovaries. They arise from the primordial germ cells, which are much the same in both sexes. After dividing many times, as the body grows and develops, the primitive egg cells, or oògonia, finally enter upon a period of growth. The female germ cell, at this stage, is known as an oöcyte, and when its growth period is completed, it is discharged from the ovary and passes, generally through an oviduct, to the outside. The growth of the oöcyte or, as we may now call it, the unfertilized egg is mainly due to the accumulation of yolk, which serves as a sort of food supply for the developing embryo. In the birds the original egg cell, which corresponds to what we call the yolk of the egg, becomes surrounded by other materials during its passage down the oviduct. First it receives a coating of albumen, the white of the egg, secreted by the glands of the oviduct, and then there is secreted around the whole the calcareous coating which

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a.ch.

FIG. 81-Section through a hen's egg. ach, air chamber; bl, blastoderm; chl, chalaza, the suspensory of the yolk; ism, internal shell membrane; s, shell; sm, external shell membrane; vt, vitelline membrane; w, white or albumen; wy, white yolk; x, layer of fluid albumen; yy, yellow yolk. (From Balfour.)

forms the shell. The egg of a bird consists, therefore, of the ovum proper, plus the surrounding coatings of nutrient and protective materials, with which all eaters of eggs are familiar.

In a great many animals, the eggs have no protective envelopes. In numerous aquatic forms the eggs are discharged into the water to take their chances of destruction or successful development. Among all the mammals, with the exception of the most primitive order of egg-laying monotremes, the eggs, which are very small, develop within the body of the parent. Such forms are called viviparous, a word which means bearing living young,

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