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CHAPTER XXIII*

DEVELOPMENT

Development is the evolution of a germ into a complete organism. The study of the changes in the developing embryo constitutes the science of Embryology; the transformations after the egg life are called metamorphoses, and include growth and repair.

The process of development is a passage from the general to the special, from the simple to the complex, from the homogeneous to the heterogeneous, by a series of differentiations.

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nu n z cb

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FIG. 361.

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Fertilization and segmentation of mammalian ovum: mo, spermatozoön; n, nucleus; nu, nucleolus z, zona radiata; cb, segmenting cell.

first the profounder distinctions, and afterward those more external. That is, the most essential parts appear first. And not only does development tend to make the several organs of an individual more distinct from one another, but also the individual itself more distinguished from other individuals and from the medium in which it lives. With advancing development, the animal, as a rule, acquires a more specific, definite form, and increases in weight and locomotive power. Life is a tendency to individuality.

The first step in development, after fertilization, is the segmentation of the egg, by a process of self-division. In the simplest form, the whole yolk divides into two parts; these again divide repeatedly, making four, eight, sixteen, etc., parts, until the whole yolk is subdivided

* See Appendix.

into very small portions (cells) surrounding a central cavity. This stage is known as the "mulberry mass," or blastula (Fig. 361, c). If the yolk is larger, relatively to the germinal vesicle, the process of division may go on more slowly in one of the two parts of the egg, first formed; or in very large eggs, like those of birds and cuttlefishes, only a small part of the yolk subdivides.

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In some form, the process of segmentation is found in the eggs of all animals, as is also the following stage. This step is the differentiation of the single layer of cells into two parts, one for the body wall, the other for the wall of the digestive tract. In the typical examples, this is accomplished by one part of the wall of the blastula turning in so far as to convert the blastula into a sort of double-walled cup, the gastrula (Fig. 362). One half of the wall of the blastula is now the outer wall of the germ, the other half that of the digestive cavity; the original blastula cavity is now new cavity formed by the infolding is the stomach and its opening is both mouth and vent (Fig. 362). Some adult animals are little more than such a sac. Hydra (Fig. 18), for instance, is little different from a gastrula with tentacles, and one of its relatives wants even these additions.

FIG. 362.
Diagram of Gastrula
of a Worm (Sagitta): a, prim-
itive mouth; b, primitive di-
gestive cavity; c, primitive
body cavity; en, endoderm ;
ec, ectoderm.

the body cavity, the

Ordinarily, however, development goes much further. From the two original layers arises, in various ways, a third between them, making the three primitive germ layers - epiblast, mesoblast, and hypoblast. This new layer is necessarily in the primitive body cavity, which it may fill up; or usually a new body cavity is formed, in different ways in different groups. In by far the

great majority of animals the digestive tract gets a new opening, which usually becomes the mouth; and the old mouth may close, or serve only the functions of the vent. From this point the development of each group must be traced in detail.

Development of a Hen's Egg. After the segmentation, the germinal disk divides into two layers, between which a third is soon formed. The upper layer (epiblast) gives rise to the epidermis, brain, spinal cord, retina, crystalline lens, and internal ear. From the lower layer (hypoblast) is formed the epithelium of the digestive canal. From the middle layer (mesoblast) come all the other organs muscles, bones, blood vessels, etc. The mesoblast

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FIG. 363.

Transverse vertical sections of an egg, showing progressive stages of development: a, notochord; b, medullary furrow, becoming a closed canal in the last.

thickens so as to form two parallel ridges running lengthwise of the germ, and leaving a groove between them (medullary furrow and ridges),164 The ridges gradually rise, carrying with them the epiblast, incline toward each other, and at last unite along the back. So that we have a tube of epiblast surrounded by mesoblast, which is itself covered by epiblast. This tube becomes the brain and spinal cord, whose central canal, enlarging into the ventricles of the brain, tells the story of its original formation. Beneath the furrow, a delicate cartilaginous thread appears (called notochord) — the predecessor of the backbone. Meanwhile the mesoblast has divided into two layers, except in the middle of the animal, beneath the spinal cord, and in the head. One of these layers remains attached to the epiblast, and

with it forms the body wall; the other bends rapidly downward, carrying the hypoblast with it, and forms the wall of the intestine. The space thus left between the layers of the mesoblast is the body cavity. At the same time, the margin of the germ extends farther and farther over the yolk, till it completely incloses it. So that now we see two cavities a small one, containing

the nervous system; and a larger one below, for the digestive organs. Presently, numerous rows of corpuscles are seen on the middle layer, which are subsequently inclosed, forming a network of capillaries, called the vascular area. A dark spot indicates the situation of the heart, which is the first distinctly bounded cavity

FIG. 364. Rudimentary Hearts, human: 1, venous trunks; 2, auricle; 3, ventricle; 4, bulbus arteriosus.

of the circulatory system. It is a short tube lying lengthwise just behind the head, with a feeble pulsation,

causing the blood to flow backward and forward. The tube is gradually bent together, until it forms a double cavity, resembling the heart of a fish. On the fourth day of incubation partitions begin to grow, dividing the cavities into the right and left auricles and ventricles. The septum between the auricles is the last to be finished, being closed the moment respiration begins. The blood vessels ramify in all directions over the yolk, absorbing its substance, and all performing the same office; it is not till the fourth or fifth day that arteries can be distinguished from veins, by being thicker, and by carrying blood only from the heart. 165

The embryo lies with its face, or ventral surface, toward the yolk, the head and tail curving toward each other. Around the embryo on all sides the epiblast and upper layer of the mesoblast rise like a hood

over the back of the embryo till they form a closed sac, called the amnion. It is filled with a thin liquid, which serves to protect the embryo. Meanwhile, another important organ is forming on the other side. From the

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FIG. 365.- Embryo in a Hen's Egg during the first five days, longitudinal view : A, hypoblast; B, lower layer of mesoblast; C, upper layer of mesoblast and epiblast united, in the last figures forming the amniotic sac; D, vitelline membrane; e, thickened blastoderm, the first rudiment of the dorsal part (in the last figure it marks the place of the lungs); h, heart; a, b, its two chambers; c, aortic arches; m, aorta; i, liver; p, allantois.

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