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tatic tubules. The gland secretes a fluid which forms part of the semen and assists in maintaining the vitality of the spermatozoa.

Semen is a complex fluid, made up of the secretions from the testicles, the vesicula seminales, the prostatic and urethral glands. It is grayishwhite in color, mucilaginous in consistence, of a characteristic odor, and somewhat heavier than water. From half a dram to a dram is ejaculated

at each orgasm.

The Spermatozoa are peculiar anatomical elements, developed within the seminal tubules, and possess the power of spontaneous movement. The spermatozoa consist of a conoidal head and a long, filamentous tail, which is in continuous and active motion; as long as they remain in the vas deferens they are quiescent, but when free to move in the fluid of the vesiculæ seminales become very active.

Origin.—The spermatozoa appear at the age of puberty, and are then constantly formed until an advanced age. They are developed from the nuclei of large, round cells contained in the anterior of the seminal tubules, as many as fifteen to twenty developing in a single cell.

When the spermatozoa are introduced into the vagina, they pass readily into the uterus and through the Fallopian tubes toward the ovaries, where they remain and retain their vitality for a period of from eight to ten days.

Fecundation is the union of the spermatozoa with the ovum during its passage toward the uterus, and usually takes place in the Fallopian tube, just outside of the womb. After floating around the ovum in an active manner, they penetrate the vitelline membrane, pass into the interior of the vitellus, where they lose their vitality, and along with the germinal vesicle entirely disappear.

DEVELOPMENT OF ACCESSORY STRUCTURES. Segmentation of the Vitellus.—After the disappearance of the spermatozoa and the germinal vesicle there remains a transparent, granular, albuminous substance, in the center of which a new nucleus soon appears; this constitutes the parent cells, and is the first stage in the development of the new being.

Following this, the vitellus undergoes segmentation; a constriction appears on the opposite side of the vitellus, which gradually deepens, until the yelk is divided into two segments, each of which has a distinct nucleus and nucleolus; these two segments undergo a further division into four, the four into eight, the eight into others, and so on, until the entire

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vitellus is divided into a great number of cells, each of which contains a nucleus and nucleolus. The peripheral cells of this “mulberry mass then


themselves so as to form a membrane, and as they are subjected to mutual pressure, assume a polyhedral shape, which gives to the membrane a mosaic appear

The central part of the vitellus becomes filled with a clear fluid. A secondary membrane shortly appears within the first, and the two together constitute the external and internal blastodermic membranes.

Germinal Area.–At about this period there is an accumulation of cells at a certain spot upon the surface of the blastodermic membranes which marks the position of the future embryo. This spot, at first circular, soon becomes elongated, and forms the primitive trace, around which is a clear space, the area pellucida, which is itself surrounded by a darker region, the area opaca.

The primitive trace soon disappears, and the area pellucida becomes guitar-shaped ; a new groove, the medullary groove, is now formed, which develops from before backward, and becomes the neural canal.

Blastodermic Membranes.-The embryo, at this period, consists of three layers, viz. : the external and internal blastodermic membranes, and a middle membrane formed by a genesis of cells from their internal surfaces. These layers are known as the epiblast, mesoblast, and hypoblast.

The epiblast gives rise to the central nervous system, the epidermis of the skin and its appendages, and the primitive kidneys.

The mesoblast gives rise to the dermis, muscles, bones, nerves, bloodvessels, sympathetic nervous system, connective tissue, the urinary and reproductive apparatus, and the walls of the alimentary canal.

The hypoblast gives rise to the epithelial lining of the alimentary canal and its glandular appendages, the liver and pancreas, and the epithelium of the respiratory tract.

Dorsal Laminæ.- As development advances, the true medullary groove deepens, and there arise two longitudinal elevations of the epiblast, the dorsal lamine, one on either side of the groove, which grow up, arch over, and unite so as to form a closed tube, the primitive central nervous system.

The Chorda Dorsalis is a cylindrical rod running almost throughout the entire length of the embryo. It is formed by an aggregation of mesoblastic cells, and situated immediately beneath the medullary groove.

Primitive Vertebræ.-On either side of the neural canal the cells of the mesoblast undergo a longitudinal thickening, which develops and ex


tends around the neural canal and the chorda dorsalis, and forms the arches and bodies of the vertebræ. They become divided transversely into foursided segments.

The mesoblast now separates into two layers; the external, joining with the epiblast, forms the somatopleure; the internal, joining with the hypoblast, forms the splanchnopleure; the space between them constituting the pleuro-peritoneal cavity.

Visceral Laminæ.—The walls of the pleuto-peritoneal cavity are formed by a downward prolongation of the somatopleure (the visceral lamine), which, as they extend around in front, pinch off a portion of the yelk sac (formed by the splanchnopleure), which becomes the primitive alimentary canal ; the lower portion, remaining outside of the body cavity, forms the umbilical vesicle, which after a time disappears.

Formation of Fetal Membranes. The amnion appears shortly after the embryo begins to develop, and is formed by folds of the epiblast and external layer of the mesoblast, rising up in front and behind, and on each side; these amniotic folds gradually extend over the back of the embryo to a certain point, where they coalesce, and enclose a cavity, the amniotic cavity. The membranous partition between the folds disappears, and the outer layer recedes and becomes blended with the vitelline membrane, constituting the chorion, the external covering of the embryo.

The Allantois. As the amnion develops, there grows out from the posterior portion of the alimentary canal a pouch, or diverticulum, the allantois, which carries blood-vessels derived from the intestinal circulation. As it gradually enlarges, it becomes more vascular, and inserts itself between the two layers of the amnion, coming into intimate contact with the external layer. Finally, from increased growth, it completely surrounds the embryo, and its edges become fused together.

In the bird, the allantois is a respiratory organ, absorbing oxygen and exhaling carbonic acid; it also absorbs nutritious matter from the interior

of the egg:

Amniotic Fluid.—The amnion, when first formed, is in close contact with the surface of the ovum ; but it soon enlarges, and becomes filled with a clear, transparent fluid, containing albumin, glucose, fatty matters, urea, and inorganic salts. It increases in amount up to the latter period of gestation, when it amounts to about two pints. In the space between the amnion and allantois is a gelatinous material, which is encroached upon and finally disappears as the amnion and allantois come in contact, at about the fifth month.

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