CHAPTER XXXVII. REPRODUCTION. MALE AND FEMALE GENERATIVE ELEMENTS. One of the chief characteristics of living beings is their power of reproduction; that is to say, organisms can, under favorable conditions, form other individuals with lives and habits similar to their own. In the lowest forms of animal life this propagation of species may take place by the division of a single cell: thus an amœba reproduces by the cleavage of its mass of protoplasm, separating the main body into two amabæ. Such a method of reproduction is purely asexual, each individual having the intrinsic power of reproduction. As we ascend the animal scale, we find that, just as other functions are executed by certain specially differentiated groups of cells, so reproduction is performed by certain collections of cells endowed with specific powers. Further, we find that the production of a new being requires the coöperation of two kinds of generative elements, each of which is produced by a distinct organ. In the higher organisms these reproductive elements are produced by different individuals of the same species, thereby dividing them into two sexes. This is termed sexual reproduction. The sexual method of reproduction is met with in all the more highly developed forms of animal and vegetable life. The male organ produces active elements-the spermatozoa; the female organ produces the ovum, which, when fertilized by the spermatozoa, develops embryo. In mammalia the uterus is a most important subsidiary organ, as it becomes modified to allow of the development and growth of the embryo; its earlier functions, however, can be performed by other organs, as seen in cases of extra-uterine fœtation, when the ovum develops in some unusual situation, such as the Fallopian tube or the abdominal cavity. The spermatozoa are formed indirectly from the cells lining the tubuli seminiferi of the testicle. These cells, cubical masses of protoplasm, give rise to others (spermatoblasts), which form another layer and undergo rapid proliferation. The nuclei divide, and from each part arises the head of a spermatozoon, the body being developed from the protoplasm of the cell. The spermatic elements escape into the tubes, and pass down the vasa. deferentia into the vesicula seminales, where they either undergo retrograde change or are cast out of the body. Section of the tubuli seminiferi of a rat. (Schäfer.) a. Tubuli in which the spermatozoa are not fully developed. b. Spermatozoa more developed. c. Spermatozoa fully developed. The ovum arises from the differentiation of a cell from the germ epithelium covering the surface of the ovary. A group of these cells entering the periphery of the ovary, becomes there embedded in a kind of capsule derived from the surrounding areolar tissue of the stroma, and forms an immature Graafian follicle. A central cell grows rapidly to form the ovum, the rest increase in number to form the small cells of the granular tunic. As the follicle develops, it works its way toward the centre of the ovary, and subsequently approaches the periphery of the organ as a fully-developed Graafian follicle. Microscopically, it is seen to be surrounded by a capsule, tunica fibrosa, which is ill-defined from the stroma of the ovary in Section of the ovary of a cat, showing the origin and the development of Graafian a. Germ epithelium. follicles. (Cadiat.) b. Graafian follicle partly developed. c. Earliest form of Graafian follicle. d. Well-developed Graafian follicle. e. Ovum. f. Vitelline membrane. h, i. Small vessels cut across. which it lies. Outside this is a layer of capillary blood vessels, tunica vasculosa, and to these two coats collectively the term tunica propria is applied. Inside the tunica propria are granular cells of small size, which occupy a considerable space in the follicle; they are heaped up at one spot round the ovum, which lies embedded in their midst. These cells receive the name of the tunica granulosa, and their projecting portion, which encircles the ovum, is called the discus proligerus. The remainder of the follicle is filled with a fluid, liquor folliculi. The surface of the ovary is covered by columnar cells, germ epithelium, continuous with the endothelial cells of the peritoneum. When the follicle is fully matured, it lies at the periphery of the ovary beneath this layer of cells, which separates it from the abdominal cavity. MENSTRUATION AND OVULATION. After puberty, at intervals averaging about four weeks, the genital organs of the female become congested, and at the same time a Graafian follicle is ruptured and its contained ovum set free. Coincidently with the rupture of the follicle, the fimbriated extremity of the Fallopian tube becomes closely approximated to the spot where the follicle lies, so that the ovum, instead of falling into the abdominal cavity, passes into the canal of the Fallopian tube, down which it is conveyed to the uterus. The usual place for the ovum to meet the spermatozoa, and to be impregnated, is the Fallopian tube. When the ovum reaches the uterus, if it be unimpregnated, it is cast out with the surface cells of the mucous membrane of the uterus, which are destroyed, and escape along with a sanious fluid. The whole of these phenomena constitute a menstrual act. If, however, the ovum become impregnated, it remains in the Fallopian tube some days, during which time the mucous membrane of the uterus becomes so hypertrophied as to retain the ovum when it reaches that organ. The human ovum is a cell consisting of a mass of protoplasm enclosing a nucleus and nucleolus, and surrounded by a cell wall. On its outer surface is an irregular layer of cells, the remains of that part of the tunica granulosa which encircled the ovum in the Graafian follicle. The cell wall of the ovum is called the vitelline membrane or zona pellucida, and the mass of granular protoplasm it encircles, the vitellus or yolk, and in this is a nucleus the germinal vesicle, which contains a nucleolus-the germinal spot. Beneath the outer covering of calcareous material of the hen's egg there is a white membrane, which encloses a transparent albuminous substance known as the white of egg. Inside this is a yellow fluid mass, the yolk, surrounded by a delicate membrane, vitelline membrane. The yolk is made up of two varieties of material of different shades of color, the white and the yellow yolk. Of these the yellow forms the greater part, the white being arranged in thin layers, which separate the yellow yolk into strata. In the centre of the yolk it forms a flask-shaped mass, with its neck turned to the upper surface, upon which a portion of the yolk rests called the cicatricula. This cicatricula, which lies between the vitelline membrane and the white yolk, is the active growing part of the egg, and out of it is developed the chick and the embryonic membranes. b Extending through the albumin from the vitelline membrane FIG. 267. Ovum. (Robin.) a. Zona pellucida or vitelline membrane. b. Yolk. c. Germinal vesicle or nucleus. d. Germinal spot or nucleolus. e. Interval left by the retraction of the vitellus from the zona pellucida. to the ends of the egg are two twisted membranous cords-the chalaza, which fix and protect the delicate yolk from shocks, but allow it to rotate, so that the cicatricula is always the uppermost part of the yolk when the egg is on its side. The main structural differences between the human ovum and that of a fowl are apparent from the above description: the essential peculiarity of the development of the hen's egg is that only a portion of the yolk is engaged in the formation of the first signs of the chick and its membranes, by far the greater part of the egg, both yolk and albumin, being utilized as nourishment during the subsequent stages of development. |