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 amoeba reproduces by the cleavage of its mass of protoplasm, separating the main body into two amoebae. 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 cooperation 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 foetation, 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 vesiculcz seminales, where they either undergo retrograde change or are cast out of the body.
Fig. 265. Section of the tubuli seminiferi of a rat. (Schafer).
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 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.
Fig. 266. Section of the ovary of a cat, showing the origin and the development of Graafian follicles.
a. Germ epithelium.
b. Graafian follicle partly developed.
c. Earliest form of Graafian follicle.
d. Well-developed Graafian follicle.
f. Vitelline membrane.
h, i. Small vessels cut across.
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 around 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 discusproligerus. 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 cell 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.