they may vary more or less from the type. It is an interesting fact, the full significance of which has not yet been worked out, that, as a rule, plants that vary profusely are those in which the characteristic 2x number of chromosomes is high (60-100).
Brief reference may be made to the cases of abnormal sexual or pseudo-sexual reproduction described above under Apogamy. Taking first the cases of true apogamy, there is clearly no need for any sexual process, for, since no meiotic division has taken place, the gametophyte is diploid; its cells, whether vegetative or contained in female organs, possess the capacity for both development and the transmission of the sporophytic characters. It is not remarkable that such a gametophyte should be able to give rise directly to a sporophyte; but it is remarkable, in the converse case of apospory, that a sporophyte should give rise to a diploid gametophyte rather than to another sporophyte. In the latter case the tendency to the regular development of the alternate form appears to override the influence of the diploid nucleus.
Turning to the various forms of pseudo-apogamy, there are first those in which fusion takes place between two apparently female organs (some Uredineae; Christman), and those in which it takes place between nuclei within the same female organ (Humaria; Blackman). If these are to be regarded physiologically as sexual acts, it must be inferred that the fusing organs or nuclei have come to differ from each other to some extent; for it is unthinkable that equivalent female organs or cells should be able to fertilize, or to be fertilized by, one another. There are finally those cases in which apparently vegetative cells take part in the sexual act, as in Phragmidium (Blackman), where the female organ fuses with an adjacent vegetative cell, and in the fern-prothallium (Farmer), where the nuclei of two vegetative cells fuse. They would seem to indicate that vegetative cells may, in certain circumstances, contain sufficient germ-plasm to act as sexual organs without being differentiated as such.
An interesting question is that of the origin of apogamy. It is no doubt the outcome of sexual degeneration; but this general statement requires some explanation. In certain cases apogamy seems to be the result of the degeneration of the male organ; as in Humaria, where there is no male organ, and in Lachnea, where the male organ is rudimentary. In others, as in the Uredineae, it is apparently the female organ that has degenerated, losing its receptive part, the trichogyne; the male cells (spermatia) are developed normally, and there is no reason to believe that they might not fertilize the female organ were there the means of penetrating it. In yet other cases the degeneration occurs at a different stage in the life-history, in the development of the spores. In the apogamous ferns investigated, meiosis is suppressed and apogamy results. In the heterosporous plants which have been investigated (e.g. Marsilia, Eu-Alchemilla) it has been observed that the microspores are so imperfectly developed as to be incapable of germinating, so that fertilization is impossible; and it is perhaps to this that the occurrence of apogamy is to be attributed. This abnormal development of the spores may be regarded as a variation; and in most cases it occurs in plants that are highly variable and often have a high 2x number of chromosomes.
It will be observed that such physiological explanation as can be given of the phenomena of reproduction is based upon the results of the minute investigation of the changes in nuclear structure associated with them. The explanation is often rather suggested than proved, and some fundamental facts still remain altogether unexplained. But it may be anticipated that a method of research which has already so successfully justified itself will not fail in the future to elucidate what still remains obscure.
Bibliography.—This article should be read in connexion with the following: Algae, Angiosperms, Bryophyta, Cytology, Fungi, Gymnosperms, Heredity, Hybridism, Mendelism, Plants, Pteridophyta.
As the bibliographies to these articles include all the publications containing the facts and theories mentioned here, it will suffice to append only a few papers of general importance: Blackman and Fraser, “Further Studies on the Sexuality of the Uredineae,” Ann. Bot. (1906) vol. xx.; Farmer, “On the Structural Constituents of the Nucleus, and their Relation to the Organization of the Individual” (Croonian Lecture), Proc. Roy. Soc. (1907) vol. 79, series B; Farmer and Digby, “Studies in Apospory and Apogamy in Ferns,” Ann. Bot. (1907) vol. xxi.; Strasburger, Die stofflichen Grundlagen der Vererbung (1905); “Apogamie bei Marsilia,” Flora (1907), vol. 97; D. M. Mottier, Fecundation in Plants (1904), Carnegie Institution, Washington.
REPRODUCTIVE SYSTEM, IN ANATOMY.—The reproductive system in some parts of its course shares structures in common with the urinary system (q.v.). In this article the following structures will be dealt with. In the male the testes, epididymis, vasa deferentia, vesiculae seminales, prostate, penis and urethra. In the female the ovaries, Fallopian tubes, uterus, vagina and vulva.
Male Reproductive Organs.
The testes or testicles are the glands in which the male reproductive cells are formed. They lie, one on each side, in the scrotum surrounded by the tunica vaginalis (see Coelom and Serous Membranes). Each is an oval gland about one and a half inches long with its long axis directed downward, backward and inward. There is a strong fibrous coat called the tunica albuginea, from which vertical and horizontal septa penetrate into the substance, thus dividing it into compartments or lobules in which the seminiferous tubes are coiled. It is estimated that the total length of these seminiferous tubes in the two glands is little short of a mile. (See fig. 1.)
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| From A. F. Dixon, Cunningham's Text-book of Anatomy. | |
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Fig. 1.—Diagram to illustrate the structure of the testis and epididymis. | |
| c.v. | Coni vasculosi. |
| c. | Globus major. |
| g.m'. | Globus minor. |
| r.v. | Rete testis. |
| s. | Septula testis. |
| s.t. | Seminiferous tubule. |
| v.d. | Vas deferens. |
| v.e. | Vas efferens. |
| v.r. | Tubuli recti. |
At the posterior part of the testis the fibrous sheath is greatly thickened to form the mediastinum testis, and contains a plexus of tubules called the rete testis (see fig. 1), into which the seminiferous tubes open. In this way the secretion of the gland is carried to its upper and back part, whence from fifteen to twenty small tubes (vasa efferentia) pass to the epididymis. Each of these is convoluted before opening, and forms what is known as a conus vasculosus.
Under the microscope the seminiferous tubules are seen to consist of a basement membrane surrounding several layers of epithelial cells, some of which are constantly being transformed into spermatozoa or male sexual cells.
The epididymis (see fig. 1) is a soft body lying behind the testis; it is enlarged above to form the globus major or head, while below is a lesser swelling, the globus minor or tail. The whole epididymis is made up of a convoluted tube about 20 ft. long, from which one long diverticulum (vas aberrans) comes off. Between the globus major and the testis two small vesicles called the hydatids of Morgagni are often found.
The vas deferens is the continuation of the tube of the epididymis and starts at the globus minor; at first it is convoluted, but soon becomes straight, and runs up on the inner (mesial) side of the epididymis to the external abdominal ring in the abdominal wall. On its way up it is joined by several other structures, to form the spermatic cord; these are the artery (spermatic) and veins (pampiniform plexus) of the testis, the artery of the vas, the ilio-inguinal, genito-crural and sympathetic nerves, and the testicular lymphatics. After entering the external abdominal ring, these structures pass obliquely through the abdominal wall, lying in the inguinal canal for an inch and a half, until the internal abdominal ring is reached. Here they separate and the vas passes down the side of the pelvis and turns
