(2995). The structure of the penis is far from being the same in all birds provided with such an organ; it offers, in fact, two types extremely different from each other, whereof the Ostrich and Drake may be taken as examples. The penis of the Ostrich is of a size proportioned to that of the bird. Its form is conical; and a deep, narrow groove runs along its upper surface from the base to the point. The vasa deferentia open into the cloaca opposite to the commencement of the groove; so that the semen flows directly into this furrow. This penis consists, first, of two solid conical bodies, entirely composed of fibrous substance, supported at their base within the sphincter of the cloaca upon its inferior wall. The fibrous cones are placed side by side, but not confounded together; and the right is smaller than the left - no doubt to allow this organ, which never becomes soft as that of quadrupeds, to be more easily folded back into the cloaca. Secondly, of a fibro-vascular body, which constitutes the bulk of the inferior aspect of the penis, and is continued to its extremity. Thirdly, of a cellular portion, capable of erection, placed beneath the skin lining the urethral groove.

This last is doubtless the first appearance of the corpus spongiosum, which in Mammifers completely encloses the canal of the urethra, while the two others represent the corpus cavemosum. The whole apparatus, when not in use, is drawn into the cloaca by two pairs of retractor muscles.

* Lemons d'Anat. Comp. torn. v. p. 108.

(2096). In Geese, Ducks, and many wading birds, such as the Stork, the structure of the male intromittent organ is totally different. When in a state of repose, it is lodged in a pouch under the extremity of the rectum, and curved, so as to describe three parts of a circle. When the penis is opened in this condition, it is found to be made up of two portions, each composing half of its substance. The parietes of one half are thick, elastic, and slightly glandular. The other presents internally a great number of transverse grooves and folds. This latter portion during erection unrolls itself outwards like a glove; and at the same time, the half first mentioned, introducing itself into the hollow cylinder formed by the second, fills it up, and constitutes the firmest part of the organ. Most of the grooves and folds visible during non-erection become much less apparent when the penis is protruded; and their direction being oblique, they prevent it from stretching out in a straight line, but oblige it to assume a corkscrew appearance.

A deep groove runs along the whole length of this singular organ; and it is into the commencement of this groove that the vasa deferentla pour the seminal secretion.

(2097). The females of species whose males possess a large penis are provided with a rudimentary clitoris of similar construction.

(2098). The female generative system in the feathered tribes offers a remarkable exception to what we have as yet seen in the vertebrate Ovipara. Instead of being symmetrically developed upon the two sides of the body, the right oviduct, and most frequently the corresponding ovarium, remain permanently atrophied, and, although they do exist in a rudimentary condition, they never arrive at such dimensions as to allow them to assist in the reproductive process.

(2099). The fertile ovarium presents in all essential circumstances the same organization as those of the Reptilia, and is in the same way attached by folds of peritoneum in the vicinity of the spine (fig. 369,f.) The contained ova are found in all stages of maturity; and being connected together by narrow pedicles, the viscus assumes a distinctly racemose appearance.

(2100). The oviduct (fig. 369, d, e) commences by a wide funnel-shaped aperture, and soon assumes the appearance of a convoluted intestine. Its lining membrane varies in texture in different parts: near the infundibular orifice it is thin and smooth; further down it becomes thicker and corrugated; and at last, near the termination of the canal, where the egg is completed by the calcification of its outward covering (g), it presents a villose texture. The oviduct ultimately opens into the corresponding side of the urethro-sexual compartment of the cloaca.

(2101). We must in the next place proceed to describe, with as much brevity as is consistent with the importance of the subject, first, the nidus, or ovisac, in which the rudiment of the future being is produced; secondly, the structure of the germ (ovulum) when it escapes from the ovary; thirdly, the additions made to the ovulum as it passes through the oviduct; and lastly, the phenomena that take place during the development of the embryo by incubation.

(2102). If the ovarium of a bird be examined whilst in functional activity, such of the pedunculated ovisacs (calyces, fig. 369, f) as have within them ovula ripe for exclusion will be found to consist of two membranes*. Of these, the exterior is very vascular, and is surrounded with a pale zone (stigma), occupying the centre of the calyx. The lining membrane of the ovisac, on the contrary, is thin and pellucid, but studded with minute corpuscles, which are probably glandular, or perhaps little plexuses of vessels. Within this ovisac the basis of the future egg (ovulum) is formed.

(2103). The ovulum produced in the ovisac, when mature, is made up of the following parts. The bulk of it consists of an orange-coloured oleaginous material, enclosed in a most delicate and pellucid membrane (membrana vitelli): this is the yelk of the future egg. Upon the surface of the yelk there is visible a slightly-elevated opaque spot (cica-tricula), wherein is lodged the reproductive germ: this last, which is apparently the most important part of the ovulum, is a minute pellucid globule, and has been named, after its discoverer, the "vesicle of Pur-Jcinje," or the germinal vesicle.

Generative apparatus of the Hen.