Immediately in front of the clowal aperture is a small conical promi nence, which, inasmuch as it becomes in the female the clitoris and in the male the penis, we may speak of as clitoro-penis (Fig. 122); on its posterior surface is a groove continued into the urino-genital sinus; and on either side of it are prominent folds of skin, h 1, which we may call labio-scrotal folds.
A very little later, in the course of the tenth week, the septum between the urino-genital sinus and the rectum grows downwards so as to reach the surface. We now have no longer a cloaca, but two perfectly distinct apertures (Fig. 123, a and e) , whereof the anterior is the urino-genital and the posterior the anus.
Up to this time the changes are the same in all embryos, but about the end of the tenth week external sexual differences become apparent. In female embryos (Fig. 124) the conical eminence remains small, and becomes the clitoris; the folds of skin surrounding it become the mona veneris in front and the labia majora at the sides, while the smaller folds, bound ing the urino-genital orifice, become the labia minors, or nymplife; the urino-genital canal shortens considerably, so as to bring the aperture of the urethra very close to the surface.
In male embryos (Fig. 125) the conical eminence elongates and becomes the penis, the groove on its posterior surface closing to form the canal of the penis or penial urethra; the folds of skin similarly unite together in the middle line behind the penis and so form the scrotum.
The above changes are usually effected, and the organs mentioned defi nitely established, by the end of the third month, but the processes may be delayed till later.
Concerning the essential organs of reproduction and their ducts there is yet something to be said. The ovary and testis are at`first absolutely indistinguishable from one another, and it is not until about the eighth week that characteristic differences appear between them. In both sexes there is a very close relation between the essential reproductive organs and the Wolffian bodies, which latter, as was noticed in the chapter on the ovary, send off outgrowths from their Malpighian bodies, forming the so-called " tubuliferous tissue," which lies very close beneath the germi nal epithelium.
In the female the "tubuliferous tissue" gradually gels separated by connective tissue from the germinal epithelium; the Wolffian body shrinks considerably, and becomes converted into the parovarium, or epiophoron, called also the organ of Rosenmiiller. The Wolffian duct is usually only recognizable in its upper portion, where it forms part of the parovarium; in ruminants, and sometimes in woman, its middle and lower portions persist as the duct of Gaertner, running in the broad ligament to the uterus. The Mfillerian ducts become in their upper portions the Fallo pian tubes, and in their lower portions unite to form the uterus and vagina. The fusion of the two ducts proceeds from below upwards, and, if it fail to extend as high as usual, may give rise to a double uterus, or even double vagina as well.
In the male the "tubuliferous tissue" becomes directly continuous with the seminal canals of the testis, forming by so doing the efferent canals for the passage of the spermatozoa to the exterior, the Wolffian body and duct becoming converted into the epididymis and vas deferens respectively.
The Mfillerian ducts in the male are of no physiological, importance; their lower united portions form the vesicula prostatica or uterus mascu linus; the middle portions usually disappear, and the upper parts may either disappear or else persist in the neighborhood of the epididymis, and give rise to the " hydatids of Morgagni." The lungs make their first appearance as a hollow median diverticulum of the ventral wall, of the oesophagus, just behind the gill clefts; the diverticulum consisting of an outer thicker wall of mesoblast, and an inner thinner lining of hypoblast, continuous with that of the alimentary canal.
The diverticulum very soon gives off two lateral outgrowths from its blind end, and so becomes bifid. Its condition at the end of the fourth week is well shown in Fig. 114. Later on, the mesoblast thickens con siderably, and becomes riddled by a number of tubular outgrowths of the hypoblastic lining. These outgrowths become ultimately the bronchi, while their blind ends dilate to form the air-cells, which lie at first close to the surface and 80 give it a granular or tubercular appearance.