In front of the urinogenital cloaca there is formed a genital prominence ([fig. 407], cp), with a groove continued from the urinogenital opening; and on each side a genital fold (ls). In the male the sides of the groove on the prominence coalesce together, embracing between them the opening of the urinogenital cloaca; and the prominence itself gives rise to the penis, along which the common urinogenital passage is continued. The two genital folds unite from behind forwards to form the scrotum.
Fig. 407. Diagram of the urinogenital organs of a Mammal at an early stage. (After Allen Thomson; from Quain’s Anatomy.)
The parts are seen chiefly in profile, but the Müllerian and Wolffian ducts are seen from the front.
3. ureter; 4. urinary bladder; 5. urachus; ot. genital ridge (ovary or testis); W. left Wolffian body; x. part at apex from which coni vasculosi are afterwards developed; w. Wolffian duct; m. Müllerian duct; gc. genital cord consisting of Wolffian and Müllerian ducts bound up in a common sheath; i. rectum; ug. urinogenital sinus; cp. elevation which becomes the clitoris or penis; ls. ridge from which the labia majora or scrotum are developed.
In the female the groove on the genital prominence gradually disappears, and the prominence remains as the clitoris, which is therefore the homologue of the penis: the two genital folds form the labia majora. The urethra and vagina open independently into the common urinogenital sinus.
General conclusions and Summary.
Pronephros. Sedgwick has pointed out that the pronephros is always present in types with a larval development, and either absent or imperfectly developed in those types which undergo the greater part of their development within the egg. Thus it is practically absent in the embryos of Elasmobranchii and the Amniota, but present in the larvæ of all other forms.
This coincidence, on the principles already laid down in a previous chapter on larval forms, affords a strong presumption that the pronephros is an ancestral organ; and, coupled with the fact that it is the first part of the excretory system to be developed, and often the sole excretory organ for a considerable period, points to the conclusion that the pronephros and its duct—the segmental duct—are the most primitive parts of the Vertebrate excretory system. This conclusion coincides with that arrived at by Gegenbaur and Fürbringer.
The duct of the pronephros is always developed prior to the gland, and there are two types according to which its development may take place. It may either be formed by the closing in of a continuous groove of the somatic peritoneal epithelium (Amphibia, Teleostei, Lepidosteus), or as a solid knob or rod of cells derived from the somatic mesoblast, which grows backwards between the epiblast and the mesoblast (Petromyzon, Elasmobranchii, and the Amniota).
It is quite certain that the second of these processes is not a true record of the evolution of the duct, and though it is more possible that the process observable in Amphibia and the Teleostei may afford some indications of the manner in which the duct was established, this cannot be regarded as by any means certain.