This mode of development only occurs however in the anterior part of the duct. In the posterior part of its course its growing point lies in a bay formed by the outer walls of the Wolffian duct, but does not become definitely attached to that duct. It seems however possible that, although not actually split off from the walls of the Wolffian duct, it may grow backwards from cells derived from that duct.
The Müllerian duct finally reaches the cloaca though it does not in the female for a long time open into it, and in the male never does so.
The mode of growth of the Müllerian duct in the posterior part of its course will best be understood from the following description quoted from the paper by Sedgwick and myself.
“A few sections before its termination the Müllerian duct appears as a well-defined oval duct lying in contact with the wall of the Wolffian duct on the one hand and the germinal epithelium on the other. Gradually, however, as we pass backwards, the Müllerian duct dilates; the external wall of the Wolffian duct adjoining it becomes greatly thickened and pushed in in its middle part, so as almost to touch the opposite wall of the duct, and so form a bay in which the Müllerian duct lies. As soon as the Müllerian duct has come to lie in this bay its walls lose their previous distinctness of outline, and the cells composing them assume a curious vacuolated appearance. No well-defined line of separation can any longer be traced between the walls of the Wolffian duct and those of the Müllerian, but between the two is a narrow clear space traversed by an irregular network of fibres, in some of the meshes of which nuclei are present.
“The Müllerian duct may be traced in this condition for a considerable number of sections, the peculiar features above described becoming more and more marked as its termination is approached. It continues to dilate and attains a maximum size in the section or so before it disappears. A lumen may be observed in it up to its very end, but is usually irregular in outline and frequently traversed by strands of protoplasm. The Müllerian duct finally terminates quite suddenly, and in the section immediately behind its termination the Wolffian duct assumes its normal appearance, and the part of its outer wall on the level of the Müllerian duct comes into contact with the germinal epithelium.”
Before describing the development of the Müllerian duct in other Amniotic types it will be well to say a few words as to the identifications above adopted. The identification of the duct, usually called the Wolffian duct, with the segmental duct (exclusive of the pronephros) appears to be morphologically justified for the following reasons: (1) that it gives rise to part of the Müllerian duct as well as to the duct of the Wolffian body; behaving in this respect precisely as does the segmental duct of Elasmobranchii and Amphibia. (2) That it serves as the duct for the Wolffian body, before the Müllerian duct originates from it. (3) That it develops in a manner strikingly similar to that of the segmental duct of various lower forms.
With reference to the pronephros it is obvious that the organ identified as such is in many respects similar to the pronephros of the Amphibia. Both consist of a somewhat convoluted longitudinal canal, with a certain number of peritoneal openings.
The main difficulties in the homology are:
(1) the fact that the pronephros in the Bird is not united with the segmental duct;
(2) the fact that it is situated behind the front end of the Wolffian body.
It is to be remembered in connection with the first of these difficulties that in the formation of the Müllerian duct in Elasmobranchii the anterior undivided extremity of the primitive segmental duct, with the peritoneal opening, which probably represents the pronephros, is attached to the Müllerian duct, and not to the Wolffian duct; though in Amphibia the reverse is the case. To explain the discontinuity of the pronephros with the segmental duct it is only necessary to suppose that the segmental duct and pronephros, which in the Ichthyopsida develop as a single formation, develop in the Bird as two independent structures—a far from extravagant supposition, considering that the pronephros in the Bird is undoubtedly quite functionless.
With reference to the posterior position of the pronephros it is only necessary to remark that a change in position might easily take place after the acquirement of an independent development, and that the shifting is probably correlated with a shifting of the abdominal opening of the Müllerian duct.