In Elasmobranchii the segmental duct undergoes a more or less similar division. “It becomes longitudinally split into two complete ducts in the female, and one complete duct and parts of a second in the male. The resulting ducts are (1) the Wolffian duct dorsally, which remains continuous with the excretory tubules of the kidney, and ventrally (2) the oviduct or Müllerian duct in the female, and the rudiments of this duct in the male. In the female the formation of these ducts takes place by a nearly solid rod of cells, being gradually split off from the ventral side of all but the foremost part of the original segmental duct, with the short undivided anterior part of which duct it is continuous in front. Into it a very small portion of the lumen of the original segmental duct is perhaps continued. The remainder of the segmental duct (after the loss of its anterior section and the part split off from its ventral side) forms the Wolffian duct. The process of formation of the ducts in the male chiefly differs from that in the female, in the fact of the anterior undivided part of the segmental duct, which forms the front end of the Müllerian duct, being shorter, and in the column of cells with which it is continuous being from the first incomplete.”
It will be seen from the above that the Müllerian duct consists of two distinct parts—an anterior part with the abdominal opening, and a posterior part split off from the segmental duct. This double constitution of the Müllerian duct is of great importance for a proper understanding of what takes place in the Bird.
The Müllerian duct appears therefore to develop in nearly the same manner in the Amphibian and Elasmobranch type, as a solid or nearly solid rod split off from the ventral wall of the segmental duct. But there is one important difference concerning the abdominal opening of the duct. In Amphibia this is a new formation, but in Elasmobranchii it is the original opening of the segmental duct. Although we admit that in a large number of points, including the presence of a head-kidney, the urinogenital organs of Amphibia are formed on a lower type than those of the Elasmobranchii, yet it appears to us that this does not hold good for the development of the Müllerian duct.
The above description will, we trust, be sufficient to render clear our views upon the development of the excretory system in Aves.
In the bird the excretory system consists of the following parts (using the ordinary nomenclature) which are developed in the order below.
1. Wolffian duct. 2. Wolffian body. 3. Head-kidney. 4. Müllerian duct. 5. Permanent kidney and ureter.
About 2 and 5 we shall have nothing to say in the sequel.
We have already in the early part of the paper given an account of the head-kidney and Müllerian duct, but it will be necessary for us to say a few words about the development of the Wolffian duct (so called). Without entering into the somewhat extended literature on the subject, we may state that we consider that the recent paper of Dr Gasser[441] supplies us with the best extant account of the development of the Wolffian duct.
The first trace of it, which he finds, is visible in an embryo with eight protovertebræ as a slight projection from the intermediate cell mass towards the epiblast in the region of the three hindermost protovertebræ. In the next stage, with eleven protovertebræ, the solid rudiment of the duct extends from the fifth to the eleventh protovertebra, from the eighth to the eleventh protovertebra it lies between the epiblast and mesoblast, and is quite distinct from both, and Dr Gasser distinctly states that in its growth backwards from the eighth protovertebra the Wolffian duct never comes into continuity with the adjacent layers.
In the region of the fifth protovertebra, where the duct was originally continuous with the mesoblast, it has now become free, but is still attached in the region of the sixth and to the eighth protovertebra. In an embryo with fourteen protovertebræ the duct extends from the fourth to the fourteenth protovertebra, and is now free between epiblast and mesoblast for its whole extent. It is still for the most part solid though perhaps a small lumen is present in its middle part. In the succeeding stages the lumen of the duct gradually extends backwards and forwards, the duct itself also passes inwards till it acquires its final position close to the peritoneal epithelium; at the same time its hind end elongates till it comes into connection with the cloacal section of the hind-gut. It should be noted that the duct in its backward growth does not appear to come into continuity with the subjacent mesoblast, but behaves in this respect exactly as does the segmental duct in Elasmobranchii (vide note on p. [634]).