16 BALFOUR AND SEDGWICK. 



urino- genital 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 Miillerian 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. 

 Miillerian 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 Miillerian 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 litera- 

 ture on the subject, we may state that we consider that the recent 

 paper of Dr. Gasser^ 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 proto-vertebrse as a slight projection from the intermediate 

 cell mass towards the epiblast in the region of the three hinder- 

 most proto-vertebrse. In the next stage, with eleven proto-ver- 

 tebrse, the solid rudiment of the duct extends from the fifth to the 

 eleventh proto- vertebra, from the eighth to the eleventh proto- 

 vertebra 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 proto-vertebra the Wolffian 

 duct never comes into continuity with the adjacent layers. 



In the region of the fifth proto-vertebra, 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 

 proto-vertebra. In an embryo with fourteen proto-vertebrse 

 the duct extends from the fourth to the fourteenth proto-vertebra, 

 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. 14), 

 * ' Arch, fiir Mic. Anat.,' vol. xiv. 



