636 EXISTENCE OF A HEAD-KIDNEY 



number of points, including the presence of a head-kidney, the 

 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. 



I. 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 literature on the subject, we may state that 

 we consider that the recent paper of Dr Gasser 1 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-vertebrae as a slight projection from the inter- 

 mediate cell mass towards the epiblast in the region of the three 

 hindermost proto-vertebrse. In the next stage, with eleven 

 proto-vertebrae, 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 meso- 

 blast, 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-ver- 

 tebrae 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 



1 Arch, filr Mic. Anat. Vol. Xiv. 



