628 EXISTENCE OF A HEAD-KIDNEY 



structure of the excretory system, but may say that one of 

 us 1 has already, on other grounds, attempted to shew that the 

 abdominal opening of the MUllerian duct in the bird is the 

 homologue of the abdominal opening of the segmental duct in 

 Amphibia, Elasmobranchii, &c., and that we believe that this 

 homology will be admitted by most anatomists. If this homo- 

 logy is admitted, the identity in position of this organ in Aves 

 and Amphibia necessarily follows. The most striking difference 

 between Aves and Amphibia in relation to these structures 

 is the fact that in Aves the anterior pore of the head-kidney 

 remains as the permanent opening of the Mullerian duct, while 

 in Amphibia, the pores of the head-kidney atrophy, and an 

 entirely fresh abdominal opening is formed for the Mullerian 

 duct. 



II. 



The Growth of the Mullerian Duct. 



Although a great variety of views have been expressed by 

 different observers on the growth of the Mullerian duct, it is 

 now fairly generally admitted that it grows in the space between 

 a portion of the thickened germinal epithelium and the Wolffian 

 duct, but quite independently of both of them. Both Braun 

 and Egli, who have specially directed their attention to this 

 point, have for Reptilia and Mammalia fully confirmed the views 

 of previous observers. We were, nevertheless, induced, partly 

 -on account of the a priori difficulties of this view, and partly by 

 certain peculiar appearances which we observed, to undertake 

 the re-examination of this point, and have- found ourselves un- 

 able altogether to accept the general account. We propose first 

 describing, in as matter-of-fact a way as possible, the actual 

 observations we have made, and then stating what conclusions 

 we think may be drawn from these observations. 



We have found it necessary to distinguish three stages in the 

 growth of the Mullerian duct. Our first stage embraces the 



1 Balfour, "Origin and History of Urinogenital Organs of Vertebrates," Journal 

 of Anat. and Phys. Vol. X., and Monograph on Elasmobranch Fishes. [This edition 

 Nos. vii. and x.] 



