On the Existence of a Head-Kidnet m the Embryo 

 Chick, and on Certain Points in the Development 

 of the MiJLLERiAN Duct. By F. M. Balfour, 

 M.A., Fellow of Trinity College, Cambridge; and 

 Adam Sedgwick, B.A., Scholar of Trinity College, 

 Cambridge. (With Plates I and II.) 



The following paper is divided into three sections. The first 

 of these records the existence of certain structures in the embryo 

 chick, which eventually become in part the abdominal opening 

 of the Miillerian duct, and which, we believe, correspond with the 

 head-kidney, or '^Yorniere^' of German authors. Tlie second 

 deals with the growth and development of the Miillerian duct. 

 With reference to this we have come to the conclusion that the 

 Miillerian duct does not develop entirely independently of the 

 "VYolffian duct. The third section of our paper is of a more 

 general character, and contains a discussion of the rectifications 

 in the views of the homologies of the parts of the excretory 

 system in Aves, necessitated by the results of our investigations. 



We have, as far as possible, avoided entering into the ex- 

 tended literature of the excretory system, since this has been 

 very fully given in three general papers which have recently 

 appeared by Semper,i Mirbinger,^ and by one of us.^ 



All recent observers, including Braun^ for Reptilia, and Egli^ 

 for Mammalia, have stated that the Miillerian duct develops as 

 a groove in the peritoneal epithehum, which is continued back- 

 ward as a primitively solid rod in the space between the Wolfiian 

 duct and peritoneal epithelium. 



^ " Das Urogenital System der Plagiostomen." 'Arbeiten a. d, ZooL- 

 Zoot. Institut. Wiirzburg.' 



- " Zur Vergl. Anat. u. Entwick. d. Excretionsorgane d. Vertebratcii." 

 'Morphologisches Jahrbuch,' vol. iv. 



3 " On the Origin and History of the Urino-genital Organs of Verte- 

 brates." ' Journal of Anat. and Thys.,' vol. x. 



* ' Arbeiten a. d. zool.-zoot. Institnt. Wiirzburg,' vol. iv. 



* ' Beitr. zur Anat. u. Entwick. d. Geschlecbtsorgane,' Inaug. Diss., 

 Zurich, 1876. 



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