XIII. ON THE EXISTENCE OF A HEAD-KIDNEY IN THE 

 EMBRYO CHICK, AND ON CERTAIN POINTS IN THE DE- 

 VELOPMENT OF THE MtJLLERIAN 'DUCT \ By F. M. BAL- 



FOUR and A. SEDGWICK. 



(With Plates 27 and 28.) 



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, corre- 

 spond with the head-kidney, or " Vorniere " of German authors. 

 The second deals with the growth and development of the Miil- 

 lerian duct. With reference to this we have come to the con- 

 clusion that the Miillerian duct does not develop entirely 

 independently of the Wolffian 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 2 , Fiirbinger 3 , and by one of us 4 . 



All recent observers, including Braun 5 for Reptilia, and Egli 6 

 for Mammalia, have stated that the Miillerian duct develops as 



1 From the Quarterly Journal of Microscopical Science, Vol. xix. 1879. 



2 "Das Urogenital-System der Plagiostomen," Arbeiten a. d. zool.-zoot. Institut. 

 Wiirzburg. 



3 " Zur vergl. Anat. u. Entwick. d. Excretionsorgane d. Vertebraten," Morpho- 

 logisches Jahrbuch, Vol. IV. 



4 " On the Origin and History of the Urinogenital Organs of Vertebrates," 

 Journal of Anat. and Pkys., Vol. x. [This Edition No. vn.] 



5 Arbeiten a. d. zool.-zoot. Institut. Wiirzburg, Vol. IV. 



6 Beitr. zur Anat. u. Entwick. d, Geschlechtsorgane, Inaug. Diss., Zurich, 1876. 



