1878.] of a Rudimentary Head-Kidney in the Embryo Chick, 4 45 



been given of the development of the Miillerian duct in the least 

 resemble what we have described, though the well known and often 

 repeated figure of Waldeyer* (copied in the "Elements of Embryo- 

 logy," fig. 51) very possibly represents a section of the anterior ex- 

 tremity of the duct in an interval between two openings. 



In the above description we have given the more important results 

 of our investigation, and it only remains for us to prove that the 

 peculiarly modified anterior extremity of the Miillerian duct is in reality 

 a rudimentary head-kidney. For this purpose it is necessary for us to 

 show, first, that it is in the situation where we might expect to find a 

 head-kidney ; and secondly, that it resembles a head-kidney in structure. 



It will be convenient to reverse the logical order, and to attempt to 

 prove, in the first instance, the second of these points. We will use as 

 our type the head-kidney in the Amphibia. 



The head-kidney of Amphibian larvaa was discovered by Johannes 

 Muller,t and is often spoken of as the Miillerian body. Its develop- 

 ment has, in recent times, been worked out in a satisfactory manner by 

 W. Muller, J Gr6tte,§ Spengel, ||and Fiirbringer.^ It developes from the 

 anterior extremity of what becomes afterwards its duct. This duct 

 is called by Eiirbringer the duct of the head-kidney, and by one of 

 us** the segmental duct. 



This duct, for which we shall retain the name segmental duct, de- 

 velopes as a groove-like invagination of the epithelium of the body 

 cavity, which soon becomes constricted into a duct, blind behind, but 

 ending in front by a groove in free communication with the body 

 cavity. The open groove at first deepens, still remaining in communication 

 with the body cavity. It next elongates, and by process of unequal con- 

 striction becomes converted into a horizontal canal, retaining its communi- 

 cation with the body cavity by a number of openings, varying according to 

 the species from two to four. 



The part of the duct following on the horizontal canal next assumes 

 an S-shaped curvature, continuous, however, with the segmental duct 

 behind, which somewhat later acquires an opening into the cloaca. We 

 need not follow the further development and final atrophy of the head- 



* " Eierstock u. Ei." Leipzig, 1870. 



t "Ueber die Wolff'schen Korper bei d. Embryonen d. Frosche u. Kroten." 

 " Meckels Archiv," 1829. 



X " Ueber d. Urogenital System der Amphioxus u. d. Cyclostomen." " Jenaische 

 Zeitschrift," Bd. ix. ; 



§ " Entwicklungsgeschichte d. Unke." 



|| " Urogenitalsystem d. Anipbibien. Arbeiten a. d. Zool-zoot. Institut zu Wiirz- 

 burg," Bd. iv. 



% " Zur. Entwicklung der Amphibienniere/' Heidelberg, 1877. " Zur verg. Anat.u. 

 Entwick. d. Excretionsorgane d. Vertebraten." " Morphologiscke Jahrbuch," Bd. vi. 



** Balfour, " Origin and History of Urinogenital Organs of "Vertebrates." 

 " Journal of Anat. and Phys.," vd. x, and " Monograph on Elasmobranch Fisbes." 



