IN THE EMBRYO CHICK. 641 



ill Aves we have only to remark, that a change in position of 

 the head-kidney might easily take place after it acquired an 

 independent development. The fact that it is slightly behind 

 the glomerulus would seem to indicate, on the one hand, that it 

 has already ceased to be of any functional importance ; and, on 

 the other, that the shifting has been due to its having a connec- 

 tion with the Mullerian duct. 



We have made a few observations on the development of the 

 Mullerian duct in Laccrta mid'alis, which have unfortunately 

 led us to no decided conclusions. In a fairly young stage in 

 the development of the Mullerian duct (the youngest we have 

 met with), no trace of a head-kidney could be observed, but the 

 character of the abdominal opening of the Mullerian duct was 

 very similar to that figured by Braun 1 . As to the backward 

 growth of the Mullerian duct, we can only state that the solid 

 point of the duct in the young stages is in contact with the 

 wall of the Wolffian duct, and the relation between the two is 

 rather like that figured by Furbringer (PI. i, figs. 14 15) in 

 Amphibia. 



DESCRIPTION OF PLATES 27 AND 28. 



COMPLETE LIST OF REFERENCE LETTERS. 



ao. Aorta, cv. Cardinal vein. gl. Glomerulus. g>\. First groove of head- 

 kidney. gr 2 . Second groove of head-kidney. gr y Third groove of head-kidney. 

 ge. Germinal epithelium, mrb. Malpighian body. me. Mesentery. i d. Mullerian 

 duct. r r First ridge of head-kidney. >' 3 . Second ridge of head-kidney. r 3 . Third 

 ridge of head-kidney. W d. Wolffian duct. x. Fold in germinal epithelium. 



PLATE 27. 



SERIES A. Sections through the head-kidney at our second stage. Zeiss 2, ocul. 

 3 (reduced one-third). The second and third grooves are represented with the ridge 

 connecting them, and the rod of cells running backwards for a short distance. 



No. r. Section through the second groove. 



No. 2. Section through the ridge connecting the second and third grooves. 

 No. 3. Section passing through the same ridge at a point nearer the third groove. 

 Nos. 4, 5,6. Sections through the third groove. 



No. 7. Section through the point where the third groove passes into the solid 

 rod of cells. 



1 Loc. at. 



