EXISTENCE OF HEAD-KIDNEY IN THE EMBRYO CHICK. 9 



After the Wolffian bodies have attained to a considerable deve- 

 lopment, the head-kidney undergoes atrophy, and its peritoneal 

 openings become successively closed from before backwards. 

 At this period the formation of the Miillerian duct takes place. 

 It is a solid constriction of the ventral or lateral wall of the 

 segmental duct, which subsequently becomes hollow, and ac- 

 quires an opening into the body cavity quite independent of the 

 openings of the head-hidney . 



The similarity in development and structure between the 

 head-kidney in Amphibia and the body we have identified as such 

 in Aves, is to our minds too striking to be denied. Both consist 

 of two parts — (1) a somewhat convoluted longitudinal canal, 

 with a certain number of peritoneal openings ; (2) a vascular 

 prominence at the root of the mesentery, which forms a glo- 

 merulus. As to the identity in position of the two organs we 

 hope to deal with that more fully in speaking of the general 

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

 has already, on other grounds, attempted to show that the ab- 

 dominal opening of the Miillerian duct in the bird is the homo- 

 logue 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 homology 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 perma- 

 nent opening of the Miillerian duct, w^hile in Amphibia, the 

 pores of the head-kidney atrophy, and an entirely fresh abdo- 

 minal opening is formed for the Miillerian duct. 



II. 



The Growth of the Miillerian Duct. 



Although a great variety of views have been expressed by 

 different observers on the growth of the Miillerian 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 Egh, who have specially directed their attention to this 

 point, have for Eeptilia 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 



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

 'Journal of Anat. and Phys.,' vol. x, and " Monograph on Elasmobranch 

 Fishes." 



