8 BALFOUR AND SEDGWICK. 



Before passing on to the other parts of this paper it is necessary 

 to say a few words in justification of the comparison we have 

 made between the modified abdominal extremity of the Miillerian 

 duct in the chick and the head-kidney of the Icthyopsida. 



Eor the fullest statement of what is known with reference to the 

 anatomy and development of the head-kidney in the lower types 

 we may refer to Spengel and Fiirbringer.^ We propose ourselves 

 merely giving a sufficient account of the head-kidney in Amphibia 

 (which appears to be the type in which the head-kidney can be 

 most advantageously compared with that in the bird) to bring 

 out the grounds for our determination of the homologies. 



The development of the head-kidney in Amphibia has been 

 fully elucidated by the researches of W. Miiller/ Gotte,^ and 

 Tiirbringer,^ while to the latter we are indebted for a knowledge 

 of the development of the Mullerian duct in Amphibians. The 

 first part of the urino-genital system to develop is the segmental 

 duct (Vornieregang of Fiirbringer), which is formed by a groove- 

 like invagination of the peritoneal epithelium. It becomes con- 

 stricted into a duct first of all in the middle^ but soon in the pos- 

 terior part also. It then forms a duct, ending in front by a groove 

 in free communication with the body cavity, and terminating 

 blindly behind. The open groove in front at first deepens, and 

 then becomes partially constricted into a duct, which elongates 

 and becomes convoluted, but remains in communication with the 

 body cavity by from two to four (according to the species) 

 separate openings. The manner in which the primitive single 

 opening is related to the secondary openings is not fully under- 

 stood. By these changes there is formed out of the primitive 

 groove an anterior glandular body, communicating with the body 

 cavity by several apertures, and a posterior duct, which carries 

 off the secretion of the gland, and which, though bhnd at first, 

 eventually opens into the cloaca. In addition to these parts there 

 is also formed on each side of the mesentery, opposite the peri- 

 toneal openings, a very vascular projection into this part of the 

 body cavity, which is known as the glomerulus of the head- 

 kidney, and which very closely resembles in structure and posi- 

 tion the body to which we have assigned the same name in the 

 chick. 



The primitive segmental duct is at first only the duct for 

 the head-kidney, but on the formation of the posterior parts of 

 tlie kidney (Wolffian body, &c.) it becomes the duct for these 

 also. 



' Loc. cit. 



2 * Jcuaiscbe ZcitscluilV vol. ix, 1875. 

 •' ' Entwickcluiigsgeschichte d. Unke.' 

 ■* Loc. cit, 



