20 BALFOUR AND SEDGWIC^. 



development of the head-kidney used to be divided into several 

 pores leading into the body cavity) remained undivided and 

 served as the abdominal aperture of the Miillerian duct. Aves, 

 finally, appear to have become differentiated along a third line ; 

 since in their ancestors the anterior pore of the head-kidney 

 appears to have become specialised as the permanent opening of 

 the Miillerian duct. 



With reference to the posterior position of the head-kidney in 

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

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

 pendent 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 connection 

 with the Miillerian duct. 



We have made a few observations on the development of the 

 Miillerian duct in Lacerta muralis, wliich have unfortunately 

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

 development of the Miillerian duct (the youngest we have met 

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

 racter of the abdominal opening of the Miillerian duct was very 

 similar to that figured by Braun.^ As to the backward growth 

 of the Miillerian duct, we can only state that the sohd 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 Fiirbringer (PI. I, figs. 14-15) in Amphibia. 



^ Loc. cit. 



