VI.] THE PERMANENT KIDNEYS. 163 



but subsequently hollow, bores its way through the meso- 

 blast, and finally appears to unite on the seventh day 

 with the Wolffian duct close to the entrance of the latter 

 into the cloaca. Later on, this state of things becomes 

 altered; the duct of Miiller opens directly into the cloaca 

 without first uniting with the Wolffian duct. Its opening 

 then lies above that of the Wolffian duct, between it and the 

 opening into the cloaca of the true urinary canal, of which 

 we shall speak directly. 



The anterior extremity of the duct of Miiller which lies 

 about on a level with the fifth protovertebra is never closed 

 in. Here the original furrow remains open, and forms a 

 funnel-shaped opening into the tube from the pleuro- 

 peritoneal cavity. In sections of the sixth day the duct 

 of Miiller is to be seen lying between the duct of the 

 Wolffian body and the pleuroperitoneal cavity. Its diameter 

 is generally smaller than that of the Wolffian duct. 



19. Between the 80th and 100th hour of incubation, the 

 permanent kidneys begin to make their appearance, and as 

 is the case with the Wolffian bodies, the first portion of them 

 to appear is their duct. Near its posterior extremity the 

 Wolffian duct becomes expanded, and from the expanded 

 portion a diverticulum is constricted off which in a trans- 

 verse section lies above the original duct, and the blind end 

 of which points forwards, that is, towards the head of the 

 chick. This is the duct of the permanent kidney or ureter. 

 At first the ureter and the Wolffian duct open by a common 

 trunk into the cloaca, but this state of things lasts for a 

 short time only, and by the sixth day the two ducts have 

 independent openings. 



The earlier state of things was overlooked by Remak, who thus came to give 

 an incorrect account of the origin of the duct of the kidneys. 



Knpffer (Untersuchung uber die Entwickelung des Ham- und GeschlecTits- 

 systems, Archiv fur Microscop. Anat. Vol. n. 1866) was the first to give a 

 correct account of the development of the duct of the permanent kidneys 

 in the chick. His observations have since been confirmed by a number of other 

 observers, including Waldeyer. 



In sections of a somewhat later period the duct of the 

 kidneys can be seen to lie above (dorsal to) the Wolffian 

 duct, and farther from it than the duct of Miiller. 



The formation of the kidneys themselves is very similar 

 to the formation of the Wolffian bodies, 



11 9 



