368 



EMBRYOLOGY. 



by means of fusion. The agreement here indicated is a not unim- 

 portant ground for my giving preference to the second rather than 

 the first view. 



As far as regards the details of the conditions, they are in the 



Chick according to the investigations of SEDGWICK, which BALFOUK 



has confirmed as follows : the ureter, which has arisen by an evagi- 

 nation from the end of the mesonephric duct, grows into that part of 

 the middle plate which is located at the end of the Wolfnan body in the 

 region of the thirty-first to the thirty-fourth primitive segment. The 

 fundament, however, is not at once and at this place converted into 

 a kidney, but first undergoes, after the ureter has penetrated into it, a 

 very considerable change in position ; to- 

 gether with the ureter it grows forward 

 on the dorsal side of the mesonephric 

 duct farther; it meanwhile gradually 

 enlarges, and begins to show internal 

 differentiation only when it has come 

 into this new position. One then sees 

 that tortuous tubules become more and 

 more distinct in the small-celled mass 

 and that in their walls Malpighian cor- 

 puscles are established. One finds, in 

 addition, that there are evaginated 

 from the end of the ureter separate 

 sacs, which grow out into collecting 

 tubes, and probably later certainty 

 in regard to this has not yet been 

 established join the tortuous tublues which have arisen in the 

 cortical portion of the kidney. 



This voluminous organ, which has soon outstripped the mesonephros 

 in size, is originally composed of individual lobes separated by deep 

 furrows (fig. 208). The lobation is retained permanently" in Reptiles, 

 Birds, and some of the Mammals (Cetacea). In most Mammals, 

 however, it disappears, in Man soon after birth. The surface of the 

 kidney acquires an entirely smooth condition ; the internal structure 

 (Malpighian pyramids) alone points to its composition out of indi- 

 vidual portions, originally also separated externally. 



For the sake of clearness the development of the three regions, 

 pro-, meso-, and metanephros, has been treated as a whole up to this 

 point. Consequently there have been left out of consideration for 

 the time being other processes which are taking place in the vicinity 



Fig. 203. Kidney and suprarenal 

 body of a human embryo at the 

 end of pregnancy. 



nn, Suprarenal body ; n, kidney ; 

 I, lobes of the kidney ; Id, ureter. 



