366 EMBRYOLOGY. 



of BALFOUB, which are also confirmed by others, the epithelium of the 

 already existing Malpighian glomeruli- is the starting-point of a proliferation. 

 Cell-buds grow out from the latter and toward the urinary tubules lying in 

 front of them, with which their blind ends fuse. After this union has been 

 effected they detach their other ends from the parent-tissue. 



Through the development of compound urinary tubules, each 

 of the branches of which is provided with a Malpighian corpuscle, 

 the primitive kidney (mesonephros) acquires a complicated structure. 

 But this is not uniform in all its parts; ordinarily the condition 

 realised in the most of the Vertebrates is this : the anterior part, 

 which afterwards enters into relation with the sexual glands, 

 retains simple tubules, and only the posterior part passes into a 

 more complicated form by the production of secondary and tertiary 

 fundaments. 



The more the mesonephros, with its tortuous tubules and its 



Fig. 207. Diagram of the original condition of the kidney in an embryo Selachian, after BALFOUK. 



pd, Mesonephric duct, which opens into the body-cavity at o, and into the cloaca at the other 

 end ; x, line along which the Miillerian duct (lying below in the diagram) is divided off from 

 the mesonephric (Wolffiau) duct^ s.t, mesonephric (segmental) tubules, which on the one 

 hand open into the body-cavity, on the other into the mesonephric duct. 



further differentiation, increases in volume, the more it becomes 

 delimited from its surroundings and emerges from the wall of 

 the body into the body-cavity as a distinctly differentiated organ, 

 where it forms a protruding band on either side of the mesentery 

 (fig. 210 WK). 



On a cross section one can recognise in the human embryo also 

 (NAGEL) two distinctly separated regions on each urinary tubule (1) 

 a larger one, which begins with the BOWMAN'S capsule and is lined 

 with large epithelial cells containing abundant protoplasm, and (2) 

 a narrower region with small cubical elements. The latter is the 

 collecting tube t which unites with other collecting tubes before it 

 opens into the mesonephric duct ; on the other hand, probably the 

 former region alone has the secretory function, as also it is best 

 developed at the time of the greatest prominence of the Wolffian 

 body. The Malpighian gloineruli, likewise, attain at this time in 

 human embryos a remarkable size (NAGEL). 



