THE ORGANS OF THE MIDDLE GERM-LAYER. 407 



undifferentiated cell-mass (the middle plate or the mesonephric 

 blastema), out of which the mesonephric tubules subsequently- 

 vvhen they become clearly distinguishable appear to have been 

 differentiated. 



15. In a part of the non-amniotic Vertebrates (some Selachians, 

 Amphibians) the primitive kidney remains in open communication 

 with the body-cavity by means of numerous ciliate funnels (nephro- 

 stomes), whereas in all Amniota the mesonephric tubules early 

 surrender their genetically established connection with the body- 

 cavity through the disappearance of the ciliate funnels. 



16. The permanent kidney (metanephros) is the latest to be 

 formed and takes its origin from two separate parts : 



(a) From an evagination of the end of the mesonephric duct, 

 which furnishes the ureters, the pelvis of the kidney, and 

 the straight urinary tubules (in other words, the efferent 

 apparatus) ; 



(&) .From a renal blastema, which represents a backward pro- 

 longation of the mesonephric blastema, has the same 

 origin as the latter, and is converted into the tortuous 

 urinary tubules with the Malpighiaii corpuscles (therefore 

 the secretory part of the kidney). 



17. The fundaments of the kidney, which have arisen far back in 

 the body, rapidly increase in size and undergo a change of position 

 by moving farther forward by the side of the primitive kidneys, 

 whereby the ureter becomes wholly detached from the mesonephric 

 duct and moves to the posterior [dorsal] surface of the allantois, the 

 future urinary bladder. 



18. In the non-amniotic Vertebrates the mesonephros also gives 

 rise by a process of fission to the Miillerian duct, which runs 

 parallel with it. 



19. In the Amniota the relation of the Miillerian duct to the 

 mesonephric duct is still uncertain, because the front end of the former 

 is established by a groove-like depression of the epithelial invest- 

 ment on the lateral face of the mesonephros, while concerning the 

 remaining part it is still undetermined whether it grows backwards 

 independently or is constricted off from the mesonephric duct. 



20. The sexual glands proceed from two fundaments :- 



(a) From a germinal epithelium, a modified part of the epithelium 

 of the body-cavity, located on the median face of the 

 primitive kidney ; 

 From the sexual cords, which grow out toward the germinal 



