THE OBQANS OF THE MIDDLE GERM-LAYEE. 359 



tion cannot be brought into unison with, the conditions of the pro- 

 nephros which have been found in the remaining and especially 

 in the lower Vertebrates (Selachians, Teleosts, Amphibia, Birds) ; on 

 the other hand allowance is made for all observations, if we sum- 

 marise them as follows : that the pronephros is developed from the 

 " middle plate,'' and that then its posterior end comes into union 

 with the outer germ-layer and in conjunction with the latter grows 

 farther backward as the pronephric duct. 



If this explanation, which has also been expressed by Wijhe and 

 RiJCKERT, is correct, then one can designate the pronephric duct at 

 its first appearance as a short canal-like perforation of the wall of 

 the body, which begins in the body-cavity with one or several inner 

 ostia and opens out upon the skin by a single external orifice. 

 Originally the outer and inner openings lay near together, later they 

 moved so far apart that the outer opening of the canal united with 

 the hind gut. It may be said, in favor of the view here presented, 

 that in the Cyclostomes the more primitive condition, that is to say, 

 the union with the skin, has been preserved. For in them the 

 mesonephric duct opens to the outside at the abdominal pore. 



That openings should arise between the cavities of the body and its 

 outer surface is in no way remarkable. I call to mind the intestinal 

 tube, at various places in the territory of which there are formed 

 openings, as mouth, anus, and branchial clefts. Still more frequent 

 are passages through the body-wall of Invertebrates. As such, arise 

 the openings at the tips of the hollow tentacles of the Actinia, on 

 the ring-canal of the Medusae, and the canals (segmental organs) 

 which in Worms lead out from the body-cavity and serve for the 

 elimination of the sexual products and the excretions, 



(6) The Mesonephros. (WolflSan Body.) 



Following upon the origin of the pronephric system there is de- 

 veloped in all Vertebrates, after the lapse of a longer or shorter 

 interval of time, a stUl more voluminous gland, serving for the secre- 

 tion of urine, the primitive kidney (mesonephros) or Wolffian body. 

 It is developed earlier in those cases in which the fundament of the 

 pronephros is from the beginning only rudimentary, as in the Sela- 

 chians and Amniota ; it appears relatively late, on the contrary, in 

 those Vertebrates in which the pronephros attains to a temporary 

 functional activity, as in the Amphibia and Teleosts. 



The mesonephros is established on the portion of the pronephric 



