i i IK DEC LN8 01 mi: lODD 



(ion cannot be brought into uni-on \\ith tin- COnditiODfl of the pro- 

 nephros \\liich have been found in the remainini: and ->priaIly 

 in t lie lo\v-r Vertebrates (Selachians. 'I 1 , -1.-..M-, Ani|i!iil)i:i, nii>U); on 

 tin- other hand allowance is made for all olervation~,. if we sum- 

 marise tin-in a> follows : that tin- pronephros is developed from the 

 'middle plate," and that then it- poxt.-rior end comes into union 

 with tin- outers-mi layer and in conjunction with th- 

 farther backward as tin- pronephric duct. 



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

 RiiCKERT, is correct, then one can d< -situate the pronephric duct at 

 its lii-st appearance as a short canal like pi-rf oration of the wall of 

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

 o.-tia and opens out upon the skin by a single external orifice. 

 < Jri-rinally the outer and inner openings lay near tog ither, later they 

 moved so far apart that the outer opening of (he 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 pon-. 



That openings ^hould arise between the cavities of the body and it - 

 outtr surface is in no way remarkable. I call to mind tin- intestinal 

 tuhe, at various places in tin- territory of which there are formed 

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

 are paa^i-s 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 Medusa-, and tin- < -egmental organ*) 



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

 elimination of the sexual products and the excretions. 



(b) The Afesonep/iros. (Wolffian 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 still more voluminous gland, serving for the se 

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

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

 prom -plii-os is from the beginning only rudimentary, as in the Sela- 

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

 those Vertebrates in which the pronephros attain^ to a t mporary 

 functional activity, as in the Amphibia and Tele 



The mesonephro> i.-- eMahlish, d on the portion of the pronephric 



