356 



EMBRYOLOGY 



funnels, in Triton and Salamander by means of two. The whole 

 fundament soon after, during the larval life, acquires ample propor- 

 tions, owing to the fact that the nephridial funnels grow out into long 

 and very tortuous tubes (pronephric canals). (Furbringer, Goette.) 



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\ wr 



In Birds, with which Rep- 

 tiles and Mammals agree, the 

 pronephros appears, much as 

 in Selachians, in a more or less 

 rudimentary form (Sedgwick, 

 Gasser, Benson, Siemerling, 

 Weldon, Mihalkovics). It 

 is first . observable in embryo 

 Chicks having eight primitive 

 segments and in the region of 

 the seventh somite ; in older 

 embryos it is developed from 

 this place backward into the 

 region of the twelfth somite. 

 At the place where the primi- 

 tive segments (fig. 200 P.v) 

 are constricted off from the 

 lateral plate {S.o), but still 

 remain for some time in con- 

 tinuity with it by means of a 

 connecting region (the middle 

 plate), there grows out from 

 the parietal lamella of the 

 middle germ - layer (somato- 

 pleure) a ridge of cells (W.d), 

 which is directed toward the 

 overlying epidermis. Later, 

 like the corresponding furrow 

 in the Amphibia, it becomes 

 detached in places from its 

 parent - tissue, and when, 

 meanwhile, the primitive seg- 

 ments have likewise wholly detached themselves from the lateral 

 plates, it is converted into a longitudinal cord, which is united with 

 the epithelium of the body-cavity by means of short transverse 

 branches. Similar conditions exist in Reptiles and Mammals. 



Finally, the pronephros subsequently acquires a peculiar condition 





