262 EMBEYOLOGY OF THE LOWER VERTEBRATES CH. 



(2) Are these elements serially homologous throughout the length 

 of pronephros and mesonephros ? 



From the facts of development as stated earlier in this chapter it 

 is clear what the answer to these two questions must be. It has 

 been shown that in Hypogeopliis and other forms the first tubule to 

 appear in each segment of the opisthouephros arises as a direct out- 

 growth from the nephrotorne exactly in the same way as the pro- 

 nephric tubule : it is clearly then a primary tubule, and its Mal- 

 pighian body, arising directly from the main part of the stalk, is 

 also primary. The evidence then seems conclusive that in Hypo- 

 geopliis the pronephric and opisthonephric tubules form a homologous 

 series, and naturally if this is true of Hypogeopliis it is, in all proba- 

 bility, true of other Vertebrates. 



Yet the view has been strongly advocated and is still held 1 y 

 many rnorphologists that there is no precise homology between the 

 units which build up pronephros and opisthonephros. Riickert, van 

 Wijhe, Field, Semon, Boveri, Felix, have been among the more 

 important protagonists of this view. They have brought forward 

 such arguments as the following : 



(1) While the pronephric tubule arises as an outgrowth of 

 somatic mesoderm, the mesonephric is derived partly from somatic 

 and partly from splanchnic. 



(2) The pronephric tubules arise relatively early and in continuity 

 with the archinephric duct, the mesonephric tubules arise much 

 later and in discontinuity with the duct. 



(3) The glomerulus of the pronephros is unsegmented and lies in 

 the general splanchnocoele : that of the mesonephros is segmental 

 and lies in a special chamber the cavity of the Malpighian body. 



These arguments however do not appear any longer to have the 

 weight which formerly attached to them. 



(1) The evidence of Hypogeopliis that opisthonephric tubules 

 arise as outgrowths of the somatic wall of the nephrotouie just as do 

 the pronephric tubules seems quite convincing. 



(2) In Hy r pogeopliis all the pronephric tubules except the first 

 three join up to the duct secondarily precisely as do the opistho- 

 nephric tubules. Further the precocious completion of the archi- 

 nephric duct is a physiological necessity, in view of the early fund inn- 

 ing of the pronephric tubules, and this in turn involves as a necessary 

 consequence that the tubules belli in 1 those \\hich first function 

 become joined to it secondarily. 



(3) The glomerulus of the pronephros /.s se-inental and the 

 pronephric chain UTS are also segmental at first in some of the more 

 archaic forms and the unsegmented condition is purely secondary. 



Another line of argument is directed not against the view that 

 prone] ihi-os and mesonephros are built, up of serially homologous units 

 DUt rather againfll tin- strict homology of the functional part- of 

 these units. Thus it is stated that in the region of the pr<>nepliros 

 in addition to tin; mam tubules there occur rudiments of other 



