THE RECAPITULATION THEORY 331 



connection with the heart, but unlike R. esculenta, 

 the efferent vessel has no connection with the heart 

 until the gills are about to atrophy. In other 

 words the continuous aortic arch, from heart to 

 aorta, is present in R. esculenta prior to the de- 

 velopment of the gills ; it becomes interrupted 

 while the gills are in functional use, but is re- 

 established when these begin to atrophy. In R. 

 temporaria, on the other hand, there is no con- 

 tinuous aortic arch until the gills begin to atrophy. 

 The difference is an important one, for it is a 

 matter of considerable morphological interest to 

 determine whether the continuous aortic arch is 

 primitive for vertebrates i.e., whether it existed 

 prior to the development of gills. This point could 

 be practically settled if we could decide which of 

 the two frogs, R. esculenta and R. temporaria, has 

 most correctly preserved its ancestral history in 

 this respect. About this there can be little doubt. 

 The development of the vessels in the newts, a less 

 modified group than the frogs, agrees with that of 

 R. esculenta, and interesting confirmation is afforded 

 by a single aberrant specimen of R. temporaria, in 

 which Mr. Bles and myself found the vessels de- 

 veloping after the type of R. esculenta i.e., in which 

 a complete aortic arch was present before the gills 

 were formed. We are therefore justified in con- 

 cluding that as regards the development of the 

 branchial blood-vessels, R. esculenta has retained a 

 primitive ancestral character which is lost in R. 

 temporaria ; and it is interesting to note that were 

 our knowledge of the development of amphibians 



