PRESIDENTIAL ADDEESS. 27 



view contradicted by the close agreement in their relations in the two 

 groups, as well as by the absence of any real break between external 

 and internal gills in Amphibians. 



It is well known that the frog and the newt differ greatly in 

 important points of their development. The two-layered condition of 

 the epiblast in the frog is a marked point of difference, which involves 

 further changes in the mode of formation of the nervous system and 

 sense organs. The kidneys and their ducts differ considerably in their 

 development in the two forms, as do also the bloodvessels. 



Concerning the early development of the bloodvessels, there are con- 

 siderable differences even between the allied species of frogs. In liana 

 esculenta Maurer finds that there is at first in each branchial arch a 

 single vessel or aortic arch, running directly from the heart to the 

 aorta ; from the cardiac end of this aortic arch a vessel grows out into 

 the gill as the afferent branchial vessel, the original aortic arch losing 

 its connection with the heart, and becoming the efferent branchial 

 vessel. Afferent and efferent branchial vessels become connected by 

 capillaries in the gill, and the course of the circulation, so long as gill- 

 breathing is maintained, is from the heart through the truncus 

 arteriosus to the afferent branchial vessel, then through the gill 

 capillaries to the efferent branchial vessel, and then on to the aorta. 

 When the pulmonary circulation is thoroughly established the branchial 

 circulation is cut off by the efferent vessel reacquiring its connection 

 with the heart, when the blood naturally takes the direct passage along 

 it to the aorta, and so escapes the gill capillaries. 



In Rana temporaries the mode of development is very different : the 

 afferent and efferent vessels arise in each arch independently and 

 almost simultaneously : the afferent vessel soon acquires 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 development 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 continuous 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 



