330 THE RECAPITULATION THEORY 



ment. 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 

 blood-vessels. Concerning the early development 

 of the blood-vessels, there are considerable differ- 

 ences even between allied species of frogs. In 

 Rana 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 main- 

 tained, is from the heart through the truncus arte- 

 riosus 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 reacquir- 

 ing 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 temporaria the mode of development is 

 very different : the afferent and efferent vessels 

 arise in each arch independently and almost simul- 

 taneously ; the afferent vessel soon acquires 



