DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 259 



fourth arch above the origin of the cutaneous artery loses its cavity 

 and becomes a solid cord, so that the fourth or pulmo-cutaneous arch 

 no longer opens into the aorta. 



The condition of the arteries is now that of the adult. 



Our description of these final stages in the formation of the arterial 

 system of the adult agrees exactly with the description given by Boas 

 in his paper quoted above, and it is a source of satisfaction to us 

 that we have been able to confirm the results of so careful a worker. 



We conclude this paper with a summary of the principal con- 

 clusions to which we have been led. 



X. EPITOME OF RESULTS. 



All our results apply to Rana temporaries alone, unless otherwise 

 stated. 



A. Development of the Heart. 



1. The heart developes before any of the vessels of the visceral 



arches, and before the dorsal aortse. 



2. The muscular wall of the heart is formed from the splanchnic 



layer of mesoblast. The inner or endothelial wall of 

 the heart is derived directly from the hypoblast of the 

 ventral wall of the pharynx and of the liver diverticulum. 



3. The heart is from the first in connection at its posterior end 



with the veins of the yolk-sac and of the liver. 



4. The heart becomes twisted on itself, and its several cham- 



bers marked off by constrictions, before its anterior end 

 acquires any connection with blood vessels. 



5. The auricle is at first single, but later becomes divided 



into right and left auricles by a septum growing down 

 from its dorsal wall. 



('). The wall of the ventricle remains thin throughout the whole 

 of development. The apparent thickening which occurs 

 is due to the development of an internal muscular reti- 

 culum. The absence of nutrient vessels in the wall of 

 the ventricle is explained by this arrangement. 



7. The valves of the truncus arteriosus are established before 

 the metamorphosis, 



