130 DEVELOPMENT OF THE FROG 



As the opercular folds develop, the first formed external 

 gills gradually shrivel up, and are replaced by a second set 

 enveloped by the ectoderm on the lower and outer side of 

 the arches. These latter are delicate thin-walled vascular 

 tufts, arranged in a double row along the ventral half of 

 each of the first three branchial arches, and in a single row 

 on the fourth branchial arch. 



The inner borders of the branchial arches are thickened, 

 and produced into processes which unite to form a kind of 

 filtering apparatus, or sieve, through which the water, taken 

 in through the mouth or nose, is strained before being passed 

 over the gills into the branchial cavity and so out. 



K. The Vascular System. 



The heart is at first a straight tube developed in the meso- 

 blast of the ventral wall of the pharynx. This soon lengthens, 

 becomes twisted into an S shape, and divided by transverse 

 constrictions into chambers. (Cf. Figs. 28, 29, and 32.) The 

 auricle is at first single, but later becomes divided by the 

 downgrowth of a septum from its dorsal wall. 



While the tadpole is breathing by means of gills, its circula- 

 tion is in all essential respects that of a fish. The venous blood, 

 returned from the body generally, enters the posterior end of 

 the heart, or sinus venosus : from this it passes into the second 

 or auricular chamber, thence to the ventricle, and from that to 

 the truncus arteriosus. From this latter arise on each side the 

 aortic arches, which carry the venous blood to the gills to be 

 aerated : from the gills the blood is collected by efferent 

 vessels, which unite above the alimentary canal to form the 

 dorsal aorta, which by its branches distributes the arterial ised 

 blood to all parts of the body. 



1. The Circulation during the time the tadpole is breathing 

 by its external gills. 



The arrangement of the bloodvessels, and the course of the 

 circulation in a 6 mm. tadpole, at a time when the external 

 gills are in full activity, is shown in Figs. 31 and 32. 



The truncus arteriosus, on reaching the anterior end of the 

 pericardial cavity, divides at once into right and left branches. 

 Each of these again divides into two, the afferent vessels for the 

 first and second branchial arches, AF X and AF 2 , which carry 



