128 DEVELOPMENT OF THE FROG 



As the opercular folds develop, the external gills gradually 

 shrivel up, and are replaced functionally hy the internal gills. 

 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. (Gf. Figs. 28, 29, and 32.) The 

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

 growth of a septum from its dorsal wall. 



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

 tion is in all essential repects 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 gUls 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 arterialised blood to all 

 parts of the body. 



1. The Circulation during the time the tadpole is breathing 

 by external gills. 



The arrangement of the bloodvessels, and the course of the 

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

 gills are in full activity, bvit the internal gills have not yet 

 formed, 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, .and AF,, which carry 



