DEVELOPMENT OF THE FROG. 



463 



when the mouth is opened, and the blastopore, which seems 

 to be plugged with yolk, is replaced by a cloacal aperture, 

 four gill-clefts open from the pharynx to the exterior. The 

 external gills shrivel, and are replaced by an internal set, 

 enclosed in a fold of skin forming a gill-chamber. Eventually 

 this gill-chamber is closed, all but a single exhalent aperture 

 on the left side. Through this aperture the water which is 

 taken in by the mouth passes outwards, having washed the 

 gills on its way. 



Here let us summarise the relations of the visceral arches, branchial 

 clefts, and blood-vessels. 



The heart develops before the vessels of the visceral arches, and before 

 the dorsal aorta. The auricle is at first single like that of a fish, but 

 becomes divided by a septum. The dorsal aorta is at first paired. At 

 an early stage in R. temporaria, the course of the circulation is like that 

 of a fish, — from the truncus arteriosus to afferent branchial vessels, thence 

 through the gills to the efferent branchial vessels, and then to the dorsal 

 aorta. But by and by a direct connection is established between the 

 afferent and efferent branchial vessels, so that blood can pass from the 

 heart to the dorsal aorta vifithout going through the gills. At the time 

 of metamorphosis an increasing quantity of blood passes by this short-cut, 

 increased work is thus thrown on the lungs as respiratory organs, and 

 the gills gradually atrophy. (These facts are taken from a research by 

 Professor Milnes Marshall and Edward J. Bles). 



The tadpole thrives on its vegetarian diet, grows rapidly 

 bigger and stronger, swims rapidly by means of its tail. 



