COMMON FROG. 



99 



ment of the membrane, or web, which is destined to form 

 its fin, is visible. About this time, also, a small projection 

 takes place on each side of the head, which is the earliest 

 indication of the branchiae, and the muscles of the spine 

 may just be seen. In figure 2 of the woodcut, I have 



given an outline of the embryo in this condition. At this 

 time the water is seen to flow in a distinct and rapid cur- 

 rent to the branchial orifice over the rudiments of the 

 branchiae, which are now just visible, the course of which 

 current I have indicated by arrows. These parts gradually 

 become more developed ; the body is more elongated, the 

 branchiae more distinct ; the fin extends round the tail and 



H 2 



