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 branchie, and the muscles of the spine 
may just be seen. In figure 2 of the woodcut, I have 
SS 
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 
branchiz, 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 
branchize more distinct ; the fin extends round the tail and 
He 
