94 RANID.E. 



begins to elongate, and a groove, which had previously di- 

 vided the upper part of it into two equal parts, begins to 

 close up. The head becomes prominent, the tail begins to 

 show itself, the little hooks by which it subsequently lays 

 hold of objects and supports itself, begin to appear. At this 

 period the examination becomes more easy, and more inter- 

 esting. At somewhat more than fifty hours in an elevated 

 temperature, the head is very well marked, the tail some- 

 what elongated, and even the rudiment 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 current to the branchial orifice over the 

 rudiments of the branchiee, 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 elon- 

 gated, the branchiae more distinct ; the fin extends round the 

 tail and partly over the back ; and the head is distinguished 

 from the body by a slight contraction at the neck ; and a 

 short period more suffices to bring it to the condition repre- 

 sented in figures 3, 4, of the woodcut, in which the branchiae 

 are seen to consist of two tubercles on each side, as yet sim- 

 ple and undivided; the holders, which are small simple 

 organs placed near the situation of the future mouth, and 

 which serve to enable the little animal to attach itself by 

 means of a viscid secretion at their extremities, have become 

 longer, the web, or fin, is also enlarged. By this time the 

 first voluntary motion of the embryo is discovered on the 

 application of any means of excitement or disturbance ; but 

 it consists only of a slight movement of the head or tail. 

 The nostrils are seen more distinctly, but the mouth is 



