204 THE FROG CHAP. 



first indication of the mouth. A similar depression 

 below the tail the proctodceum (pcdm) marks the 

 anus : both at first are mere blind pouches and have 

 no communication with the enteric cavity. The head- 

 region is further marked by two pairs of vertical ridges 

 separated by depressions : the ridges are the branchial 

 or gill arches (], br. a), and the depressions the branchial 

 clefts. 



Further elongation takes place (Fig. 64, L), the head 

 becomes distinctly marked off, the tail extends consider- 

 ably beyond the anus, a thickening appears which 

 marks the position of the eye (e), and a depression that 

 of the ear (just above br 2 in the figure) ; and from each 

 branchial arch arises a little tuft, the rudiment of one of 

 the external gills (br 1 , br 2 ). In this condition the tad- 

 pole is hatched : it is still unable to feed, the stomodaeum 

 not yet being in communication with the enteric cavity ; 

 it is nourished therefore entirely by the yolk with which 

 a large portion of the body is still filled. 



Up to the stage shown in Fig. 64, I, the cells of 

 which the tadpole is composed, although distinguishable 

 into ectoderm, endoderm, and mesoderm, are all more 

 or less similar : there are no muscles, no cartilages, 

 bones, or connective-tissue. But shortly after the stage 

 referred to, the permanent tissues begin to be formed : 

 the outer ectoderm cells take on the form of epiderm, 

 the endoderm cells become the epithelium of the enteric 

 canal, and from offshoots of the latter all of course 

 lined by endoderm are formed the lungs, the liver, the 

 pancreas, and the urinary bladder. 



The mesodermi undergoes much more extensive changes, 

 giving rise to the connective-tissue, cartilage, bone (after 

 metamorphosis), and muscle, as well as to other parts 

 i.e., to by far the greater portion of the permanent tissues. 

 In certain places the embryonic mesoderm cells, hitherto 



