204 THE FROG chap 



pole attaches itself to weeds (J and L, sk). Just above the 

 sucker a depression — the mouth-pit or stomodceum (L, stdm) 

 — makes its appearance, and is the first indication of the 

 mouth. A similar depression below the tail — the proctodcBum 

 {pcdm) — marks the anus : both at first are mere blind pouches 

 and have no communication with the enteric canal. The 

 head-region is further marked by two pairs of vertical ridges 

 separated by depressions : the ridges are the branchial or 

 gill arches (J, br.ct), and the depressions the branchial clefts. 



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

 becomes distinctly marked off, the tail extends considerably 

 beyond the anus, a thickening appears which marks the 

 position of the eye («), and a depression that of the ear (just 

 above br ^ in the figure) ; and from each branchial arch arises 

 a little tuft, the rudiment of one of the external gills ibr'^, 

 br''). In this condition the tadpole is hatched : it is still 

 unable to feed, the stomodaeum not yet being in com- 

 munication w^ith 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 ecto- 

 derm, endoderm, and mesoderm, are all more or less similar : 

 there are no muscles, no cartilage, no bone, no 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 epidermis, the endoderm cells become the 

 epithelium of the enteric canal, and from offshoots of the 

 enteric cavity — all of course lined by endoderm — are formed 

 the lungs, the liver, the pancreas, and the urinary bladder. 



The mesoderm undergoes much more extensive changes, 

 giving rise to all the connective tissue, cartilage, bone, and 

 muscle, as well as to other parts — i.e., to by far the greater 



