DEVELOPMENT OF FROG 673 



the somatic or outer layer being applied to the ectoderm ; 

 the space between the two layers is the body cavity. 

 The body now becomes distinctly divided into regions, the 

 eyes bud out from the brain, a rudiment of the gills appears, 

 and the larva, still within its gelatinous case, exhibits 

 peculiar lashing movements of the tail. 



Eventually, about a fortnight after the eggs are laid, the 

 larva escapes from the surrounding jelly and swims in the 

 water. At this stage and for some time the ectoderm is 

 ciliated. There is a cloacal opening, but the mouth is 

 not yet more than a dimple. A glandular crescent, often 

 misnamed a sucker, lies on the under surface of the head, 

 and secretes a sticky slime, by means of which the tadpole 

 attaches itself to foreign objects. The protruding gills 

 soon become branched. There are three of them on each 

 side, the first the largest. They are covered with ecto- 

 derm, and are borne on the outside of the first three 

 branchial arches. The mouth, which has previously 

 been merely a blind pit, opens into the gut, the gut itself 

 lengthens rapidly, and becomes coiled like a watch-spring ; 

 the larvae feed eagerly on vegetable matter and increase in 

 size. The glandular crescent forms two small discs, 

 which gradually disappear as the power of locomotion 

 increases. About the time when the mouth is opened, 

 four gill-clefts open from the pharynx to the exterior. 



A second period, the true tadpole stage, now begins. 

 A skin-fold or operculum covers the external gills, which 

 then atrophy, and are replaced by " internal " gills 

 developed on the ventral halves of four branchial arches. 

 These gills, though called internal, are covered with 

 ectoderm like their predecessors, and are comparable 

 not to ordinary fish-gills, but to the external gills of 

 Polypterus, Protopterus, and Lepidosiren. The mouth 

 acquires horny jaws, and the fleshy lips bear horny papilla?. 

 By the continued growth of the opercular fold the gill- 

 chambers are closed, with the exception of a single exhalant 

 aperture on the left side. Through this opening, the 

 water which is taken in by the mouth in respiration passes 

 outwards, having washed the gills on its way. 



In the third period the rudiments of the limbs appear. 

 The fore-limbs are concealed within the gill-chambers, and 



43 



