54 LABORATORY STUDJES IN ZOOLOGY 



hatching all four meet corresponding cvaginations of the ento- 

 derm of this region thus forming the functional gill slits. The 

 nostrils appear as a pair of external depressions or pits a little 

 above the rudiment of the mouth. It is difficult to make out 

 the beginning of the eyes which are indicated on each side by an 

 ectodermal thickening, above the nasal pits. Soon after this 

 stage, life in the jelly of the egg mass is abandoned. This marks 

 the end of embryonic existence and the beginning of larval life 

 as a free-swimming organism. 



9. Young Larva (Two to Four Days after Hatching).— The 

 features adapting the larva to an aquatic life are easily recognized. 

 Note the vertically flattened tail useful in swimming. The sides 

 of the tail show the zigzag muscle segments. Identify the eyes 

 on the sides of the head, the ventrally located mouth, the two 

 nasal openings cephalad of the mouth, and the external gills which 

 appear as long, branching tufts extending from the side of the 

 "neck region". The gill slits show beneath. This stage in 

 the larval life is of a few days' duration only. The disappearance 

 of the external gills is accompanied by the caudal growth of a 

 flap of skin, the operculum (meaning "cover") beginning in front 

 of the first external gill. This cover fuses along its edges with 

 the integument of the head, thus conceaUng the region of the 

 gill slits along with the surface of the "shoulder region" from 

 which the arms are formed. The edges of the caudally growing 

 opercular flap, throughout its extent, fuses with the surface of 

 the body excepting at one place on the left side where it remains 

 open as the spiracle. The external gills are thus covered over 

 and disappear. Internal gills formed within the margins of the 

 gill shts become the functional respiratory organs. With the loss 

 of the external gills and the completion of the tail crest the early 

 larva enters the tadpole stage. The true tadpole stage endures 

 until development is completed in which state the larva is said 

 to be mature. During the tadpole stage the legs may be seen 

 growing from the base of the tail. The arms, however, are not 

 to be found upon the exposed surface. If a slit were made 

 through the spiracle and thence cephalad across the opercular 

 flap the relations of the spiracle and gill slits and the location 

 of the arms would be revealed. In the leopard frog larval 

 maturity is reached from 60 to 80 days after the eggs are laid. 

 At this time the larva passes into its third stage, namely, metamor- 

 phosis. That is, there ensues a relatively short period during 



