A SURVEY OF THE LATER EMBRYO OR LARVA 157 



ing external gills. This is called the operculum. On the left side of 

 the head the operculum remains open at its posterior margin, to allow 

 the egress of water. This opening is known as the spiracle. The oper- 

 cular flaps from the two sides fuse ventrally to envelop the gill or 

 opercular chamber within. Surrounding the mouth are a pair of 

 horny jaws and lips, covered by horny rasping papillae. These are 

 derived from the corneum and consist of rows of tooth-like horny 

 denticles which are replaced frequently. Cornification begins at the 

 1 1 mm. stage. 



Feeding becomes important as the yolk is being consumed more 

 rapidly. The tadpole begins to use its oral accessories to obtain food, 

 which is almost exclusively vegetation, until after metamorphosis. 

 The intestine, developed from the midgut, is a long, thin, coiled tube 

 having the appearance, through the thin abdominal wall, of a watch- 

 spring. If stretched out straight, this larval gut often measures about 

 nine times the length of the body of the tadpole. 



Metamorphosis. 



Under normal conditions of temperature (i.e., 20°-25° C.) and 

 food supply, the tadpole of Rami pipiens will reach metamorphosis 

 in 75 to 90 days. This period can be extended by keeping the 

 larvae in an environment cooler than normal or it may be shortened 

 by keeping them warmer and feeding them thyroid hormone or dilute 

 iodine which tends to accelerate the changes attending metamorpho- 

 sis. 



There are four major areas of change during metamorphosis. 

 First, the respiratory system, which has already gone through an ex- 

 ternal and an internal gill phase, now changes to a lung type of res- 

 piration. This develops concurrently with the change from an aquatic 

 to a terrestrial environment or habitat, characteristic of amphibia. 

 Second, the horny jaws are lost, the mouth widens, and the gut 

 shortens to about two or three times the length of the body. There 

 are parallel changes in the histology of the gut to take care of the 

 change in diet. Third, the two pairs of legs develop and the tail is 

 lost by regression. The hind legs appear some time before metamor- 

 phosis and the forelimbs are pushed through the opercular membrane 

 just before emergence of the tadpole from the water. Fourth, certain 

 endocrine glands function actively and the definitive gonads appear. 

 There are also those changes v/hich are necessarv in the transforma- 



