248 COMPARATIVE ANATOMY 



system. The notochord is formed as a median rod of tissue 

 underlying the neural groove, and in the frog is apparently 

 derived from the continuous sheet of mesoblast, and not from 

 the hypoblast. The lateral sheets of mesoblast, after the 

 formation of the notochord, are divided into mesoblastic 

 somites. Meanwhile the embryo has elongated, the anterior 

 end of the neural tube is dilated and divided by constric- 

 tions into a fore, a mid, and a hind brain. The limits of 

 the gut become defined, and the liver is formed as an out- 

 growth from its lower side. The heart is formed in the meso- 

 blast on the ventral side of the anterior end of the gut. The 

 gills are formed from thickenings on the sides of the head, 

 known as the gill-plates, and the sense organs are developed 

 from thickenings known as sense-plates, situated in front of 

 the gill-plates. After these and other changes are completed, 

 the embryo escapes from its gelatinous capsule as an elongate 

 free swimming larval form, destitute of limbs, but provided 

 with a long tail. This larva is the well-known tadpole. 



The tadpole, when hatched, has no mouth, but lives for 

 some days on the still abundant remains of the food -yolk 

 stored in the floor of the gut. It has a sucker on the lower 

 side of the head, by which it attaches itself to weeds and 

 other objects, and it breathes by three pairs of tuft-like 

 external gills situated on the sides of the neck. It is 

 interesting to note, in this connection, that the embryo dog- 

 fish is similarly furnished with tufted external gills. In the 

 space of a few days a mouth is formed ; it is provided with 

 a fringe-like upper and lower lip, and is furnished with a pair 

 of horny jaws, which are eventually cast off, and are not to 

 be confounded with the jaws of the adult. On the establish- 

 ment of the mouth, the tadpole begins to feed on a purely 

 vegetable diet, and the gut, which was previously short and 

 wide, becomes very long, and is coiled up inside the abdomen 

 like a watch-spring. Posteriorly it opens by the anus, which 

 was established somewhat early in embryonic life. The liver 

 becomes histologically differentiated, and the pancreas is 

 formed. 



While these changes are in progress the gill-plates on either 

 side of the neck are perforated by four slit-like openings, the 

 gill-slits, leading from the pharynx to the exterior. The original 

 external gills shrivel up and are replaced by a new series formed 



