AMPHIBIA 207 



lary plate is clearly defined. At a time when the blastopore is nearly- 

 closed the dorsal parts of the embryo show the broad primitive groove, 

 flanked on both sides by two pairs of medullary folds, inner and outer. 

 The outer folds fade away, but the inner ones arch over the groove 

 and meet in the region of the future neck, the closure proceeding 

 thence backwards. Thus the groove is converted into the neural tube. 

 The anterior part of the tube soon becomes differentiated into the 

 primitive brain, with the three primary brain lobes representing the 

 primordia of the fore, mid, and hind brain. During these changes the 

 embryo has been elongating and before hatching has reached a length 

 nearly three times its breadth. 



3. Larval Period (Fig. 120, 1-10).— At the time of hatching the 

 larva is a somewhat fish-like creature with a fairly long vertically 

 flattened tail. The mouth is ventral in position and is soon surrounded 

 by a chitinous rim or scraper, which is used as a larval organ in scrap- 

 ing off nutritive scum from lily pads, etc. Two pairs of branching 

 external (larval) gills are the first functional respiratory organs. In 

 addition to the external gills, internal gills, homologous with those of 

 adult fishes, are formed and take over the respiratory function for a 

 considerable period. Soon the external gills disappear, and a fold of 

 skin grows backward from in front of their original location, forming 

 an operculum under which he the internal gills. The operculum has 

 but one outlet, a small unpaired spiracle on the left side. Some 

 writers have interpreted this operculum as the equivalent of the 

 atrium of Amphioxus, but the homology has not been fully estab- 

 lished. The hind limbs are the first to appear, closely followed 

 by the fore limbs, which for some time are concealed beneath 

 the operculum. Only in the later stages of larval life are the lungs 

 developed, and as long as the larva uses the gills the lungs remain 

 very small. 



Metamorphosis (Fig. 120, 11-15). — The period of metamorphosis 

 is really a part of the larval period and cannot be sharply marked off 

 from the latter, since the change is a gradual one. Toward the close 

 of the larval period the tail begins to be resorbed and its materials 

 are stored up in the liver. The long, spirally coiled intestine shortens. 

 The mouth loses its chitinous rim and grows much wider. The giUs 

 disappear and the lungs grow rapidly in size and the larva comes 

 frequently to the surface to breathe air. When these changes are 

 complete the animal is no longer a larva, but an adolescent frog. 



