DEVELOPMENT OF RANA 197 



already seen becomes the pulmonary. The lateral dorsal 

 aorta between the dorsal ends of the carotid and systemic 

 arches (the ductus caroticus) disappears, as also does the 

 connexion between the pulmonary artery and the lateral dorsal 

 aorta (ductus arteriosus, or Botalli). After this change, the 

 organism is perfectly adapted to breathe in air after the manner 

 of land-animals. 



The gills disappear ; the gill-slits close up ; the animal 

 ceases feeding, and the horny teeth drop off. The mouth 

 becomes wider and its angle moves farther back. The tongue 

 develops, and the eyes become more prominent and bulge 

 out from the top of the head. The lateral-line organs disappear 

 and the skin is shed. Glands appear which will keep it moist 

 on land. Internally, great changes take place in the intestine, 

 which loses its watchspring-like coils, and becomes relatively 

 much shorter. This is an adaptation to the carnivorous habits 

 of the frog, for less surface is required for the digestion of a 

 meal of animal food. Lastly, the tail becomes reduced and 

 finally completely absorbed, its debris being ingested by 

 wandering white blood-corpuscles, or phagocytes. 



This astonishing and comparatively rapid change is brought 

 about by the secretion of the thyroid gland, which has been 

 increasing until it reaches a size sufficient to " pull the trigger " 

 of metamorphosis. During the process of change the weight 

 of the body actually decreases, but after coming out on land 

 and recommencing to feed, the size of the young frog increases. 



Literature 



Jenkinson, J. W. Vertebrate Embryology. Oxford, at the Clarendon 

 Press, 191 3. 



Kellicott, W. E. Chordate Development. Henry Holt, New York, 



1913. 

 Morgan, T. H. The Development of the Frog's Egg. Macmillan, New 



York, 1897. 



