132 DEVELOPMENT OF THE FROG. 



modification ; but shortly after the metamorphosis the hyoman- 

 dibular cleft, which has at no period opened to the exterior, 

 is stated to widen somewhat and form the Eustachian passage, 

 while the layer of integument closing its outer end becomes the 

 tympanic membrane. There is some reason, however, for thinking 

 that the Eustachian passage developes independently in the frog, 

 and not from the hyomandibular cleft. The columella, which 

 has been described with the skull, is formed still later, (fif. 

 Fig. 11, p. 58). 



Special Sense Organs. During the tadpole stage, while the 

 animal is leading an aquatic life, special sense organs in the 

 form of small epidermal papillae, supplied by branches of the 

 trigeminal and pneumo-gastric nerves, are found arranged in 

 rows along the body, and round the eyes, and in other parts of 

 the head. These are lost at the time of the metamorphosis. 



The mouth of the tadpole is also provided with small rounded 

 papillse, which are probably organs of taste. (See Fig. 30.) 



H. Development of the Alimentary Canal. 



The alimentary canal is developed in three lengths: (1) the 

 mesenteron, which is formed by splitting apart of the yolk cells 

 as described above ; this gives rise to nearly the whole length 

 of the alimentary canal; and from it are developed the gill-slits, 

 the lungs, the thyroid, the liver, the pancreas, and the bladder; 

 as well as the notochord : (2) the stomodseum, which is a 

 pitting-in at the anterior end of the body, from which the 

 mouth and pituitary body are formed : and (3) the proctodseum, 

 which is a similar pitting-in at the hinder end of the body to 

 form the anal or cloacal opening. 



From the mode of their formation it follows that the mesen- 

 teron is lined by hypoblast, and the stomodseum and procto- 

 dseum by epiblast. 



1. The mesenteron. The early development of the mesen- 

 teron has already been described. 



The anterior end of the mesenteron, in the head region, is 

 considerably dilated from the first : and at the hinder end of the 

 embryo a similar, though much smaller, expansion takes place. 

 In this way {cf. Fig. 28), the mass of the food-yolk becomes 

 confined to the ventral portion of the body region, not extend- 

 ing into either the head or the tail. 



The hypoblast, which is a definite layer of cells, at first confined 



