64 THE BIOLOGY OF THE FROG chap. 



is slightly movable, and if it be pressed upward, the valves 

 of the nostrils become closed, and prevent the passage of air 

 through the nares. Pass a bristle through the nostril and 

 it will be found to emerge into the mouth through one of a 

 pair of rounded openings, the internal nares, situated some- 

 what behind the corresponding external openings. The 

 anus, or opening of the cloaca, lies somewhat dorsal in posi- 

 tion at the posterior end of the body. 



On the upper side of the head, in front of the eyes, there 

 usually occurs a small, light-colored mark, the brow spot. 

 In some specimens this spot may be entirely concealed by 

 pigment ; but in most cases it may be detected, although it 

 is often quite inconspicuous. The brow spot is a feature of 

 considerable interest, from the fact that in the embryonic 

 development of the frog it is in connection with a peculiar 

 outgrowth of the brain known as the epiphysis or pineal 

 gland. When the outer or distal portion of this structure 

 becomes constricted off, after the bones of the skull have 

 developed, it is quite widely separated from the basal 

 portion, which persists in connection with the brain even in 

 the adult frog. The pineal gland is found in almost all ver- 

 tebrates, including man, in whom it was given by certain 

 ancient philosophers the important function of being the 

 seat of the soul. It has been ascertained that this structure 

 is a rudiment of a stalk which formerly connected with a 

 median eye ; in fact, there are certain reptiles (Hatteria) in 

 which this eye is fairly well developed, containing a cornea, 

 lens, retina, coats of pigment, and other structures charac- 

 teristic of a well-developed visual organ ; but in most verte- 

 brates the eye no longer appears, or is represented by the 

 merest rudiment. The connection of the pineal gland with 

 the surface in the development of the frog, and the persis- 

 tence of the brow spot marking the point of this former 



