DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 223 



between the stomatodasum and pharynx, and the tadpole first com- 

 mences to eat. As might be expected, the exact time at which 

 the perforation is effected is subject to some variation ; but in all 

 tadpoles that we have examined measuring 8| mm. or under, the 

 septum is still intact, while in many 9 mm. specimens, and all 9^ mm. 

 ones, perforation has occurred, as shown in Plate XV., Figs. 11 

 and 12, Z. 



As regards external characters, the tail has increased considerably 

 in length, and the body has become much more slender owing to 

 absorption of the yolk, the diminution in bulk being very apparent 

 when the animal is viewed from the ventral surface. The dorsal fin 

 is large, and extends forwards to about the level of the ear. There 

 is a well-marked rectal papilla, opening at the edge of the ventral 

 fin, the cloacal opening being almost exactly at the middle of the 

 length of the tadpole. The head kidney swellings are still very 

 prominent, and immediately in front of them is the laterally con- 

 stricted neck. 



The head is broad, rounded anteriorly, and widest opposite the gills. 

 The olfactory sacs are conspicuous, their depressions being deeper 

 than at the former stages, and their posterior lips being particularly 

 well marked. Both eye and ear are visible on the surface as small 

 swellings, circular in outline. The mouth is a deep depression on 

 the under surface of the head, a little distance from its anterior end ; 

 it is transversely elongated, and bounded by distinct lips, within 

 which lie the horny jaws. The sucker on the under surface of the 

 head is still single, and of the same shape as before, but is distinctly 

 smaller. 



The external gills of the first and second branchial arches are 

 greatly developed, and about equal in length to the transverse 

 diameter of the head at their base (Fig. 12). In the living animal 

 the gills are carried projecting outwards and backwards from the 

 head at an angle of about 45° with the axis of the body. Each gill has 

 distinct muscles of its own, by means of which the entire gill or its 

 several lobes can be moved freely and independently. Each gill 

 consists of from five to seven main lobes, decreasing in size from 

 above downwards, and each lobe gives off minor lobes along its 

 posterior border. The circulation of the blood in the gills can be well 

 studied in the living animal. Each lobe, and each of its minor lobes, 



