162 



THE FKOG. 



open to the exterior. Almost directly after the opening out 

 of the gill-clefts, the internal gills begin to form, as a series 

 of small papilla3 along their margins, ventral to the external 

 gills : the tadpole now begins to breathe in the typical fish-like 

 manner, taking in water at its mouth, and passing it through 

 the gill-clefts, and so over the internal gills, into the opercular 

 cavity, from which it escapes by the opercular spout. 



Fig. 75. — Transverse section through the head of a 12 mm. Tadpole ; the sec- 

 tion passing through the auditory organs, the pharynx and internal gills, 

 the glottis and laryngeal chamber, and the heart, x 40. 



A, aorta. AF, afferent blood-vessel of second branchial arch. BH, hind-brain. 

 BR.1, .2, .3, .4, first, second, third, and fourth branchial arches. CH, notochonl. 

 CP, pericardia] cavity. EA, anterior vertical semicircular canal EP, efferent blood- 

 vessel of second branchial arch. EH, horizontal semicircular canal. ER, recessus 

 vestibuli. EV, vestibule of ear. G-I, internal gills. HC2, second branchial cleft. 

 LC, laryngeal chamber. LT, glottis. LY, lymphatic space. OP, opercular cavity. 

 RA. auricle of heart. RV, ventricle. TP, pharynx. V.4, fourth ventricle. X'. 

 choroid plexus of fourth ventricle. VIII, auditory nerve. 



The internal gills rapidly increase in size, and branch so as 

 to form plumose tufts arranged in a double row along the 

 ventral half of each of the first three branchial arches, and a 



