DEVELOPMENT AND METAMORPHOSIS. 85 



XII. Development and Metamorphosis. 



In the first condition, when the embryo has become 

 distinct from the vitelline sac, the head is large and 

 distinct from the elongate body, the tail absent or 

 rudimentary. The head is usually cleft below by a 

 median longitudinal groove, in the middle of which a 

 transverse or rhomboidal depression represents the 

 first rudiments of the mouth ; on each side, in front 

 of the mouth, a pit indicates the nostril ; behind it is 

 a grooved fold, the cephalic crescent, which develops 

 into a single or paired prominence, the holder — 

 "crochets "of Rusconi — acting as an adhesive apparatus 

 by means of which the helpless embryo fixes itself at 

 first to the outer surface of the mucilaginous envelop 

 of the egg, and later to weeds or submerged objects. 

 Eyes are absent. A small bud-like tubercle on each 

 side of the posterior border of the head is the rudi- 

 ment of the external gills, and vertical grooves in 

 front and behind the bud represent the visceral clefts, 

 the intervals between which will later become converted 

 into the four branchial arches. 



The adhesive apparatus mentioned above varies con- 

 siderably, and its conformation affords the means of 

 distinguishing genera or even species at a period when 

 the tadpole characters are not yet developed. It is 

 mainly through the observations of Heron-Royer and 

 the more scientific researches of Thiele that we have 

 become acquainted with its modifications in most of 

 the European forms. Thiele has shown that the term 

 " sucker," which has been bestowed on this organ by 

 various authors, is a misnomer. There is no muscular 

 suctorial apparatus developed in connection with it ; 

 it is glandular and secretory, producing a sticky 

 mucus or slime, which serves to fasten the larva to its 

 resting-place.* The mode of its development and 



* From this apparatus the ventral adhesive disk of certain tadpoles 

 living in mountain streams of the Himalayas, Burma, and the Malay 

 Archipelago is no doubt developed. 



