BATRACHIA. 193 



Reproduction takes place in the spring. Copulation is confined to 

 an external approximation of the two sexes, and almost always takes 

 place in the water. The male, which sometimes has a wart-like 

 elevation on the thumb (Rana) or gland on the arm (Pelobates) 

 embraces the female from the back, usually behind the front limbs, 

 and pours out the seminal fluid over the spawn as it issues in strings 

 or in clumps. The individual eggs are surrounded by a viscous layer 

 of albumen which swells up in the water. 



The upper half of the ovum is of a darker colour than the lower. 

 The process of segmentation begins in the upper part, and the con- 

 strictions which lead to the formation of the segmentation spheres 

 proceed more rapidly in this region than at the lower pole. With 

 the end of segmentation a cavity the segmentation cavity appears 

 in the mass of cells ; it is placed nearer to the upper pole than to the 

 specifically heavier lower pole. . The germ [blastoderm], with medullary 

 plate and folds, arises on the upper half ; it quickly, even before the 

 closure of the medullary canal to form the medullary tube, grows 

 round the yolk. After development of the branchial arches and before 

 the mouth is formed, the embryos which have a short tail leave 

 their egg membranes as tadpoles at a stage of development which 

 varies with the species. They then attach themselves by means of 

 two suckers to the gelatinous remains of the spawn (similar suckers 

 are present on the throat of the Triton-larvae, where however they 

 are stalked). Most larvae leave the egg membranes with more or less 

 developed rudiments of three pairs of branched external gills (vol. i., 

 fig. 111). The body gradually increases in length and the fin-like 

 tail developes. Later the mouth is formed and the larva begins 

 to feed. Soon the external branchial appendages disappear, while 

 the skin grows over the gill slits like an operculum in such a manner 

 that only one gill aperture is left, through which the water flows out 

 of the branchial chambers on either side. 



During these processes fresh lancet-shaped gill-plates are developed 

 in double rows along each branchial arch. The mout 1 ! is armed with 

 a horny beak, which is used in gnawing vegetable anl also animal 

 substances. The intestine has become very long and much coiled, 

 and the lungs have grown out of the pharynx in the form of long- 

 sacs. As development proceeds the hinder extremities first make 

 their appearance on the body of the Tadpole close to the attachment 

 of the strongly- developed swimming tail. As the pulmonary respi- 

 ration increases, the branchial apparatus becomes more and more 

 reduced, and the animal undergoes an ecdysis, with which is con- 



VOL. n. 13 



