12 



FROG 



(Lond. 1886); Life of Froebel, by Emily Shirreff 

 (Lond. 1887); and his Letters, translated by Moore 

 and Michaelis (1890). 



Frog, a genus ( Rana ) of tailless Amphibians ; 

 but the name, usually with some prefix or other, is 

 often extended to the members of related genera or 

 even of related families e.g. to the obstetric frog 

 (Alytes), to the tree-frogs (Hylidae), or to the peep- 

 ing frogs (Hy lodes). The common frog in Britain 

 is Rana temporaria, distinguished from the edible 

 frog, R. esculenta, which has been introduced into 

 Britain, by slight differences in colouring, by the 

 presence of a dark, triangular patch extending 

 backwards from the eye, and by the absence of the 

 dilatable sacs ( at the back corners of the mouth ) 

 which intensify the croaking of the ' Cambridge- 

 shire Nightingales.' The general shape is an 

 elongated oval, of which the head occupies about 

 a third ; a hump on the back marks the end of the 

 distinct vertebrae, and the beginning of an un- 

 segmented portion known as the urostyle. The 

 tail has completely disappeared, the young animal 

 having literally lived upon it during part of its 



Common Frog ( Rana temporaria). 



metamorphosis. The arms are short, the fingers 

 four and unwebbed, and the innermost is swollen 

 in the males ; the hind-legs are long and muscular, 

 well adapted for both leaping and swimming, with 

 an elongated ankle, five webbed toes, and an 

 internal ' tarsal tubercle ' like a hint of a sixth. 

 The skin is soft and glandular, with pigment 

 cells admitting by their changes of a slight altera- 

 tion in colour. The external nostrils are situated 

 near the tip of the snout ; the eyes have a movable 

 lower lid ; the tympanum or drum of the ear is 

 readily seen somewhat farther back. 



General Life. The frog, aquatic in its youth, 

 generally remains near water. In dry weather it 

 hides itself, and great numbers are often seen to 

 issue forth on the welcome return of rain. Their 

 leaping and swimming deftness need no remark. 

 The adults feed upon living animals, insects, and 

 slugs. These are caught on the large viscid tongue, 

 which being fixed in front of the mouth and free 

 behind, can be thrown rapidly outwards, and even 

 more rapidly retracted. In winter the frog ' hiber- 

 nates ' or lies torpid, buried in the mud at the 

 bottom of the pool, and great numbers of in- 

 dividuals may be dug up in winter all clustered 

 together. During this season certain 'fatty bodies,' 

 situated on the top of the reproductive organs, and 

 apparently degenerate portions of the kidney, be- 

 come reduced in size, being probably the ovaries 

 and testes, which become functional in the month 

 of March. Then it is that the frogs congre- 

 gate together for breeding purposes, and that the 

 males with their vigorous croaking serenade their 

 more weakly-voiced mates, preceding the birds in 



announcing the approach of spring. The titles bull- 

 frog, blacksmith-frog, sugar- miller, &c., applied to 

 certain species, obviously refer to their notable 

 vocal powers. 



The frog generally contains some interesting 

 parasites a hermaphrodite threadworm or Nema- 

 tode ( A ngiostonium nigrovenosum ) in the lungs, a 

 fluke or Trematode with many suckers (Polystomum 

 integerrimum) in the bladder, and a ciliated In- 

 fusorian with many nuclei (Opalina ranarum) in 

 the hindmost part of the alimentary canal. 



Life-history. The eggs of the frog are familiar 

 to almost all ; each is a little dark ball enclosed in 

 a glutinous sheath which swells in the water into a 

 clear round globe. The egg has most black pig- 

 ment in its upper half, the heavier yolk sinking 

 for the most part to the lower hemisphere. They 

 are fertilised just as they leave the female, which 

 the male is at the same time embracing. The 

 division of the ovum is complete but unequal, the 

 upper hemisphere with the ' formative protoplasm ' 

 soon exhibiting a larger number of smaller cells 

 than the lower portion, which chiefly consists of 

 yolk to be gradually absorbed by the embryo (see 

 EMBRYOLOGY ). 



By the tenth day after the eggs are laid the 

 head, body, and tail of the young frog may be dis- 

 tinctly seen. Following the lines of its ancestral 

 history (why or how is a difficult question), the 

 animal becomes fish-like, with a long tail and with 

 three pairs of external gills on its neck. About a 

 fortnight after the laying the young tadpoles are 

 hatched, and, jerking themselves out of the gelatin- 

 ous mass, 'swim freely in the water. They are 

 still mouthless, and live on their still unexhausted 

 capital of yolk. They have a paired sucker under- 

 neath their head, by means of which when tired 

 they attach themselves to water-weeds or other 

 objects. In a few days, however, they gain a 

 mouth, ' bordered by a pair of horny jaws, and 

 fringed with fleshy lips provided with horny 

 papillae.' The whole arrangement reminds one of 

 that of the lamprey. As the tadpole hungrily 

 feeds on fresh-water weeds (algte, &c. ), the hitherto 

 short alimentary canal becomes elongated, furnished 

 with a liver and pancreas, and, when the animal is 

 big enough to dissect, may be readily seen coiled up 

 like a watch-spring. About the time when mouth 

 and anus have been opened the four gill-slits or 

 clefts, opening from the pharynx to the exterior, 

 may also be seen, and very soon the original ex- 

 ternal gills shrivel, and are replaced by an internal 

 set. As the latter develop, a fold of skin grows 

 over them, forming a gill-Chamber which by-ami -by 

 closes so much that only a single exit aperture 

 remains, and that on the left side. Through this 

 the water taken in for respiration by the mouth 

 passes to the exterior, after washing the gills on its 

 way. 



The tadpole thrives on its vegetarian diet, and 

 rapidly grows bigger and stronger ; the large tail is 

 a powerful swimming organ, and the adhesive 

 suckers are less and less used. The limbs bud 

 forth, but the anterior pair, hidden by the gill- 

 covers above referred to, are longer of becoming 

 distinctly visible. By the end of the second month 

 the tadpole has attained to the level of the double- 

 breathing fishes or Dipnoi (see FISHES) ; in other 

 words, the lungs become useful, the gills for a 

 while persist, but, as the animals get into the habit 

 of coming oftener to the surface to breathe, these 

 latter organs gradually degenerate. 



Two or three weeks more, and a remarkable 

 change a metamorphosis occurs, in which the 

 tadpole rises above the fish level and becomes a 

 distinct amphibian ( see AMPHIBIA, for figures, &c. ). 

 The tadpole ceases to feed upon algae , and begins 

 to live at the expense of its tail, from which 



