THE FROG 107 



feeding the alimentary canal grows rapidly, and is always to be seen 

 through the semi-transparent abdominal wall as a coiled tube. 

 Two pairs of comb-shaped external gills grow out from the side of 

 the neck to be followed shortly afterwards by a third pair. A fold 

 of skin, termed the operculum, starts to grow out from just in front of 

 the gills and then to grow back over them. Between, in front, and 

 behind the external gills slits appear, which lead through into the 

 pharynx, and on their walls the internal gills are developed as a series 

 of folds, and with their coming the external gills gradually get smaller. 

 The operculum grows backwards and its hinder edge fuses with the 

 body wall, until at the end of the fourth week it is completely closed, 

 save for a spout-like opening on the left-hand side, which serves for 

 the egress of the water used in respiration. 



Tiny little projections, the limb-buds, appear at the base of the 

 tail during the sixth or seventh week ; they grow slowly, until a 

 fortnight or so later they are miniature limbs with joints and digits. 

 The front limbs develop at the same time as the hind ones, but they 

 are hidden by the operculum, and do not appear externally until 

 they are practically fully formed. One of them, the left, protrudes 

 through the spout and the other actually bursts through the oper- 

 culum itself. 



Somewhere about the twelfth week a marked and deep-seated 

 change takes place in the tadpole, which up to now has been in many 

 ways extremely fish-like in its internal anatomy. The casting off 

 of the skin allows the eyes to come to the surface, the horny jaws 

 disappear, and the mouth and tongue increase in size. Lungs develop 

 internally, and for a short time the animal breathes by both gills and 

 lungs, but soon by the latter alone ; a change that necessitates a 

 radical alteration in the circulatory system. The limbs become 

 stronger and more useful, and with the disappearance of the tail, 

 which has been gradually getting shorter and shorter, the animal 

 becomes a tiny frog. This last rather quick series of changes, whereby 

 the water-dwelling fish-like tadpole becomes transformed into the 

 lung-breathing land-living frog, is termed the metamorphosis. A 

 similar metamorphosis is characteristic of all land-dwelling Amphibia. 

 The remainder of the year is passed in feeding till the winter, when 

 this young animal hibernates. It comes forth the following spring, 

 but it does not breed until a year later. Thus the frog passes through 

 a series of changes in a regular sequence, finally reaching the adult 

 stage, i.e. not the stage of full growth, but the stage at which it is 

 able to reproduce, and this development is termed the life-history 

 or life-cycle. As long as the young animal is within the egg envelope 

 and entirely independent of the outer world in the matter of food we 

 term it an embryo. When it hatches, however, and lives a perfectly 



