THE DEVELOPMENT OF THE TADPOLE. 215 



commences about the same time as that of the hind-leg, and pro- 

 ceeds equally with it, although the front-legs themselves are 

 hidden under the skin for some time, breaking out at about the 

 same time as the hind-legs are capable of useful motion. 



As in the hind-limb, so in the fore, the first visible sign of 

 structure consists in the formation of a protuberance, which, 

 however, instead of growing outwards, increases in length inwards, 

 and ultimately almost fills a cavity contained under the skin in 

 the region of the lungs. The abdominal region here extends 

 outwards to so great an extent that, proceeding upwards from the 

 outer skin of the abdomen, each section, upwards to the middle 

 plane of the lungs, is found to be quite surrounded by a layer of 

 skin, thus giving the false appearance of a diaphragm stretching 

 between the abdominal and thoracic regions. 



Before the deposition of colour-cells in the skin, so deceptive 

 is this appearance, that it requires some amount of thought to 

 dispossess one's mind of the idea of a true diaphragm, instead of 

 merely the false appearance of one. The first change of the pro- 

 tuberance into an arm is seen in the formation of a joint almost 

 close to the skin by the production of two transverse layers of 

 cells proceeding from the outer covering of the protuberance. 

 The outer layer, of somewhat elongated cells, bends first outwards 

 and next inwards across the whole width of the protuberance, thus 

 dividing it into two parts : a very short proximal and a much 

 longer distal one, which is in every part surrounded by the inner 

 layer of cells forming the covering. The thicker middle layer, 

 between these two, bends round also, so as to form a kind of 

 division between these two transverse layers (see Fig. 8, PL XXIL, 

 and Fig. 9, PI. XXIL). 



Much acute speculation has been spent on the homologies 

 existing between, and derivation of, the vertebrate limb from the 

 fin of the fish. It is impossible for me to express any opinion on 

 either of these theories, except to make the bare statement that 

 whatever may have happened in past ages, at the present time we 

 never find a foetal fin or ichthyopterygium becoming a vertebrate 

 hand or cheiropterygium, nor do we find in the two structures 

 such analogous and close agreement in detail, or, indeed, apparently 

 in development, as would be likely to impress difterent observers 

 with similar ideas as to their close identity. 



