442 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



with the rudiment of an external gill, but in a large proportion of 

 the Vertebrates it is narrower in a dorsi-ventral than iu an antero- 

 posterior direction so as to have the form of a short longitudinally- 

 running ridge. 



As is usual where active increase of surface is about to take place 

 the projecting limb rudiment is foreshadowed by a thickening of the 

 ectoderm and by a condensation of the underlying mesenchyme. 



In Torpedo, one of the Rays — fishes characterized by the great 

 antero-posterior extension of the large pectoral fin — the two limb 

 rudiments which are at first distinct (Rabl, 1893) become for a time 

 joined together by a transitory ectodermal thickening. This phase, 

 in which the two fin rudiments are as it were parts of a continuous 

 ridge, was the earliest stage observed by Balfour (1878) and it 

 afforded him an embryological basis for the lateral-fold view of the 

 phylogenetic origin of the Vertebrate limbs (see p. 445). 



As the limb rudiment develops it shows in many cases character- 

 istic changes in its position. First it shows a movement of rotation. 

 This is well illustrated by the case of Ceratodus as described by 

 Semon (1898). Here the pectoral limb rudiment becomes rotated in 

 such a way that its originally pre-axial or headward edge becomes 

 dorsal and its originally lower or ventral surface comes to face in 

 a headward direction. In other words, if one could observe the 

 developing left pectoral limb from a point away to the animal's left 

 side the limb would be seen to undergo a clockwise rotation. It 

 results from this that when the fully developed limb is folded back 

 alongside the body its outer surface is that which was originally 

 ventral. A rotation similar in direction though varying in angular 

 extent in different forms occurs also during the development of the 

 pectoral fin in Crossopterygians and Actinopterygians. In Tetrapoda, 

 on the other hand, a rotation of the limb rudiment in the opposite 

 direction takes place — the pre-axial edge becoming ventral. Not 

 improbably this may be regarded as a secondary modification fore- 

 shadowing the pronate position of the fore-limb characteristic of 

 terrestrial progression. 



The pelvic fin in Ceratodus undergoes a similar rotation but in 

 the opposite direction to that of the pectoral : the left pelvic fin 

 regarded from a point away on the animal's left would be seen to 

 undergo a counter-clockwise rotation. The result is that the origin- 

 ally dorsal surface comes to face headwards or, when the fin is 

 folded back alongside the body, outwards. 



The corresponding rotation of the pelvic limb in other fishes 

 and in the lower Tetrapods appears to stand in need of further 

 investigation. 



It is clear from the facts of Comparative Anatomy that the paired 

 limbs have undergone extensive shiftings along the surface of the 

 vertebrate body in adaptation to its general form and its method of 

 movement (see below, p. 448). It is of interest — though not necessary 

 for establishing the fact of such phylogenetic shifting — to enquire 



