350 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



to decide which is the most nearly primitive of the various forms of 

 paired fin met with in surviving fishes. The present writer takes the 

 view that undoubtedly the most primitive type of paired fin known 

 to occur in existing Vertebrates is the paddle-like limb of Ceratodus. 

 Physiologically this type of fin is as clumsy and archaic an organ in 

 comparison with the paired fin of a Shark or Actinopterygian fish as 

 is the most primitive type of savage's paddle compared with a racing 

 oar. Further we know from the data of palaeontology that the 

 Ceratodus type of limb is of great antiquity and that it was a 

 common type of fin amongst the more ancient Sharks and Ganoids 

 as well as amongst the Lung-fishes. There are only two possible 

 explanations of its occurrence in the three groups mentioned. Either 

 (1) it is an archaic type of fin inherited from the common ancestors 

 of those groups or (2) it has been evolved independently in the three 

 groups. The latter explanation seems very improbable — for such a 

 type of organ would become evolved independently in different 

 groups, only if it were physiologically very efficient. But it seems 

 quite impossible with knowledge of the structure of the limb of 

 Ceratodus and its use in the living animal to regard it as an organ 

 of great locomotor efficiency. Apart from this consideration we have 

 the historical fact that this type of fin has vanished away entirely 

 in the two successful types of fish, the Sharks and the Teleostomes, 

 and has persisted unchanged only in one of the surviving Lung-fishes. 

 There seems then no escaping the first of the two possible con- 

 clusions mentioned above, that the paddle-like fin of Ceratodus and 

 the ancient Lung-fishes, Ganoids and Sharks is a common heritage 

 from the ancestral group out of which these fishes evolved and is 

 therefore the most archaic of the known types of paired fin. 



Development of the Pectoral Limb Skeleton in Ceratodus. — 

 The skeleton of the limb makes its first appearance (Semon, 1898) 

 about stage 45 (Fig. 201) as a rod-like condensation of connective tissue 

 along the axis of the limb which tapers gradually towards the apex. 

 Histological differentiation, by which this rod-like structure passes 

 through a prochondral into a completely chondrified condition, 

 proceeds from the base towards the apex. While in the prochondral 

 condition the rod is a continuous structure, chondrification takes 

 place from separate centres, with the result that the rod becomes 

 converted into a series of blocks of cartilage, each separated from its 

 neighbours by a thin layer of unchondrified tissue. The basal block, 

 lying within the body wall and spreading in a ventral direction, is 

 the rudiment of the pectoral girdle ; the rest of the series forms the 

 axis of the limb. 



The lateral rays make their appearance later, the development 

 again proceeding from base towards apex, and those on the preaxial or 

 definitively dorsal side preceding those on the postaxial or definitively 

 ventral. Each ray spreads out from the prochondral tissue between 

 two segments of the axis and it is noteworthy that rays develop 

 from the .first (proximal) of these intersegmental joints although in 



