352 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



The prochondral rudiment of the axial cartilage extends out 

 into the limb-rudiment, forming a broad plate which tapers off 

 posteriorly (Fig. 168). The condition is in its essentials the same as 

 that in Geratodus except that here the axial rudiment is laid back 

 along the side of the body. The prochondral fin-rays arise, as 

 in Ceratodus, in the form of outgrowths from the axial element. 

 These are restricted to the outer (preaxial) side of the limb. They 

 develop in Spinax (Braus) in series from before backwards except 

 that anteriorly, in the region which will give 

 rise to mesopterygium and propterygium, a few 

 rays develop in the opposite sequence from 

 behind forwards. 



The chondrification of the limb skeleton 

 appears to take place in Mustelus and Torpedo 

 continuously but in Spinax it sets in first in 

 the axial portion and then in the rays in the 

 same succession as they first appear. The 

 separate segments of the rays in Spinax also 

 fig. 168.— Section through develop in succession as separate centres of 



pectoral fin of Torpedo chondrification. 



embryo, parallel to sur- ^o understand the morphological relations 



MoUie? 1 1894.) tCT °f these early stages it is advisable to refer 



back to the paddle type of limb as it exists 



The prochondral rudiment . ^, . ■*-. , -. n .-, t-, 7 



of the skeleton is shaded. in the ancient sharks of the genus Pleura- 

 canthus. Here (Fig. 169, B) we find a limb 

 resembling generally that of Geratodus but differing from it in two 

 conspicuous details. (1) The skeletal axis has become relatively 

 larger and clumsier, its original elements having probably under- 

 gone extensive processes of fusion both with one another, as 

 shown by the fact that the cartilages of the axis are in places 

 less numerous than the lateral rays, and also with the basal portions 

 of these lateral rays. (2) The rays on the postaxial side of the 

 limb are much reduced in number, only a few persisting towards 

 the limb apex. 



The tendency of the postaxial rays to disappear in these archaic 

 sharks (and the same tendency is seen in Lung-fishes) justifies us in 

 believing that the external side of the pectoral limb-axis in the 

 young Elasmobranch is morphologically preaxial. This conclusion 

 raises the interesting question — Are there any vestiges of postaxial 

 rays to be found in existing Elasmobranchs ? This question has to 

 be answered in the affirmative. In Centrophorus (Fig. 169, C) 

 Braus finds a number of postaxial rays near the tip of the fin in a 

 late stage of development; in Spinax at least one similar piece of 

 cartilage occurs ; and even in the adults of various Sharks Gegenbaur 

 and Bunge found similar vestiges. As vestigial organs are notori- 

 ously variable more extended investigations into the occurrence of 

 such vestigial postaxial rays are very desirable. They should be 

 carried out on as many different species of Shark as possible and 



