440 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



flexure after the manner of an Eel. To secure greater efficiency the 

 body, more particularly its purely motor post-anal portion, becomes 

 compressed from side to side, the compression being most marked 

 near the margin of the body where the thin almost membranous 

 median fin is produced. 



In development the median fin arises as a projecting fold of slightly 

 thickened ectoderm into which later on mesenchyme penetrates. In 

 what the evidence points to as being the primitive condition this fin 

 rudiment is continuous and extends round the hind end of the body. ' 

 In such relatively primitive Vertebrates as Crossopterygians, Lung- 

 fishes, and some Elasmobranchs, it extends forwards on the dorsal 

 side practically to the head-region, while on the ventral side it 

 reaches the anus and may even be continued onwards as a pre-anal 

 median fin, though possibly this has originated in pbylogeny in- 

 dependently of the main fin-fold. 



In the Lung -fishes the median fin-fold during the course of 

 development never loses its continuous and practically symmetrical 

 arrangement round the tip of the tail. It retains throughout life the 

 primitive symmetrical (protocercal) form. In the Crossopterygians . 

 (apart from the anterior portion of the dorsal fin which becomes divided 

 up into a series of finlets) the same holds until a very late stage in 

 development, the tail of the adult becoming very slightly asym- 

 metrical though the term protocercal is usually and justifiably still 

 applied to it. A similar protocercal tail occurs in the Amphibian 

 larva while the tail of the adult Newt or Crocodile is simply a pro- 

 tocercal tail in which there is no longer a membranous fin-fold present. 



It is however characteristic of the fishes in general that, in 

 accordance with their high specialization as expert swimmers, the 

 median fin during ontogeny loses its homogeneous character — cer- 

 tain portions of it, probably those portions which are in mechanic- 

 ally the most favourable positions, becoming enlarged while the 

 intervening portions become reduced to the point of complete dis- 

 appearance. The result is that the place of the originally continuous 

 fin -fold is taken by a series of separate fins — one or two dorsal, a 

 caudal or tail fin, and on the ventral side, an anal fin. Of these the 

 caudal fin — most favourably situated of all the series to serve as a 

 propelling organ — becomes specially enlarged. 1 1 is also characteristic 

 of the more efficient swimmers that the part of the caudal fin lying 

 on the ventral side of the axis becomes particularly developed. We 

 may probably associate this with the function of rotating the body 

 about its long axis as may be seen in a shark when it seizes its prey. 

 The unsymmetrical condition of the tail so produced is termed hetero- 

 cercal. When carried to its extreme the tip of the vertebral axis 

 becomes tilted upwards and, so far as external appearance goes, a 

 (secondarily) symmetrical condition is arrived at, as is seen in the 

 homocercal tail of the Teleostean fishes. It will be understood that 

 the protocercal, heterocercal and homocercal conditions merge into 

 one another and cannot be distinguished by any rigid definition. 



