LIMBS. 



85 



the paired and the unpaired limbs. Both arise in Fishes as 

 linear proliferations of the epiblast, which form four folds or ridges 

 a dorsal and a ventral, extending from the head backwards 

 to the tail, and two lateral (Fig. 68, A, D, S, /S 1 ). 1 Mesoblastic 

 elements extend into them later. 



a. UNPAIRED LIMBS. 



The unpaired limbs, which are characteristic of Fishes, are 

 developed from the dorsal and ventral ridges. They either remain 

 continuous in their further development, as in some Fishes and 

 tailed Amphibians, or else certain parts undergo degeneration, so that 



Bt' An 



FlG. 68. DlAGBAM SHOWING (A) THE ^DIFFERENTIATED CONDITION OF THE 



PAIRED AND UNPAIRED FINS IN THE EMBRYO, AND (B) THE MANNER IN 

 WHICH THE PERMANENT FINS ARE FORMED FROM THE CONTINUOUS FOLDS. 



D, dorsal fin-fold : S. S, lateral folds, -which unite together at S 1 to form the ventral 

 fold; RF, FF, dorsal fins; SF, tail-fin; AF, anal fin; BrF, pectoral fin; 

 BF, pelvic fin ; An, anus. 



they only persist in certain regions, which are spoken of as dorsal, 

 caudal, and anal fins (Fig. 68, A, B) : in these regions muscles 

 and skeletal parts (fin-rays) become formed. 



After the formation of the epiblastic fin-folds, the next parts of the unpaired 

 fins which appear are the muscles, and then follow the supporting cartilages ; 

 these latter arise, like the skeletal parts of the paired fins, entirely independently 

 of the axial skeleton. The connection between the latter and the unpaired fins 

 is only secondary. This holds good also for the caudal fin, in which the 

 relation between the axial skeleton and that of the fin is a very close one. 



The caudal fin is the principal organ of locomotion in most Fishes, and it 

 acts in a horizontal direction : the paired fins play only a secondary part, and 

 are principally concerned in rising and sinking in the water. 2 



1 See also p. 86. 



2 The curious suctorial disk on the dorsal side of the head of the Eemora 

 (Echeneis), by means of which it attaches itself to foreign objects, arises in the 

 embryo from the anterior portion of the dorsal unpaired fin, and this is indicated 

 throughout life by the arrangement of the blood-vessels, nerves, and skeletal parts. 



