86 GENERAL SKETCH OF 



from the tip of the notochord of the larval tail, is the first 

 sign of the permanent organ. The true rays form in the lower 

 region of the tail, the whole generally pushing the notochord 

 with the larval tail upwards, and giving it a characteristic bend 

 or slope, the boundary between the permanent and the larval 

 tail being marked by a notch. 



In some fishes, as in the cod and the haddock, though the 

 ventral thickening is most distinct, the tip of the notochord 

 remains median, and rays develop on both sides of it, making a 

 feather-tip. 



Paired fins. 



When the embryo is first outlined, an alar expansion, con- 

 sisting of epiblast and hj^poblast, resting on periblast, stretches 

 away on each side along the whole trunk. No mesoblasfc 

 apparently extends into it. Soon two flattened oval pads, 

 consisting of a double fold of epiblast, are differentiated from 

 the rest of the expansion. Then the outer border of the cell- 

 mass (mesoblast) near the Wolflian ducts sends a process 

 between the layer and spreads out radially, but does not quite 

 reach the distal margin. The fin gradually becomes discon- 

 nected from the covering of the yolk-sac, the central meso- 

 blast assumes a columnar character, and later towards the 

 trunk a stout peduncle is formed, cartilage-cells are developed, 

 radial structures appear and lastly pigment may be seen on its 

 surface. The fin, moreover, leaving its primitive horizontal 

 position, becomes more or less vertical. During the third week 

 after hatching, the rotation of the fin has made it obliquely 

 vertical, and then the basal attachment is placed almost dorso- 

 ventral. Embryonic and permanent rays develop as in the 

 dorsal fin. A pectoral bar appears on each side, the first part 

 of the girdle, and then the various elements of the latter, as 

 shown in a special research of Prof Prince \ are outlined. 



The ventral fins of the lesser weever appear in the egg 

 shortly after the pectoral, and from the same alar expansion, 

 but as a rule, in the forms with pelagic eggs, the ventral fins 



' "On the Development and Morphology of Limbs of Teleosteans." Glasgow, 

 1891. 



