Subclass Dipneusti, or Lung-fishes 617 



shark there then exists no unpaired fin; the gill-slits (five ?) 

 are well separated and there is an abrupt cephalic flexure. 

 In this stage pronephros (primitive kidney) and primitive 

 segments are well marked, and are outwardly similar to those 

 structures in Ganoid; the mouth is on the point of forming its 

 connection with the digestive cavity; the anus is the persistent 

 blastophore; the heart, well established, takes a position, as 

 in Cyclostomes, immediately in front of the yolk material. 



In a later stage the unpaired fin has become perfectly 

 established, the tail increasing in length; the gill-slits have 

 now been almost entirely concealed by a surrounding dermal 

 outgrowth, the embryonic operculum; a trace of the pectoral 

 fin appears; the lateral line is seen proceeding down the side 

 of the body ; near the anal region the intestine * becomes nar- 

 rower, and the beginnings of the spiral valve appear. In a 

 larva of two weeks a number of developmental advances are 

 noticed; the fish has become opaque; the primitive segments 

 are no longer seen; the size of the yolk mass is reduced; the 

 anal fin-fold appears; sensory canals are prominent in the head 

 region; lateral line is completely established; the rectum be- 

 comes narrowed; and the cycloidal body-scales are already 

 outlined. Gill-filaments may still be seen beyond the rim of 

 the outgrowing operculum. In the ventral view of a some- 

 what later larva the following structures are to be noted: the 

 pectoral fins, which have now suddenly budded out,f reminding 

 one in their late appearance of the mode of origin of the anterior 

 extremity of urodele; the greatly enlarged size of the opercular 

 flap; external gills, still prominent; the internal nares, be- 

 coming constricted off into the mouth-cavity by the dermal 

 fold of the anterior lip (as in some sharks) ; and finally (as in 

 Protopterus and some batrachian larvae) the one-sided position 

 of the anus. 



* The yolk appears to be contained in the digestive cavity, as in Ichthyophis 

 and lamprey. 



t The abbreviated mode of development of the fins is most interesting; 

 from the earliest stage they assume outwardly the archipterygial form; the 

 retarded development of the limbs seems curiously amphibian-like; the pec- 

 torals do not properly appear until about the third week, the ventrals not 

 until after the tenth. 



