6 1 8 Subclass Dipneusti, or Lung-fishes 



The larva of six weeks suggests the outline of the mature 

 fish; head and sides show the various openings of the tubules 

 of the insunken sensory canals; and the archipterygium of 

 the pectoral fin is well defined. The oldest larva figured is 

 ten weeks old; its operculum and pectoral fin show an increased 

 size; the tubular mucous openings, becoming finely subdivided, 

 are no longer noticeable; and although the basal supports of 

 the remaining fins are coming to be established, there is as yet 

 little more than a trace of the ventrals. 



The early development of a lung-fish has thus far been 

 described (Semon) only from the outward appearance of the 

 embryo. The egg of Neoceratodus has its upper pole distin- 

 guished by its fine covering of pigment. From the first fine 

 planes of cleavage it will be seen that the yolk material of the 

 lower pole is not sufficient to prevent the egg's total segmenta- 

 tion. The first plane of cleavage is a vertical one, passing 

 down the side of the egg as a shallow surface furrow, not appear- 

 ing to entirely separate the substance of the blast omeres, al- 

 though traversing completely the lower hemisphere. A second 

 vertical furrow at right angles to the first is seen from the upper 

 pole. The third cleavage is again a vertical one (as in all other 

 fishes, but unlike Petromyzori), approximately meridional; its 

 furrows appear less clearly marked than those of earlier cleavages, 

 and seem somewhat irregular in occurrence. The fourth cleavage 

 is horizontal above the plane of the equator. Judging from 

 Semon's figure, at this stage the furrows of the lower pole seem 

 to have become fainter, if not entirely lost. In a blastula 

 showing complete segmentation the blastomeres of the upper 

 hemisphere are the more finely subdivided. In the earlier 

 stage the dorsal lip of the blastopore is crescent-like; in the 

 later the blastopore acquires its oblong outline, through which 

 the yolk material is apparent ; its conditions may later be com- 

 pared to those of a Ganoid. 



The next change of the embryo is strikingly amphibian-like ; 

 the medullary folds rise above .the egg's surface, and, arching 

 over, fuse their edges in the median dorsal line. The medullary 

 folds are seen closely apposed in the median line; hindward, 

 however, they are still separate, and through this opening the 

 blastopore may yet be seen. At this stage primitive segments 



