22O DEVELOPMENT OF FISHES 



gill slits, five (?), GS, are well separated, and there is an 

 abrupt cephalic flexure. In this stage pronephros and 

 primitive segments, PS, are well marked, and are out- 

 wardly similar to those structures in Ganoid ; the mouth, 

 S, is on the point of forming its connection with the 

 digestive cavity ; the anus is the persistent blastopore ; 

 the heart, well established, takes a position, as in Cyclo- 

 stomes, 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, PF, appears ; the lateral line is seen pro- 

 ceeding down the side of the body ; near the anal region 

 the intestine* becomes narrower and the beginnings of 

 the spiral valve appear. In a larva of two weeks (Fig. 292), 

 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 

 becomes 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 (Fig. 293), 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 



* The yolk appears to be contained in the digestive cavity as in Ichthy- 

 ophis and lamprey. 



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

 from the earliest stage they assume outwardly the archiptervgial form ; the re- 

 tarded 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. 



