DEVELOPMENT OF LUNG-FISH 2OI 



theory of the embryo's concrescence. In the hinder region 

 of the same embryo (Fig. 242) the blastopore is still 

 apparent, BP, reduced to a narrow, fissure-like aperture ; 

 around it is the tail mass, corresponding generally to CF 

 of Fig. 226 ; and encircling all is the hinder continuation 

 of the medullary folds. 



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. 

 In Fig. 243, the tail region of a slightly older embryo, this 

 process is clearly shown ; the medullary folds, MF, are 

 seen closely apposed in the median line ; hindward, how- 

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

 blastopore, BP, may yet be seen. At this stage primitive 

 segments are shown at PS ; in the brain region in Fig. 

 244 the medullary folds are still slightly separated (cf. CP, 

 Fig. 226). 



Two views of an 

 older embryo are fig- 

 ured (Figs. 245 and 

 246), where the fish- "^ 

 like form may be rec- 



Oo^nized. The medul- Fig. 248.— Embryo of Ceratodus, near the time 



^ of hatching. 



lary folds nave com- qS. GUI slits. M. Mouth pit. op. Optic vesi- 



pletely fused in the ""^J^^;^ ^''"^''''^ ^*'^"''"' p^^^^p^^^^' ^- ^^^'' 

 median line, and the 



embryo is coming to acquire a ridge-like prominence ; 

 optic vesicles and primitive segments are apparent, and 

 at BP the blastopore appears to persist as the anus. The 

 continued growth of the embryo above the yolk mass, 

 V, is apparent in Fig. 247; the head end has, however, 

 grown the more rapidly, showing gill slits, GS, auditory, 

 optic, and nasal vesicles, AU, OP, and O, at a time when 



