DEVELOPMENT OF LUNG-FISH 201 
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, I/F, 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- 
ognized. The medul- ‘Fig. 248.— Embryo of Ceratodus, near the time 
4 of hatching. 
lary folds have com- Gs, Gillslits. 44. Mouth pit. OP. Optic vesi- 
: cles. PN. Primitive kidney, pronephros. 7. Tail 
pletely fused in the oF ice, 
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, 
Y, 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 
