DEVELOPMENT OF GANOID 205 
surface as a plug-like mass; at SC is the segmentation 
cavity. The dorsal lip of the blastopore is seen to be far 
longer than the ventral lip; its rim is the more inflected, 
at KV occurring a recessus which the writer compares 
to the Kupffer’s vesicle of Teleost development; the 
cavity, C, coelenteron, between the wall of the blastopore 
and the yolk mass is in this region the largest. The 
germ layers in this stage, EC, MES, EN, are seen to 
be confluent at the blastopore’s rim; at the termina- 
tion of the ccelenteron, entoderm and mesoderm are 
merged; the ectoderm forms the roof of the segmenta- 
tion cavity. 
The form of the embryo next becomes more definitely 
established. In Fig. 257 the blastopore, much reduced 
in size, is seen at BP; its thickened rim is whitish in 
colour; the darkened area, whose boundary is LC, is the 
cceelenteron, seen faintly through the translucent margin 
of the blastopore; the embryo is the opaque area of the 
blastopore’s dorsal lip, terminating anteriorly in the dilated 
tract, H, the head region. In a sagittal section of a 
slightly later stage (Fig. 258), the relations of germ 
layers, EC, MES, EN, coelenteron, C, and yolk mass, 
Y, may be compared with those of the section (Fig. 256), 
wherein the region YP corresponds to that of VC. A 
thin ectoderm will now be seen to have enclosed the 
entire egg; the segmentation cavity has disappeared ; the 
rim of the blastopore, becoming continually constricted, 
causes the yolk material to recede from the surface, and 
leaves the blastopore disappearing, as the blunt diver- 
ticulum of VC. The neurenteric canal, WC, is the last 
communication between the surface of the egg and the 
ceelenteron ; this has become established before the blas- 
topore closes in the stage of Fig. 257 at its dorsal lip; 
