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, 

 2X 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 coelenteron, 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 

 coelenteron, 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 NC A 

 thin ectoderm will now be seen to have enclosed the 

 entire Qgg ; 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 NC The neurenteric canal, NC, is the last 

 communication between the surface of the Qgg and the 

 coelenteron ; this has become established before the blas- 

 topore closes in the stage of Fig. 257 at its dorsal lip; 



