DEVELOPMENT, 549 
blastoderm, or segmented area, some nuclei (so-called 
“merocytes”) are formed in the outer part of the subjacent 
yolk (Fig. 290, ~.). It seems most probable that these 
are hypoblast elements which assist in the preparation of 
the yolk for absorption, and eventually degenerate in the 
empty external yolk-sac. 
At the close of segmen- 
tation the blastoderm is a 
lens-shaped disc with two: 
strata of cells. It is thicker 
at one end—where the em- 
bryo begins to be formed. 
‘Towards the other end, be- 
tween the blastoderm and 
the yolk, lies a segmentation 
cavity (Fig. 290, 5g.c.). 
At, the embryonic end 
the outer layer or epiblast 
undergoes a slight invagina- 
tion (Fig. 290, x.), beginning 
to form the roof of the 
future gut (g.); in other 
words, establishing the hypo- 
blast. This inflected arc of 
the blastoderm corresponds 
to the blastopore or mouth een F ire 
of the _gast tula, which is Gua et hich 
much disguised by the pres- has been cut open to show con- 
ence of a large quantity of tents. 
yolk. As the invagination eg. ‘‘External” gills; dA, dorsal fin 
proceeds, the segmentation {14s 2, yelksacs at, Salk of yolk 
cavity is obliterated. The case By mene Ot niet it is ed 2 
floor of the gut is formed by ee ee ee 
infolding of the lateral walls. 
Along the mid-dorsal line of the epiblast a medullary 
groove appears—the beginning of the central nervous 
system. Its sides afterwards arch towards one another, and 
meet to form a medullary canal (Fig. 290, #.c.). A posterior 
communication between this dorsal nervous tube above and 
the ventral alimentary tube persists for some time as the 
neurenteric canal (Fig. 290, 7e.c.). 
