DEVELOPMENT OF SHARK 197 
portion has been roofed over by the coalesced sides, and 
the process of enclosing the groove is being continued 
anteriorly, although the head end of the embryo is now 
flattened out as the prominent cephalic plate. 
In the stage figured in 227, the form of the embryo has 
‘been acquired; the head in the manner already outlined, 
the tail by the coalescence and subsequent outgrowth 
s of the tail folds; C# The entire embryo now rises above 
the blastoderm, as this continues to enclose the yolk. In 
the figure the yolk has thus been more than half enclosed ; 
its final appearance is seen in the oval space outlined by a 
dotted line behind the embryo. 
The origin of the germ layers is not as readily traced 
as in the Cyclostome. Ectoderm is the most clearly 
marked ; even in the blastula (Fig. 221) it has appeared 
‘as an outer single-celled stratum clearly differentiated 
from the underlying cells. Entoderm is only to be 
seen on the dorsal wall of the ccelenteron: the ventral 
entoderm (cf. Fig. 222) is merged with the yolk. Meso- 
derm takes its origin from the inner layer on either side 
of the median line, but it arises as a solid cell mass 
instead of as the pouch-like diverticula in Petromyzon. 
Cross-sections of an embryo represented by Fig. 224 
have been figured in Figs. 228 and 229; the former is of 
the hinder region and illustrates the mode of growth of the 
mesoderm, J7ES; the latter across the head region, 
shows that in this region the mesoderm is separated 
from the inner layer. Both sections show the simple 
character of the medullary groove, and the latter section 
_ the mode of origin of the notochord, CH, z.e. as an axial 
_ thickening of the entoderm. 
An embryo of about the stage of Fig. 227 is extremely 
delicate and may readily be viewed as a transparent object. 
