DEVELOPMENT OF SHARK 



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 

 of the tail folds, CF. 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, MES ; 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, i.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. 



