192 DEVELOPMENT OF FISHES 
passing down the back of the embryo ; the notochord is aris- 
ing from the differentiating cells of the roof of the gut. In 
the cross-section shown in Fig. 214, the subsequent con- 
ditions of these structures may be seen; the medullary 
nerve cord, JV, is now in section elliptical, separated dor- 
sally from the ectoderm, and its cellular elements are of 
more uniform size, arranged with bilateral symmetry, its 
central lumen having not as yet appeared; the notochord, 
now constricted off from the wall of the gut, takes upon 
it its characteristic form and structure. It is, however, 
in the differentiation of the walls of the gut that this 
section is of especial interest; the gut is seen to have 
greatly enlarged, and at the expense of the yolk material ; 
its lining membrane, entoderm, ZW, is now directly ap- 
posed to the outer germ layer, ectoderm, EC. The middle 
germ layer, mesoderm, MES,—out of which cartilage, 
muscular and connective tissue, are formed, —is now seen 
taking its origin as paired evaginations of the dorsal wall 
of the gut. The mesoderm shortly loses its connection 
with the entoderm, and by the rapid increase of its cellular 
elements rapidly invests the remaining embryonic struct- 
ures ; its segmental character may be seen in the surface 
view shown in Fig. 210, its dorsal portions appearing as 
the primitive segments. 
Later developmental stages are shown in the sagittal 
sections, Figs. 211, 212. These may best be compared 
with Fig. 209. In Fig. 211 the head end of the body has ~ 
greatly elongated, and with it the gut cavity has dilated ; 
entoderm is now composed of very minute cells, whose 
nuclei are suggested by dots; the yolk has become more 
definitely restricted to the region of the hinder gut; the 
blastopore is still seen; at its lips the germ layers are 
alone fused. 
