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, M, 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, EN, 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. 



