2o6 DEVELOPMENT OF FISHES 



the medullary furrow of the embryo has here been the 

 deepest, and has been bridged over by a coalescence of 

 its margins. At the anterior end of the embryo the 

 inner, EN, and middle, MES, germ layers become 

 greatly thinned, in the region where the heart is shortly 

 to arise. 



The next stage of development is represented in Figs. 

 259, 260, showing front and hinder regions of the same 

 embryo. The curiously flattened mode of growth char- 

 acteristic of the sturgeon is here very apparent ; the 

 embryo has surrounded over three-fourths of the egg's 

 circumference, yet has not risen above its surface curva- 

 ture ; the head region is especially flattened ; mouth, M, 

 heart, H, gill slits, GS, brain, and optic vesicles are broadly 

 spread out : the fourth ventricle at MC, the pronephros 

 at PN, the primitive segments at PS. In the tail region 

 the medullary folds appear at M, the pronephric duct at 

 PN, the neurenteric canal at NC. A favourable section 

 through the hinder body region of an early embryo is 

 shown in Fig. 261 ; it illustrates the mode of origin of the 

 following structures : the notochord as an axial thickening 

 of entoderm, EN, immediately under MC \ the medullary 

 canal, as an infolding of (an under, or formative layer of) 

 the ectoderm, its sides, folding over dorsally, coming to fuse 

 in the median Hne; the mesoderm, MES, as in sharks, 

 arising (partly) from the entoderm on either side of the 

 notochord. 



The later stage, shown in Figs. 262 and 263, may be con- 

 trasted with Figs. 259 and 260; the head region, though 

 still greatly flattened out, is now rising above the surface ; 

 the trunk region is becoming prominent ; the tail is bud- 

 ding out, and separating from the egg surface ; sense 

 organs are well outlined, and pectoral fins, P, elasmobran- 



