206 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, JZ 
heart, H, gill slits, G.S, brain, and optic vesicles are broadly 
spread out: the fourth ventricle at J/C, the pronephros 
at PN, the primitive segments at PS. In the tail region 
the medullary folds appear at J/, the pronephric duct at 
PN, the neurenteric canal at VC. 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 WC; 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 line; the mesoderm, J7ES, 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, ¥, elasmobran- 
