G20 



ANATOMY OF VERTEBRATES. 



largest at one end, and impressed by a slight longitudinal fissure. 

 The rudiments of the neural axis are first recognisable in the two 

 parallel longitudinal elevations ('primitive trace' or 'laminae 

 dorsales,' ib. m, n) bordering the fissure. Beneath these is at the 

 same time forming the notochordal rudiment of the vertebral 



O 



column, ib. c. The albuminous principle is concentrated in 

 m, the gelatinous one in c : this chemical differentiation does 

 not affect n. The polyhedral cells extend the vertebral layer 

 oil each side of the e primitive trace,' which also increases 

 in length : the neural columns, at first flat and horizontal, 



o ' 



rise at their outer margins, approximate, and ultimately unite 

 above, where they are covered by the peripheral cell-layer, a : 

 they are also defended by the nascent neurapophyses, ib. n. 

 Meanwhile the ( animal ' layer is extending laterally, ib. b, beneath 



the investing membrane, a ; and the cephalic 

 end of the embryo enlarges and raises itself 

 from the yolk-bed. A section of the ovum 

 just prior to the coalescence of the e laminae 

 dorsales ' to form the neural axis, as in fig. 

 428, shows, a, the dark investing membrane, 

 or f cambium : ' b, the musculo-tegumentary 

 layer, inclosing the whole yolk, v ; m, the 

 myelonal columns ; c, the notochord ; n, the 

 blastema, in which cartilaginous rudiments 

 of the neurapophyses begin ; h, the cavity, 

 beneath the germ due to solution of the 

 yolk-substance. On the ventral aspect of the embryo layers of 

 cells have been forming two parallel ridges projecting into the 

 yolk ; and the intermediate space is converted by liquefaction of 



cells into a primitive alimentary 

 groove. But all the systems and 

 organs for the support of the 

 embryo begin to be developed 

 after the main basis of the neural 

 and vertebral parts has been 

 established. 



Figure 429, A, gives a view of 

 the embryo of the Frog from 

 the dorsal aspect, showing the 

 myelonal columns at the period 



of their meeting above the myelonal canal and the commencing 

 encephalic expansion, the extension of the neuro-vertebral tracts 

 outward, and the indication of lucmal arches of the cephalic 



Section of yolk arid embryo, 

 Frog, magn. LXXIV. 



429 



B 



Embryo of the Frog, ccxxxvm. 



