(5-20 



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 

 column, ib. e. The albuminous principle is concentrated 



in 



m 



gelatinous one in c : this chemical differentiation does 



n. The polyhedral cells extend the vertebral layer 



side of the ' primitive trace,' which also increases 



and 



the 

 not affect 

 on each 



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

 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. 

 Mean while 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 ' laminae 

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

 428, shows, a, the dark investing membrane, 

 or ' cambium : ' b, the musculo-tegumentary 



428 



layer, inclosing the whole yolk, v 

 myelonal columns ; c, the notochord 



n, 



Section of yolk and embryo, 

 Frog, ni.ign. lxxiy. 



429 



the 



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 hamial arches of the cephalic 



Embryo of the Frog, ccxxxyjh. 



