0-20 



ANATOMY OF VERTEBRATES. 



428 



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 ' laminaj 

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

 same time forming the notochordal rudiment of the vertebral 

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

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

 not aifect n. The polyhedral cells extend the vertebral layer 

 on each side of the ' primitive trace,' which also increases 

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

 rise at their outer margins, approximate, and ultimately unite 

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

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

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

 the investing membrane, a ; and tlie cephalic 

 end of the embryo enlarges and raises itself 

 from the yolk-bed. A section of the ovum 

 just jirior to the coalescence of the ' lamina} 

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

 428, shows, a, the dark investing membrane, 

 or ' cambium : ' b, the museulo-tegumentary 

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

 myelonal columns ; c, the notochord ; n, the 

 blastema, in which cai'tilaginous rudiments 

 of the ncurapophyses begin ; /(, the cavity, 

 beneath the germ due to solution of the 

 yolk-sul)stance. On the ventral aspect of the embryo lavers 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 

 cmliryo begin to be developed 

 after the main basis of tlie neural 

 and vertebral parts has been 

 establislicd. 



Figure 429, a, gives a view of 



the embryo of the Frog from 



the dorsal aspect, showing the 



myelonal cohunns at the jieriod 



of their meeting above the myelonal canal and tlie commcncino- 



cnceplialic expansion, tlie extension oi" the neuro-vcrtebral tract's 



outward, and the indication of luemal arches of the cephalic 



clioTi nf yi)]]; anil cinl)i-yo, 

 Froi:', iiia:^'u. lxxi\'. 



429 



Embryo oi: Iho Fnig. coxxxvut. 



