286 



THE EVOLUTION OF MAN. 



tebrate, the Amphioxus, this persists throughout life in this 

 very simple form, and permanently constitutes the whole 

 internal skeleton (Fig. 151, i,vol. i. p. 420 ; Plate XI. Fig. 15). 

 But even in the Mantle Animals (Tunicata), the nearest 

 invertebrate allies of Vertebrata, we find this same noto- 

 chord; transitorily in the transient larval tail of Ascidia 

 (Plate X. Fig. 5, cA) ; permanently in the Appendicularia 

 (Fig. 162). The Mantle Animals, as well as the Acrania, 

 have undoubtedly inherited the notochord from a common 

 worm-like parent-form, and these primaeval worm ancestors 

 are the Chorda Animals (Chordonia, p. 91). 



Long before any trace of a skull, limbs, etc., appears in the 

 human embryo or in that of any of the higher Vertebrates 

 in that early stage when the whole body is represented only 

 .A by the lyre-shaped germ-discin the cen- 

 tral line of this latter, directly under the 

 primitive groove or medullary furrow, ap- 

 pears the simple chorda dorsalis. (Of. Figs. 

 84-87, vol. i. pp. 297, 298, surface view; 

 Figs. 66-70, 89-93, transverse section ; also 

 Plates IV., V, ch.} As a cylindrical chord it 

 traverses the longitudinal axis of the body, 

 and is equally pointed at both ends. The 

 cells which compose the chord (Fig. 257, &) 

 come, in common with all the other cells of 

 the skeleton, from the skin-fibrous layer. 

 They most resemble certain cartilage cells ; 

 a special " chordal tissue " is often said to 

 exist; but this must not be regarded as 

 more than a special form of cartilaginous tissue. At an 

 early period the notochord envelopes itself in a structureless 

 sheath (a) as clear as glass, which is secreted by its cells. 



Pio. 257. For- 



lion of notochord 

 (chorda dorso2is) of 

 an embryo sheep : 

 a, sheath ; b, cells. 

 (After Koelliker.) 



