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, di) ; 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 

 $ by the lyre-shaped germ-disc in the cen- 

 tral line of this latter, directly under the 

 primitive groove or medullary furrow, ap- 

 pears the simple chorda dorsalis. (Cf. Figs. 

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

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

 Plates IV., V., c/i.) 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, 6) 

 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. 



FIG. 257. For. 

 tion of notochord 

 (chorda, dorsalis) of 

 an embryo sheep : 

 a, sheath ; b, cells. 

 (After Koelliker.) 



