44^ THE EVOLUTION OF MAN. 



a cylindrical cord, pointed at both ends, and composed of 

 large, light-coloured vesicular cells, appears in the middle 

 line of the skin-fibrous layer, directly over the intestinal 

 tube (d) and below the nerve-tube (m), (and therefore 

 along the long axis of the body). This is the chorda 

 dorsalis, or notochord (Plate X. Fig. 11, 12, ch). The lateral 

 portions of the skin-fibrous layer, which lie on both sides 

 of the notochord, and which we may in this case also call 

 " side-layers," or " side-plates," split into two strata, a thin 

 leather-skin (corium) and an underlying muscle-plate. 

 The latter soon breaks up into a number of homogeneous 

 sections, lying one behind another. These are the side 

 muscles of the trunk, which indicate the first articulation 

 or metameric structure of the body (Fig. 12, mp). 



By these separations the gastrula of the Amphioxus has 

 changed into a vertebrate body of the simplest form, with 

 the characteristic disposition of the fundamental organs 

 which belongs exclusively to Vertebrates. Directly below the 

 skin we find, at the dorsal side of the medullary tube, on the 

 ventral side of the intestinal tube, and between the two 

 tubes, the firm axis of the body, the notochord; and, on 

 either side of this, the regular series of muscle-plates. If 

 we now look at the larva of the Amphioxus from one side 

 (Plate X. Fig. 11, 12), we see that on the top lies the 

 medullary tube, still open anteriorly (ma) ; directly under 

 this lies the strong notochord (ch), and under this the 

 much broader intestinal tube (d). The latter also has an 

 opening at one end, the original mouth of the gastrula (o). 

 It is, however, a very singular and important fact that this 

 primitive mouth does not afterwards become the permanent 

 mouth-opening of the Amphioxus. On the contrary, it soon 



