VERTEBRATE NATURE OF THE LaNCELET. 449 



closes. The future permanent mouth is formed only second- 

 arily, from the outside, and at the opposite end of the body 

 (near ss, Fig. 12). At this point, a groove-like depression 

 originates in the outer skin (epidermis), and this grows 

 inwards and breaks a way through into the closed intestine. 

 Similarly, the anal opening forms behind (in the neighbour- 

 hood of the closed gastrula-mouth). We saw that in Man 

 and in all higher Vertebrates mouth and anus originate 

 as shallow grooves in the outer skin ; and that these also 

 break through inwards, thus gradually communicating with 

 both blind ends of the intestinal tube. (Cf. p. 338.) 



Between the intestinal and the nerve tubes we find the 

 notochord as a cartilaginous cylindrical rod, traversing 

 the entire length of the larval body. On each side of the 

 notochord lie the muscle-plates, already broken up into 

 a number of separate pieces, or primitive vertebral seg- 

 ments (10 to 20 on each side) ; these are separated from 

 each other by simple oblique, parallel lines of demar- 

 cation. In the fully-formed animal each of these divid- 

 ing lines describes an acute angle forwards (Plate XI. Fig. 

 15, r). The number of separate muscle-plates indicates 

 the number of metamera of which the body consists. At 

 first this number is small, but it afterwards increases 

 considerably in the direction from front to rear. This 

 is owing to that same terminal budding in virtue of 

 which the chain of primitive vertebral segments grows 

 in the human embryo. Here, too, the foremost metamera 

 are the oldest, and the terminal ones the most recent. To 

 each metameron corresponds a definite segment of the 

 medullary tube and a pair of spinal nerves, which pass from 

 it out to the muscles and to the skin. Of all the organic 



