THE LATER EMBRYONIC DEVELOPMENT. 



67 



MS 



(Fig. 33) they acquire the >-like shape so characteristic of the 

 adult (Fig. 11. x). 



The walls of the somites soon u-ndergo important changes. 

 At the time of separation from the archenteron (Fig. 29, MS), 

 the wall of each somite consists of a single layer of cells, 

 somewhat irregular in shape and size, but showing no marked 

 differences in different parts. As the somites extend down the 

 sides of the body they become 

 somewhat triangular in trans- 

 verse section. In each somite 

 there may now be distinguished 

 (Fig. 32) an outer or parietal 

 wall, next the external epiblast; a 

 visceral wall, in contact with the 

 hypoblast of the archenteron ; 

 and a notochordal wall, forming 

 the base of the triangle, and in 

 contact with the notochord and 

 the nerve cord. The cells of the 

 parietal and visceral walls are 

 slightly flattened, but show no 

 special peculiarities ; those of the 

 notochordal wall, on the other 

 hand, show marked changes. 

 Each cell (Fig. 32, ML) is much flattened dorso-ventrally, and 

 elongated in a direction parallel to the axis -of the embryo 

 (Fig. 31); and is undergoing changes by which it becomes con- 

 verted into a muscle cell or fibre. This differentiation of muscle 

 cells begins at a stage with about nine pair of somites, and 

 proceeds rapidly ; the muscles, at a stage with eleven pairs of 

 somites, beginning to contract and cause lateral undulations of 

 the body. The mass of muscle cells, formed in this way by 

 modification of the notochordal wall of a somite, is called a 

 myotome : the myotomes, being formed from the somites, are, 

 like these, arranged segmentally from their first appearance; 

 they increase rapidly in size, and become the great lateral muscles 

 or myotomes of the adult Amphioxus (Fig. 12, x). Each 

 muscle cell extends the whole length of the somite to which it 

 belongs. 



In the higher Vertebrates it will be found that the earliest 



F 2 



FIG. 82. Transverse section through 

 the middle of an Amphioxus 

 embryo with nine pairs of meso- 

 blastic somites, x 435. (After 

 Hatschek.) 



CH, notochord. I, spinal cord. 

 ML, muscle layer. MS, cavity of 

 mesoblastic somite. T, mesenteron. 



