68 AMPHIOXUS. 



muscles to appear in the development of the embryo correspond 

 in mode of formation, and in relations, to the myotomes of Am- 

 phioxus. The formation of muscles, as indeed of all other tissues, 

 by direct modification of epithelial cells, is a further point of very- 

 great and general interest, indicating that the epithelial cell 

 is a more primitive type of structure than muscle, connective 

 tissue, nerve tissue, or any of the other histological elements of 

 which the body of an adult animal is composed. 



The cavities of the somites give rise to the ccelom or body 

 cavity of the adult. After their separation from the archenteron 

 they are completely closed, and remain so for some time ; the 

 anterior and posterior walls of adjacent somites becoming closely 

 applied to one another, and forming septa which separate the 

 cavities of successive somites from one another (Fig. 31). To- 

 wards the close of the embryonic period, the ventral portions of 

 these septa disappear, so that the somites open into one another : 

 and the body cavity, which up to this time has been represented 

 by a series of isolated chambers, now becomes continuous from 

 end to end of the animal. The dorsal portions of the somites, 

 however, remain separate from one another throughout life. 



The first somite (Fig. 27, s i) is a little distance from the an- 

 terior end of the body : from its anterior and dorsal border, at a 

 stage with about nine pairs of somites, a hollow conical process is- 

 given off towards the anterior end of the embryo (Figs. 30, 31) ^ 

 the walls of this process undergo changes similar to those de- 

 scribed above as occurring in the body of the somite itself. 



At the time of their first appearance the somites are paired ; 

 the two somites of each pair being exactly opposite each other, 

 and the whole embryo being bilaterally symmetrical. At a stage 

 with nine pairs of somites this symmetry becomes disturbed (Fig. 

 31), the somites of the right side becoming situated a little behind 

 the corresponding ones of the left side, and ultimately alternating 

 with these. This curious lateral asymmetry is preserved in all 

 the later stages, and in the adult animal as well. The fact that 

 the somites are at first symmetrically arranged shows that it is 

 a secondary and not a primitive feature, and the further fact 

 that it appears just at the time when the great lateral muscles 

 are being formed, and are coming into use for swimming, suggests 

 that the explanation of the asymmetry is to be found in some 

 mechanical advantage gained by the alternating arrangement of 



