26 CHORDATE ORGANIZATION n. 3- 



considering the body as consisting of two tubes, the outer skin 

 (ectoderm) and the inner alimentary canal (endoderm), with a space 

 between (the coelom) lined by a third layer (the mesoderm). This 

 arrangement is actually found during the course of the development 

 (Fig. 18). The mesoderm at first forms thin layers, the somatopleure 

 applied to the outer body wall and the splanchnopleure to the gut. 

 Very soon the inner layer becomes much thickened where it is applied 

 to the nerve-cord and notochord, and here it forms the myotomes, or 

 muscle-blocks. In this dorsal part of the mesoderm the coelom, 

 known here as the myocoele, soon becomes obliterated, leaving the 

 ventral splanchnocoele around the gut. Besides the muscle that forms 

 in the myotomes, non-myotomal muscles develop in the somatopleure 

 and splanchnopleure. These are not divided into segments and are 

 innervated by the dorsal nerve-roots, the ventral roots supplying only 

 the myotomes. 



4. Movement of amphioxus 



The adult myotomes are blocks of striated muscle-fibres, running 

 along the body, separated by sheets of connective tissue, the myo- 

 commas. This repetition or segmentation is characteristic of the 

 organization of all chordates. The myocommas do not run straight 

 down the body from dorsal to ventral side but are V-shaped (Fig. 5). 

 However, each muscle-fibre runs straight from before backwards, and 

 the contraction of the whole myotome therefore bends the body. A 

 full discussion of the means by which forward motion is achieved by 

 such a system will be given later (p. 133). Essentially, contraction of 

 the myotomes results in transverse motion of the body inclined at 

 varying angles in such a way as to result in forward propagation. Each 

 myotome must therefore contract after that in front of it — the effect 

 being to produce an S-bend that moves backwards through the water 

 as the fish moves forward. 



For our present purpose the point is that the contraction is serial, 

 that is to say, it depends on the breaking up of the longitudinal muscle 

 into blocks. It was probably the need for division of the musculature 

 that led to the development of the segmentation, and this, affecting 

 primarily the muscles, has come to influence a great part of chordate 

 organization. 



Contraction of the longitudinally arranged muscle-fibres will only 

 produce a sharp bending of the body if there is no possibility of short- 

 ening of the whole. To prevent telescoping, an incompressible and 

 elastic rod, the notochord, runs down the centre of the body. It 



