MOKPHOLOGY OF CKANIAL MUSCLES IN SOME VERTEBRATES. 177 



the added myotomes necessarily lie beneath the anterior body 

 myotomes, with their upper ends at varying distances from the 

 ventral surface of the latter, with which they may or may 

 not agree in antero-posterior extent. In Ran a esculent a, 

 with five to six somites, the most anterior trunk myotome 

 lies above the third branchial, i.e. the first added myotome ; 

 a little later there is a relative shifting forward of the trunk 

 myotomes, so that the first trunk myotome comes to lie above 

 the first branchial myotome (see Coming's figures, Taf. ix, 

 figs. 7, 11, and 2G). In Scyllium this overlapping is, second- 

 arily, antedated in development, so that the first branchial 

 myotome never lies in front of the first trunk myotome. In 

 the rabbit it does so. 



The backward growth of the head into the body by this 

 process of metameric increase leads to the nou-development 

 of the coelomic portion of the anterior trunk-somites. In 

 Scyllium, for instance, the first four trunk-somites have no 

 coelomic portions. 



In the head, as in the body, each myotome is at first an 

 epithelium-lined cavity, which is continuous below with the 

 coelom. The differences between the myotomes of the body 

 and those of the head are : (1) Whereas in the body the 

 myotomes extend dorsally to the mid-dorsal line, and, subse- 

 C[uently, ventrally outside the somatopleure wall of the coelom 

 to the mid-ventral line, neither of these secondary phenomena 

 takes place in the head ; (2) whereas the coelom is large in 

 the body and contains the alimentary canal and other viscera, 

 it is small in the head and lies entirely ventral to the alimen- 

 tary canal; (3) in correlation with this the myotomes of the 

 body lie, at first, dorso-lateral to the alimentary canal, those 

 of the head lie dorso-lateral and lateral to it ; (4) the sclero- 

 tome elements of the body-myotomes are formed by ventro- 

 medial outgrowths, those of the head from scattered cells 

 given off from the premandibular, mandibular, and hyoid 

 myotomes. These differences are intimately associated with 

 the development of gill-clefts and a cranium in the head, of 

 viscera and a vertebral column in the body. 



