THE MUSCULAR SYSTEM. 67 I 



ating rather granular columnar cells, enclosing a small kernel of spherical 

 cells. They are not, as may be seen in the above figure, completely 

 separated from the ventral (or lateral as they are at this period) parts of the 

 mesoblastic plate, and the dorsal and outer layer of the cortex of the 

 somites is continuous with the somatic layer of mesoblast, the remainder of 

 the cortex, with the central kernel, being continuous with the splanchnic 

 layer. Towards the end of the second and beginning of the third day the 

 upper and outer layer of the cortex, together probably with some of the 

 central cells of the kernel, becomes separated off as a muscle-plate (fig. 1 16). 

 The muscle-plate when formed (fig. 117) is found to consist of two layers, 

 an inner and an outer, which enclose between them an almost obliterated 

 central cavity ; and no sooner is the muscle-plate formed than the middle 

 portion of the inner layer becomes converted into longitudinal muscles. 

 The avian muscle-plates have, in fact, precisely the same constitution as 

 those of Elasmobranchii. The central space is clearly a remnant of the 

 vertebral portion of the body cavity, which, though it wholly or partially 

 disappears in a previous stage, reappears again on the formation of the 

 muscle-plate. 



The remainder of the somite, after the formation of the muscle-plate, 

 is of very considerable bulk ; the cells of the cortex belonging to it lose 

 their distinctive characters, and the major part of it becomes the vertebral 

 rudiment. 



In Mammalia the history appears to be generally the same as in Elas- 

 mobranchii. The split which gives rise to the body cavity is continued to 

 the dorsal summit of the mesoblastic plates, and the dorsal portions of the 

 plates with their contained cavities become divided into somites, and are 

 then separated off from the ventral. The later development of the somites 

 has not been worked out with the requisite care, but it would seem that they 

 form somewhat cubical bodies in which all trace of the primitive slit is lost. 

 The further development resembles that in Birds. 



The first changes of the mesoblastic somites and the forma- 

 tion of the muscle-plates do not, according to existing statements, 

 take place on quite the same type throughout the Vertebrata, 

 yet the comparison which has been instituted between Elasmo- 

 branchs and other Vertebrates appears to prove that there are 

 important common features in their development, which may be 

 regarded as primitive, and as having been inherited from the 

 ancestors of Vertebrates. These features are (i) the extension 

 of the body cavity into the vertebral plates, and subsequent 

 enclosure of this cavity between the two layers of the muscle- 

 plates ; (2) the primitive division of the vertebral plate into an 

 outer (somatic) and an inner (splanchnic) layer, and the formation 

 of a large part of the voluntary muscular system out of the inner 



