THE MUSCULAR SYSTEM. 



553 



Fig. 378. Hoiuzontal section through 

 the tkunk of an embryo of scyllium con- 

 siderably younger than 28 f. 



The section is taken at the level of the 

 notochord, and shews the separation of the 

 cells to form the vertebral bodies from the 

 muscle-plates. 



ch. notochord; ep. epiblast; Vr. rudiment 

 of vertebral body; mp. muscle-plate; mp'. 

 portion of muscle-plate already differentiated 

 into longitudinal muscles. 



somites the sjjlit between the somatic and splanchnic layers becomes to a 



large extent obliterated, though in 



the anterior somites it appears in 



}>art to persist. The somites on the 



second day, as seen in a trans vei-se 



section (fig. 115, P.v.), are somewhat 



quadrilateral in form but broader 



than they are deep. 



Each at that time consists of a 

 somewhat thick cortex of radiating 

 rather granular columnar cells, en- 

 closing a small kernel of spherical 

 cells. They are not, as may be 

 seen in the above figure, completely 

 separated from the ventral (or late- 

 ral 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. IIG). The muscle-plate when fox-med (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, pre- 

 cisely the same constitution as those of Elasmobi-anchii. The central 

 space is clearly a i*emnant of the vertebral portion of the body cavity, 

 which, though it wholly or partially disappears in a previous stage, re- 

 appears 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 mnjor 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 formation 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 Elasmobranchs and 

 other Vertebrates appears to prove that there are important common 



