VI.] THE MUSCLE-PLATES. 187 



it appears in part to persist. The somites on the second 

 day, as seen in a transverse section (Fig. 34, 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, 

 enclosing a small kernel of spherical cells. They are 

 not, as may be seen in the above figure, completely 

 separated from the ventral (or rather at this period 

 lateral) 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 dorsal and 

 outer layer of the cortex, together probably with some 

 of the central cells of the kernel, becomes separated 

 off as a special plate. From this plate, which is 

 shewn in the act of being formed in Fig. 64, ms, the 

 greater part of the voluntary muscular system of the 

 trunk is developed. When once formed the muscle- 

 plates have in surface views a somewhat oblong form, 

 and consist of two layers, an inner and an outer, which 

 enclose between them an almost obliterated central 

 cavity (Fig. 65, mp). No sooner is the muscle-plate 

 formed than the middle portion of the inner layer be- 

 comes converted into longitudinal muscles. The central 

 space in the muscle-plates is clearly a remnant 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. 



It is especially interesting to note that the first 



