4 8 



THE MESOBLAST. 



somatic layer adjoining the epiblast. 

 Between these two layers there is 

 soon developed in the region of the 

 head a well-marked cavity (fig. 20 A, 

 //) which is subsequently continued 

 into the region of the trunk, and 

 forms the primitive body cavity, equi- 

 valent to the cavity originating as an 

 outgrowth of the archenteron in Am- 

 phioxus. The body cavities of the 

 two sides are at first quite inde- 

 pendent. 



Coincidentally with the appear- 

 ance of differentiation into somatic 

 and splanchnic layers the mesoblast 

 plates become in the region of the 

 trunk partially split by a series of 

 transverse lines of division into meso- 

 blastic somites. Only the dorsal 

 parts of the plates become split in 

 this way, their ventral parts remain- 

 ing quite intact. As a result of this 

 each plate becomes divided into a dorsal portion adjoining the 

 medullary canal, which is di- 

 vided into somites, and may 

 be called the vertebral plate, 

 and a ventral portion not so 

 divided, which may be called 

 the lateral plate. These two 

 parts are at this stage quite 

 continuous with each other ; 

 and the body cavity origi- 

 nally extends uninterrupted- 

 ly to the summit of the ver- 

 tebral plates (fig. 21). 



The next change results 

 in the complete separation 

 of the vertebral portion of 

 the plate from the lateral 



*7> 



FIG. 21. TRANSVERSE SEC- 

 TION THROUGH THE TRUNK OF 

 AN EMBRYO SLIGHTLY OLDER 

 THAN FIG. 28 E. 



nc. neural canal ; pr. posterior 

 root of spinal nerve ; x. subnoto- 

 chordal rod ; ao. aorta ; sc. soma- 

 tic mesoblast ; sp. splanchnic me- 

 soblast ; mp. muscle-plate ; mp' . 

 portion of muscle-plate converted 

 into muscle ; Vv. portion of the 

 vertebral plate which will give 

 rise to the vertebral bodies ; al. 

 alimentary tract. 



FIG. 22. HORIZONTAL SECTION THROUGH 

 THE TRUNK OF AN EMBRYO OF ScYLLIUM 

 CONSIDERABLY 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. 



