THE MUSCLE-PLATES. 



a mass of cells, placed between the muscles and the notochord 

 ( Vr\ These cells form the commencing vertebral bodies, and 

 have at first (fig. 378) the same segmentation as the somites 

 from which they sprang. 



After the separation of the vertebral bodies from the somites 

 the remaining parts of the somites may be called muscle-plates ; 

 since they become directly converted into the whole voluntary 

 muscular system of the trunk (fig. 379, mp}. 



According to the statements of Bambeke and Go'tte, the Amphibians 

 present some noticeable peculiarities in the development of their muscular 

 system, in that such distinct muscle-plates as those of other vertebrate types 

 are not developed. Each side-plate of mesoblast is divided into a somatic 

 and a splanchnic layer, continuous throughout the vertebral and parietal 

 portions of the plate. The vertebral portions (somites) of the plates soon 

 become separated from the parietal, and form independent masses of cells 

 constituted of two layers, which were originally continuous with the 

 somatic and splanchnic layers of the parietal plates (fig. 79). The outer or 

 somatic layer of the vertebral plates is formed of a single row of cells, but 

 the inner or splanchnic layer is made up of a kernel of cells on the side of 

 the somatic layer and an inner layer. The kernel of the splanchnic layer 

 and the outer or somatic layer together correspond to a muscle- plate of other 

 Vertebrata, and exhibit a similar segmentation. 



Osseous Fishes are stated to agree with Amphibians in the development 

 of their somites and muscular 

 system 1 , but further observations 

 on this point are required. 



In Birds the horizontal split- 

 ting of the mesoblast extends at 

 first to the dorsal summit of the 

 mesoblastic plates, but after the 

 isolation of the somites the split 

 between the somatic and splanch- 

 nic layers becomes to a large ex- 

 tent obliterated, though in the an- 

 terior somites it appears in part 

 to persist. The somites on the 

 second day, as seen in a trans- 

 verse section (fig. 115, P.?'.), are 

 somewhat quadrilateral in form 

 but broader than they are deep. 



Each at that time consists of 

 a somewhat thick cortex of radi- 



FlG. 378. HORIZONTALSECTION THROUGH 

 THE TRUNK OF AN EMBRYO OF SCYLL1UM 

 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. 



1 Ehrlich, " Ueber den peripher. Theil d. Urwirbel." Archiv f. mikr. Anal., 

 Vol. XI. 



