552 



THE M USCL E-FLA TE8. 



Very early (fig. 377) the inner or splanchnic wall of the somites 

 loses its simple constitution, owing to the middle part of it under- 

 going peculiar changes. The meaning of the changes is at once 

 shewn by longitudinal horizontal sections, which prove (fig. 378) that 

 the cells in this situation {mp) have become extended in a longi- 

 tudinal direction, and, in fact, form typical spindle-shaped embryonic 

 muscle-cells, each with a large nucleus. Every muscle-cell extends 

 for the wliole length of a somite. The inner layer of each somite, 

 immediately within the muscle-band just described, begins to pro- 

 liferate, and produce a mass of cells, placed between the muscles 

 and the notochord (Fr). 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 be- 

 come directly converted into the whole 

 voluntary muscular system of the trunk 

 (fig. 379, mp). 



According to the statements of Bambeke 

 and Giitte, 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 ai'e not developed. Each side-plate of 

 mesoblast is divided into a somatic and a 

 s})lanchnic layer, continuous throughout the 

 verteVjral and parietal portions of the plate. 

 Tlie vertebral portions (somites) of the plates 

 soon become separated from the parietal, and 

 form independent masses of cells c instituted 

 of two layers, which were originally con- 

 tinuous 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 Verte- 

 brata, and exhibit a similar segmentation. 



Osseous Fishes are stated to agree with 

 Amphibians in the development of their 



somites and muscular system', but further observations on this point are 



requii'ed. 



In Birds the horizontal splitting of the mesoblast extends at first to the 



dorsal summit of the mesoblastic plates, but after the isolation of the 



Fig. 377. Transverse sec- 

 tion THROUGH THE TRUNK OF AN 

 EMBRYO SLIGHTLY OLDER THAN 

 EIG. 28 E. 



nc. neural canal ; pr. pos- 

 terior root of spinal nerve ; x. 

 subnotochordal rod ; ao. aorta; 

 Hc. somatic mesoblast ; sp. 

 splancbnic mesoblast ; mp. 

 muscle plate ; mp'. poition of 

 muscle -plate converted into 

 muscle ; Vv. portion of the 

 vertebral plate whicli will give 

 rise to the vertebral bodies; al, 

 alimentary tract. 



1 Ehrlich, " Ueber den peripher. Tlieil d. Urwirbel." Archiv f. micr. Aitat., Vol. xi. 



