THE M UtiCL E-PLA TEX. 



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 (nip") 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 whole 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 ( IV). 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 

 (tig. 379, >np). 



According to the statements of Bambeke 

 and Giitte, the Amphibians present some 

 noticeable peculiarities iu the development 

 of their muscular system, in that such distinct 

 muscle-plates as those of other vertebrate 

 tvpes 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 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 la> er and 

 an inner layer. The keriu-1 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 1 , but further observations on this point are 



required. 



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 



1 Ehrlich, " Ueber den peiiplicr. Tlicil d. T T r\\hbel." An-hic f. inici: An:it., Vol. xi. 



al 



FIG. 377. TRANSVERSE SEC- 

 TION THRoniH THE TRUNK OF AN 

 KJIHRYO SLIGHTLY OLDER THAN 

 FIG. 28 E. 



in 1 , neural canal ; jir. pos- 

 terior root of spinal nerve ; .r. 

 subnotochordal rod ; ao. aorta; 

 sc. somatic mesoblast ; sp. 

 sphuicLnic mesoblast ; nip. 

 muscle-plate ; nip', portion of 

 muscle -plate converted into 

 muscle; J>. portion of the 

 vertebral plate which will give 

 rise to the veitebral bodies; al. 

 alimentary tract. 



