350 EMBRYOLOGY. 



chorda. The differentiation of muscle-fibres follows at a much later 

 stage of development, in comparison with Amphioxus and the Cyclo- 

 stomes. The inner layer of the muscle-plate is converted into 

 longitudinal muscle-fibres, the outer contributes to the formation of 

 the corium (fig. 202). 



Let us now consider somewhat more in detail the original condition 

 of the musculature. It shows at the beginning complete uniformity 

 in all classes of Vertebrates. Everywhere there appears as its 

 foundation a very simple system of longitudinal contractile fibres, 

 which first appear near the chorda and neural tube and spread 

 themselves out thence dorsally toward the back and ventrally in the 

 wall of the belly. The muscle-mass is divided in a very uniform 

 manner into separate segments or myomeres by means of connective- 

 tissue partitions (ligamenta intermuscularia), which run transversely 

 or obliquely to the vertebral column. In the lower Vertebrates this 

 condition persists, in the higher ones it gives place to a more 

 complicated arrangement. 



We cannot recount more precisely the details of the manner in 

 which the groups of muscles of the higher Vertebrates, so various in 

 form and position, are derived from the original system, especially 

 since this field of embryology has been as yet little cultivated ; let 

 attention be here called to only two points, which come in question 

 in the differentiation of the groups of muscles. 



First, a very important factor is furnished in the development of 

 the skeleton, which with its processes affords points of attachment 

 for muscle-fibres. Some of these find in this way opportunity to 

 detach themselves from the remaining mass. 



Secondly, the development of the limbs, which arise as protuberances 

 at the side of the trunk (figs. 157 and 158), operates toward a 

 greater differentiation of the musculature. The limbs likewise ac- 

 quire their musculature, which in the higher Vertebrates has a very 

 complicated arrangement, from the primitive segments, as has been 

 learned through the investigations of KLEINENBERG and BALFOUR, as 

 well as recently through the very convincing accounts of DOIIRN. 



In the Selachians, in which the processes are most clearly recog- 

 nisable, cell-buds sprout forth out of the still hollow primitive segments 

 and grow into the paired and median fins, in which they become meta- 

 morphosed into muscle- fibres. The fact that always from a large 

 number of primitive segments buds are given off to a fin is worthy of 

 attention, because it demonstrates that the extremity is a structure 

 that belongs to several somites. 



