THE ORGANS OF THE MIDDLE GERM-LAYER. 347 



The characteristic feature of these consists in their being simple somatimes 

 cubical, sometimes cylindrical, sometimes thread-like epithelial cells, the 

 outer ends of which ordinarily reach the surface of the epithelium and are 

 here provided with cilia, whereas their basal ends lie upon the sustentative 

 lamella (Stiitzlamelle) of the body and are there differentiated into one or 

 several either smooth or transversely striped muscle-fibrillas. Inasmuch as the 

 fibrillEe of numerous cells lie parallel and close to one another, muscle-lamella 

 arise, by the activity of which the changes in the form of the body are 

 produced. In Ccelenterates loth the outer and the inner germ-layers can 

 develop muscle-cells. 



When one turns to the Vermes it is seen, in those groups in which a body- 

 cavity (an enterocoel) is formed by an infolding of the inner germ-layer, that 

 the parietal wall of the body-cavity, or the parietal lamella of the middle 

 germ-layer, has assumed the production of the entire musculature of the 

 trunk. Here also, for example in the Chaetognatha, etc., the epithelial cells 

 differentiate at their basal ends, which are directed toward the surface of 

 the body, a lamella of muscle-fibrillse, whereas their other ends bound the 

 body-cavity. Thus from the lower to the higher animals the capability of 

 producing muscles is, with the progressive differentiation of the body, more and 

 more restricted to a limited special territory of the total epithelial investment 

 of the tody. 



This process has proceeded furthest in the Vertebrates, for in them the 

 musculature of the trunk is no longer furnished by the whole parietal lamella 

 of the middle germ-layer, but by only a small detached part of it, the primitive 

 segments. Consequently in Vertebrates the musculature spreads out from a 

 small region where it originates, distributes itself first in the trunk, and then 

 from the latter grows out into the extremities. 



In the Vertebrates we recognised two different forms of voluntary musculature, 

 the muscle-layer (and the Muskelkastchen derivable from it) and the primitive 

 bundle (Muskelprimitivbiindel). Parallels to this are found in the Inverte- 

 brates, both in Coalenterates and in Worms. In Ccelenterates both forms are 

 derived from the primitive smoothly outspread muscle-lamella by the forma- 

 tion of folds, and are to be explained in the same way as the formation of those 

 folds which in epithelial lamellae play such an important part in the origin of 

 the most various organs. When certain tracts of a muscle-lamella are called 

 upon to execute additional labor, this can be effected only by an increase 

 in the number of the fibrillEe lying parallel to one another. But a greater 

 number of fibrillse can be brought into a circumscribed territory only in one or 

 the other of two ways : either by their coming to lie in several layers one above 

 another, or if the more simple arrangement of lying side by side is to be 

 retained by the folding of the muscle-lamella. The folding exhibits two 

 modifications. Sometimes there are produced parallel daughter-lamellae placed 

 side by side and perpendicular to the mother-lamellas ; sometimes the folded 

 lamella? become wholly detached from the parent-layer and converted into 

 muscle-cylinders, which imbed themselves in the underlying sustentative 

 lamella. 



With the conception here presented of the origin of the transversely striped 

 muscle-fibres of Vertebrates, it must be assumed as very probable that 

 subsequently an increase in their number will take place as a result of 

 constriction and detachment into two parts, as was first maintained by 

 WEISMANN. 



