346 EMBRYOLOGY. 



continually being formed more fibrillse (fig. 193), which gradually fill 

 the interior portion of the cylinder. Only in the axis of the latter 

 are there places left free, in which the small nuclei (ink) come to lie ; 

 these, formed by division of the single mother-nucleus, increase 

 considerably in number. Moreover, connective tissue with blood- 

 vessels now penetrates between the muscle-fibres or the primitive 

 bundles (pb), as the finished elements are subsequently called. 



If we consider from a general point of view the facts here presented, 

 which have been acquired in the study of the lower Vertebrates, 





Fig. 193. 

 Fig. 192. Cross section through the musculature of the trunk of a larva of Triton tseniatus 



5 days old. Magnified 500 diameters. 

 ink, Nuclei of muscle-cells ; mf, muscle-fibrillse cut crosswise ; dk, yolk -granules. 



Fig. 193. Cross section through the musculature of the trunk of a larva of Triton tseniatus 

 10 days old. Magnified 500 diameters. 



pb, Primitive bundle of muscle-fibrillse (Musktlprimitivburidel) ; mf, muscle-fibrillse cut cross- 

 wise ; mk, nuclei of muscle cells. 



we arrive at two propositions of importance concerning the origin of 

 the musculature : 



(1) In Vertebrates the elements of the musculature of the trunk are 

 developed out of epithelial cells which are derived from a circumscribed 

 ten*ritory of the epithelium of the body-cavity, a territory that is con- 

 stricted off from the latter to form the primitive segments. 



(2) The epithelial products become surrounded and enveloped on all 

 sides by connective tissue, just as do the glands and gland-ducts that 

 bud forth from an epithelium. 



A comparison with the condition and development of the musculature of some 

 classes of Invertebrates leads to a still better comprehension of the above 

 propositions. In most of the Ccelenterates the muscular elements are components 

 of the epithelium, not only during their development, but also in the adult 

 animal, so that the designation epithelio-muscular cells is suitable for them. 



