THE ORGANS OF THE MIDDLE GERM-LAYER. 345 



Finally, three alterations of the Muskelkastchen take place. The 

 homogeneous cementing substance, which was indicated during the 

 first stage by only a line line between the two fibrillae-sheets of 

 a muscle-layer, increases and produces the partition by means of 

 which the individual Muskelkastchen are separated from each other, 

 and in which afterwards connective-tissue cells and blood-vessels are 

 also to be found. Secondly, the protoplasmic matrix of the formative 

 cells is almost completely consumed in the continued production of 

 numerous fine fibrilla3, which finally fill the whole interior of the Kast- 

 chen. One can now distinguish two different kinds of fibrillse -those 

 that are centrally located, and those that are firmly attached to the 

 partitions. Thirdly, there are to be found scattered between the 

 fibrilla? numerous small nuclei, which pro- 

 bably are descended from the original 

 single nucleus of the formative cell by 

 frequently repeated division. 



The development of the muscle-seg- 

 ments takes place in the remaining Ver- 

 tebrates in a somewhat different manner 

 from that of Amphioxus and the Cyclo- 

 stomes. For the study of this process Ti s> J 91 --* M sect j" throu / h 



the trunk-musculature of a 



the tailed Amphibia furnish the mOSt larva of Petromyzon Planeri 



, i T m L ic. i A/" 6 weeks old. Magnified r>00 



instructive obiects. In Iriton (figs. 106, 



J diameters. 



105 lisll) each of the primitive segments A 1 , Muskelkastchen; ml; nuclei 



ill ., i i of muscle-cells; iiif, muscle- 



contains a considerable cavity, which is nbrmfe cut crosswise, 



bounded on all sides by large cylindrical 



epithelial cells. In somewhat older embryos active cell-multiplication 

 takes place in the part of the epithelium which is adjacent to the 

 chorda and neural tube, and which, therefore, corresponds to the 

 previously described muscle -forming layer of Amphioxus and the 

 Cyclostomes. By this growth the cavity of a primitive segment 

 becomes entirely filled. At the same time the cells lose their original 

 arrangement and form ; they are converted into longitudinally ar- 

 ranged cylinders, which correspond in length to a primitive segment 

 and are located by the sida of and above one another on both sides of, 

 and parallel to, the spinal cord and chorda dorsalis (fig. 192). Each 

 cylinder, which in the beginning exhibits only a single nucleus (mk), 

 becomes surrounded with a mantle of the finest transversely striped 

 fibrillre (mf) ; it is now comparable with a Muskelkastchen of the 

 Cyclostomes (fig. 191). A series of further alterations also takes 

 place in this instance as in the former. In older larvae there are 



