'NIK 





Finally, threo aid rations of the Moakelk&Btohen t;tk<- place. The 

 homoir 'in -ous cementing substance, which \v;is indicated during the 

 first stage by only ;t line line between the two fibrilla- 

 a muscle-layer, incn :i>, s and produce the partition by me. 

 which the individual M uskelkiistchen :ire separated from each other, 

 and in which afterwards connect ive-f issue cells and blord-ves-i K are 

 also to be found. Secondly, the protoplasmic matrix of the format iv 

 cells is almost completely consumed in the continued production of 

 numerous fine fibrilhc, which finally fill the whole interior of the !^ 

 chen. One can now distinguish two dill, rent kinds of iihrilhc 

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

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

 fibrilhe 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 

 the tailed Amphibia furnish the most 

 instructive objects. In Triton (figs. 106, 

 105 ush) each of the primitive segments 

 contains a considerable cavity, which is 

 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 

 Cyclcstomes. 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 side of and above one another on both sides of, 

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

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

 becomes surrounded with a mantle of the finest transversely striped 

 fibrillae (mf) ; it is now comparable with a Muskelkiistclun 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 



mf mk 



Tig. 191. Cross section through 

 the trunk-musculature of a 

 larva of Petromyzon Planeri 

 6 weeks old. Magnified 500 

 diameters. 



k, Muskelkastchen ; mk, nuclei 

 of muscle-cells ; mf, muscle- 

 fibrillse cut crosswise. 



