996 
ment. Probably it has a slightly altered chemical constitution, and 
may be regarded as having not a contractile but only a mechanical 
function and therefore representing to a certain extent a sort of 
intracellular tendon-fibril. This noncontractile part of the myofibril 
is formed as a differentiation of the extremity of the myofibrilla, is 
therefore entirely continuous with the striated part of it, is lying 
completely intracellular, inside the sarcolemma and attached to it at 
its end. It is this relation between apparently different sorts of fibrils 
which is visible in many of the figures of the paper by ScHuLTzE 
mentioned above. According to the mode of development described 
here, one is not entitled to draw from it the conclusions SCHULTZE 
and his followers draw from their preparations. 
Until now we had to do only with the intracellular fibrillar 
differentiations. But at this point begins the second phase of the 
development of the muscle-fibres, the forming of the tendon-fibrillae. 
The thin layer of homogeneous gelatinous substance of the primitive 
myoseptum between the myotomes begins to thicken, connective- 
tissue cells lying between the epithelium and the myotome begin to 
migrate slowly in between the myotomes and so step by step the 
connective-tissue septum of the leptocephali is built up. Afterwards 
these cells of the secondary myoseptum send cells down between the 
muscle plates into the myotome, tater on blood capillaries follow, 
and so gradually the features of the adult myotomes are laid down. 
Even at the first thickening of the primitive myoseptum the differen- 
tiation sets in which interests us here most of all, viz. the forming 
of the tendonfibrillae, a fibrillisation of the substance of the myoseptum , 
whether under the influence of immigrating connective tissue-cells 
or directly under the influence of the growing and expanding myotomes 
is not to be determined. The fact that interests us chiefly here is 
that in the process of formation of these primary tendon-fibrillae 
essentially the same features are shown as in the formation 
of the myofibrillae in the adjacent myotomes. Here likewise we see 
the fibrillae, the homologa of the collagenous fibrillae of the connection- 
tissue myoseptum of older forms, differentiated in direct connection 
with the ends of the myofibrillae of the muscle-plates of the adjacent 
myotomes. Exactly at the point, where inside the sarcolemma (which 
is still visible with the same clearness around the extremity of the 
muscle fibre as before) a myofibrilla is attached to it, a tendon- 
fibrilla appears attached to the outside of the sarcolemma. And at 
whatever point we study the differentiation of the tendon-fibrillae, 
whether in the regular clearly defined myoseptum between the two 
rows of parallel and close standing muscle-cells of the adjacent 
