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myoseptum is formed. In the developmental stage, which we are 
studying now, this myoseptum is already to be seen as an extremely 
thin (0,5—1 w) layer of a homogeneous gelatinous substance (not 
an effect of shrinkage of the elements in the sections), that fills up 
entirely the room between the two opposite rows of sarcoblastendings. 
This homogeneous layer or film of gelatinous substance remains 
in the same form and conditions throughout the whole of the 
forelarval period. It is only at the end of this first period of 
development, when the yolk is nearly absorbed and the small 
forelarvae begin to migrate towards the bottom of the sea and to 
grow out to leptocephali, that the formation of tendon fibrillae sets in. 
The next step in the differentiation of the sarcoblasts consists of 
the formation of the striated myofibrillae in the cytoplasm. 
I will not enter here into details about the relations between the 
mitochondria ‘and the myofibrillae and about the first signs of 
eytoplasmatie differentiation, and only mention here, that we find 
the first traces of myofibrillar differentiation in the cytoplasm of the 
sarcoblasts near the extremity of the cell-bodies. And here we meet 
with an important phenomenon, which in the end, as we shall see 
later on, gives us the clue to the understanding of the most important 
facts of the myofibrillar differentiation and the solution of the 
problem of the union of the tendon- and muscle-fibrillae, which was 
the starting-point of the present paper, viz. that, as the first traces 
of the myofibrillae appear as small dots er rods, stained black by 
the iron-haematoxylin, in the two rows of sarcoblast-ends of two 
different myotomes, separated by the thin layer of gelatinous substance 
of the primitive myoseptum, these dots or rods are lying exactly 
opposite to each other in the protoplasm of the two sets of sarcoblasts, 
In the developmental stages following the one described here, we 
find the myotibrillae as long delicate threads, showing the typical 
cross striation, and running parallel to each other straight from one 
end of the sarcoblast to the other, congregated into two distinct 
bundles, thin and flat, leaving a narrow median band of cytoplasm 
between, in which the nuclei lie imbedded in the protoplasm. By 
a longitudinal splitting of the myofibrillae and later on of the small 
bundles, new systems of fibrils are formed *), and this accumulation 
of fibrils is continued until each sarcoblast seems to be a mass of 
fibrils with a central median thin layer of sarcoplasm containing 
the nuclei. In the course of this accumulation-process the extremities 
of the sarcoblast lining the myoseptum get broader and flatten 
1) This process is described very fuily in the paper by Dr. SuNteR mentioned above. 
66 
Proceedings Royal Acad. Amsterdam, Vol. XVII. 
