992 
at a rate which greatly increases the number of nuclei without 
dividing the cell body. This latter grows longer until it reaches 
from the anterior to the posterior boundary of the myotome, while 
its numerous nuclei are stretched in a single row (in a sagittal 
section through the body of the larva) from one end to the other. 
The cells have changed from mononucleated embryonic cells into 
the elongated and multinucleated sarcoblasts. 
In these sarcoblasts the muscle fibrillae, the myofibrillae, now 
begin to appear in the protoplasm. At this stage each sarcoblast 
appears as a flat, multinucleated, plate-like cel!body, surrounded on 
all sides by a very delicate but clearly defined thin membrane, the 
sarcolemma. In each myotome these flat, plate-like sarcoblasts are 
arranged very regularly in a row as the leaves of a book, so that 
their broad sides are lying in the frontal plane of the body. They 
all reach from one end of the myotome through its whole length to 
the other boundary of the myotome. At the end of the myotome 
the rows of the thin rounded edges of the muscleplates nearly 
touch the homologous extremities of the sarcoblasts of the foregoing 
or following myotome. 
When we now study these boundary-surfaces between the myotomes, 
where the corineetive-tissue myoseptum is found in older forms and 
in the full-grown leptocephali, in thin sagittal sections through the 
larval body, in which sections therefore the sarcoblasts are cut 
longitudinally and at right angles to the broad surface of the plate- 
like cellbody, it is always the same sort of picture we get to view. 
The protoplasmatic’) cellbodies of the sarcoblasts, appearing, when 
cut in the direction mentioned above, as long regular rod-shaped 
elements, are separated from each other by very thin but clearly 
defined boundary-lines, running exactly parallel to each other, 
and at the extremity of the myotome these boundary-lines follow 
the rounded ends of the sarcoblast in an extremely regular curve, 
which runs very clearly and distinctly around the entire end of the 
sarcoblast. There is no interruption of this boundary-line whatever 
to be seen. Thus all the sarcoblasts of the two myotomes in question are 
standing with their rounded and perfectly isolated endings in two 
opposite rows, and they are generally arranged in such a manner, 
that the ends of the sarcoblasts of one row alternate with those of 
the sarcoblasts of the other row, belonging to the second myotome, 
and thus the space between the two rows of sarcoblasts is reduced 
to a minimum; in this space in a later period of development the 
1) Before the appearance of the myofibrillae. 
