No. 2.] VENTRAL ABDOMINAL WALLS IN MAN. 351 
rison! in combating the notion that all muscles arise directly 
from the myotomes. Harrison studied the development of the 
fin muscles in the trout, and finds that, with the exception of 
those in the pectoral fin, they arise directly through buds from 
the myotomes. In this one fin the muscle bud is small, and is 
so closely blended with the mesenchym that it is impossible to 
show whether or not the muscles of this fin arise from the 
muscle bud or mesenchym or from both. It appears to me that 
if all the body muscles of sharks and all of them in the trout 
arise directly from the myotomes, with the exception of those 
of the pectoral fin, it is better for the present to ascribe the 
muscles of this fin to the myotomes rather than to mesenchym, 
when mesenchym and buds are so completely blended. 
Not only does Harrison? deny that all the skeletal muscles 
arise from the myotomes, but he also attempts to prove that 
the segments of a muscle, like the rectus, do not indicate that 
it has arisen from a corresponding number of muscle plates. 
He has shown that in teleosts the rectus is first split from the 
muscle plates as a band of tissue, and then undergoes secondary 
segmentation. After the muscle is laid down its front end 
remains connected with the myotomes until it has segmented, 
and hence these segments of the rectus correspond with the 
myotomes, while behind they do not. That the segments in 
the posterior end are formed after the muscle is separated from 
the myotomes indicates that there are mechanical causes which 
favor segmentation, but it is not proved that these segments 
do not correspond to the myotomes until it is shown that the 
nerve supply of the two are different. Even if it were shown 
that the succession of segments in the rectus did not correspond 
fully with the segmental nerves it would not prove anything 
special other than the history of this muscle. We must only 
recall the serratus anticus, which is composed of beautiful 
segments, all of which are of secondary formation. In its 
development the muscle wanders down from the neck, attaches 
itself to the scapula, and then successively to the ribs. During 
all this process of growth it retains its original nerve connec- 
1 Harrison, Arch. f. mikr. Anat., Bd. xlvi, p. 500. 
2 Harrison, Johns Hopkins University Circular, No. 3, 1894. 
