. 447 
In an embryo of Petr. Planeri of 5 cm, I find that the first 
myotome whose ventral portion remains connected with the dorsal 
portion is the 13th. Anterior to this the dorsal and ventral portions 
of the lateral musculature are separated from each other by the cleft 
in which the spiracula lie. While there are anterior to the 13th myotome 
12 segments in the dorsal portion, there are only 9 in the ventral‘). 
Now if each of these ventral segments represents a single bud from 
a myotome then it is clear that the first must have been derived 
from the 4th myotome of the dorsal series. But having no evidence 
that either this or the two following myotomes (d—6) produce ventral 
buds, I conclude that the segments of the M. par. subbran- 
chialis do not represent primary myotomic segments, at least so 
far as the anterior eight segments are concerned. The anterior eight 
segments (derived in all probability from buds of the 7th—11th myo- 
tomes inclusive) I hold to be secondary and segmented in adaptation 
to the segmentation of the visceral arches. In embryos of the 15th to 
the 18th days of development, when the M. par. subbranchialis 
is clearly segmented, its segmentation corresponds exactly with the 
segmentation of the visceral arches. This primitive correspondence 
between myomerism and branchiomerism, which was also true in 
early stages of the dorsal myotomes, becomes disturbed during develop- 
ment by the great posterior extension of the branchial basket and by 
a concomitant crowding forward of the myotomes. A comparison of 
embryos of 5 mm and of 5 cm shows that the dorsal portion of 
myotomes 7—12 which in the earlier stage lay behind the last visceral 
cleft, in the later lie anterior to this. Since, however, the segments 
of the M. par. subbranchialis, except the last, retain their 
primitive correspondence with the visceral arches, there appears in 
the later stages of development the incongruence in the muscle segments 
above and below the “Kiemenfurche”. 
While these facts of development explain satisfactorily the discrep- 
ancy in the number of myotomic segments above and below the 
“Kiemenfurche”, on the other hand there are variations in the number 
of segments, which need other explanation. For example, I find in a 
specimen of Petromyzon fluviatilis just after metamorphosis, that 
anterior to the 15th myotome there are on the left side 14 myotomes 
in the dorsal series and 13 in the ventral; while on the right side 
1) Juni (’87) has obtained the same result from the study of 
Ammocoetes. HarscHer’s (’92) Fig. 9, p. 148, shows one segment less in 
the M. par. subbranchialis. 
