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value, it is clearly an assumption, based on the assumption that the 
ectodermic folds in Amphioxus and Craniota are morphologically the 
same, to say that the former growth is the more primitive of the two. 
In the branchial region of Petromyzon the myotomes grow as far 
ventrad as the branchial furrow permits. Posterior to this they grow 
ventrally and meet in the mid-ventral line. In Amphioxus also the 
myotomes behind the branchial region grow farther ventrad than those 
in the branchial region, although they never come to meet in the mid- 
ventral line. I therefore see in the ventral growth of the myotomes 
in Amphioxus and in the ventral extension of the myotomes into the 
ventral musculature of Petromyzon a difference in degree and not in 
kind. The growth of buds from the myotomes into the lateral 
appendicular folds of Vertebrates possessing such may indeed be 
regarded as a secondary modification of such myotomic development. 
Moreover, it seems to me to be more in accordance with the facts to 
state that the myotomes of Amphioxus grow toward the peribranchial 
folds (that is to say, ventrad) rather than into these folds. 
Furthermore, since I fail to find in Petromyzon embryos any growth 
which would lead me to infer that this animal ever possessed paired 
appendages, I conclude that its development furnishes us with evidence 
that the growth of myotomic buds into the appendicular folds did 
not take place in the Vertebrate series “ehe die parietale Musculatur 
den Körper ventral umwachsen hatte”. I also infer that as the branchial 
basket became reduced in extent, myotomes progressively farther for- 
ward grew ventrad to meet in the mid-ventral line. The growth of 
the anterior of these myotomes forward below the branchial basket 
into a primitively “fremdes Gebiet” I regard as a still later extension 
of this growth. 
It has already been noted that in Petromyzon and Squalus buds 
from myotomes three segments removed come to occupy the same 
relative position in these two forms anterior to the hyoid arch. If 
we grant the primitive correspondence between branchiomerism and 
mesomerism, as we have good reason to do !), the greater posterior 
extent of the branchial basket in Petromyzon does not entirely account 
for this difference, for we have to do here with a difference of three 
mesomeres and only two branchiomeres. I believe, however, that this 
discrepancy is to be explained in the following way: While in Petro- 
myzon the visceral pouches correspond segmentally with the myotomes, 
1) I would call the attention of the reader to the recent evidence 
given by Price (’96) bearing upon this question. 
