45 
stage Il, it was found in the fourth and in an embryo in 
stage O in the seventeenth trunksegment. This shows that 
the growth of the anterior region of the gut surpasses that 
of the rest of the body, a process which causes the hind- 
most gill-slits to be found finally under myotomes which 
were originally situated behind them. But this does 
not mean tosay that all the myotomes found 
above the gill-slits have only secondarily 
acquired thís position. That myotomes behind 
the auditory capsule have been, so to speak, crushed out 
of existence by the strong development of this capsule can 
not be doubted. FüRBRINGER (1897, p. 440), however, 
assumes a “stetiges Vorrücken” and dissolution of somites 
in the occipital region, for which embryology has not yielded 
serious evidence. On the contrary, no embryologist, with 
the exception of BRAUS (1899), has assumed such a process, 
and GOODRICH (1918, p. 13), the last to have worked on 
the subject, asks: what definite evidence is there that such 
a procession of myotomes which plunge one after the other 
below the capsule and vanish in a cloud of mesenchyme 
really occurs. 1 think with GOODRICH that there is good 
reason to believe that for the most part myotomes once 
laid down persist. 
We have seen that in Petromyzon, as a consequence of 
the backward extention of the branchial basket, the post- 
otic myotomes 7—12 become secondarily epibranchial; in 
the same way this occurs with a considerable number of 
myotomes in Selachians but of these only the anterior one 
or two, as we have seen, belong to the skull. The other 
occipital myotomes, in front of these, and their nerves belon 
from the beginning to the branchial region, justlikethe anterior 
epibranchial myotomes in Petromyzon. In Selachians, however, 
this epibranchial musculature is much less developed than in 
the latter, though in the more primitive froms with the greatest 
number of gill-slits it is more developed. In other groups 
of Vertebrates, and even in rays, it is no longer found. 
FüRBRINGER's rule applied to the hypoglossus. — Petromyzon 
shows us that the opinion that the hypoglossus ab origine 
has nothing to do with the vagus (vago-accessorius) is not 
incorrect. If we compare the situation of the hypoglossus- 
roots in the main groups of Vertebrates (cf. Plate I), we 
see that there can be no question about an individual 
homology of the segments to which they belong in the 
