456 Julia B. Platt 
that the myotomes successively grow ventrad as the reduction of 
the branchial arches permits them to do so, were it not that the 
myotome which gives rise to the anterior bud of the hypo- 
glossus musculature in Necturus lies above — not posterior 
to — the second vagus arch (see also fig. 25). As straws show 
which way the wind blows, this simple fact appears to me to justify 
FÜRBRINGER’S theory regarding the replacement of occipital somites 
by somites originally belonging to a posterior region. The myotome 
which gives rise to the anterior part of the ventral hypoglossus 
musculature in Necturus has migrated forwards in phylogeny from 
its original position posterior to the branchial clefts. 
On the evidence furnished by Amphioxus and Bdellostoma 
(see Price 96) we may surely assume the ancestral Vertebrate to 
have had a great number of gill clefts, which were by no means 
confined to the region we now call the head. It is also probable 
that these gill clefts lay below the myosepta (Bdellostoma, Petro- 
myzon, Necturus) which separated the individual members of 
the row of axial muscle segments that extended throughout the length 
of this primitive Vertebrate. With the development of the ear from 
an integumentary sense organ in this Amphioxus-like creature, two 
axial muscle segments, which correspond in position to v. WIJHE’s 
fourth and fifth somites, appear to have been crushed out of existence. 
Thus the line of myotomes with their associated nerves and supportive 
tissue was interrupted, and it seems highly probable that as the 
branchiomeres became reduced posteriorly, the myotomes with their 
nerves and supporting tissues slipped forwards and became corre- 
spondingly reduced immediately posterior to the auditory vesicle. 
If this supposition be correct, each postotie segment of the head 
is of heterogeneous composition, and the sixth somite of v. WIJHE 
does not belong primarily to the segment in which it lies, or to the 
palaeocranium, but like all of the somites posterior to the ear has 
wandered into a region to which it was primarily foreign. The 
primitive postotic segments of the head are represented alone by the 
branchial arches, the glossopharyngeal nerve, and the nerves of the 
two vagus roots. A corollary is that the number of segments included 
in the skull between the trabeculae and the last occipital arch is 
equal to the number of branchial arches possessed by the ancestral 
vertebrate. 
