The Development of the Cartilaginous Skull etc. in Necturus. 455 
the previous existence of which comparative anatomy renders most 
probable, do not appear, even passingly, in embryonic development. 
To me (FÜRBRINGER), it seems that one may draw this conclusion 
with better right than with SEWERTZOFF assume that the occipital 
arch which fuses with the palaeocranium in the Amphibia corresponds 
to the first occipital vertebra of Acanthias, while the first three 
free vertebrae in the Amphibia correspond to the last three occipital 
arches taken into the skull of this Selachian. SEWERTZOFF hereby 
assigns the Amphibia a much more primitive position than the 
Selachii, thus inclining towards the view of STÖHR, against which 
GEGENBAUR (87, page 72) has already protested«. 
However, when FÜRBRINGER adds that to him the so-called occi- 
pital arch of the Amphibia, presupposing its phylogenetic reality, 
seems rather the rudiment of a multiplum of primary occipital 
vertebrae, I find that the relation of the occipital arch to the myo- 
tomes in Necturus speaks against such an assumption, but I think 
that FÜRBRINGER may be right in homologizing the boundary between 
the skull and vertebral column in the Amphibia with that in the 
Selachii. 
From the comparison of the development of the m. parietalis 
ventralis branchialis in Petromyzon with that of the hypoglossus 
musculature in Acanthias, NEAr (97, page 460) infers »that as the 
branchial basket became reduced in extent, myotomes progressively 
farther forward grew ventrad to meet in the mid-ventral line«, but 
calls attention to the fact that while five myotomes take part in the 
formation of the branchial part of the ventral longitudinal muscle 
in both forms, the myotome which gives rise to the anterior part of 
the muscle in Acanthias lies three segments anterior to that which 
gives rise to the corresponding part of the muscle in Petromyzon. 
Thus a difference of three mesomeres is found with a coincident 
reduction of but two branchiomeres. Acanthias has six visceral 
clefts, and Petromyzon eight. 
But three myotomes take part in the formation of the hypo- 
glossus musculature in Necturus, unless the second somite also 
contributes a few scattered cells, which is doubtful. From the table 
(page 454) it is seen that the anterior of these three myotomes lies 
one segment anterior to that which gives rise to the anterior hypo- 
glossus bud in Acanthias. Since Necturus has one branchial arch 
less than Acanthias, this relation of the myotomes to the hypo- 
glossus musculature might be regarded as supporting NEAL’s view 
