VISCERAL ARCHES OF THE GNATHOSTOME FfSHES. 385 
Squalus acanthi as. In fig. 20 of that work Scammon 
gives a reconstruction of the head somites in a 9 mm. embryo 
of Squalus, and I have reproduced it in the accompanying 
Text-fig. 1. In this figure it is seen that while the coelomic 
cavity might properly be considered to be prolonged into the 
short united portion of the hyal and mandibular stalks, and 
even beyond the hyal stalk for a short distance into the 
ventral end of the mandibular stalk, it can no more be con- 
sidered to be prolonged into the basal portion of the hyal stalk 
than also into the basal portions of the stalks of the branchial 
myotomes. The case is strictly similar to that of the truncus 
arteriosus and the afferent arteries that arise from it. The 
truncus arteriosus cannot be considered as in any way con- 
tinued into any of these arteries excepting only into the 
afferent mandibular artery. With regard to this latter artery 
there is no line of demarcation to indicate where the truncus 
arteriosus ends and the mandibular artery begins, and in my 
discussion of these arteries in embryos (Allis, 1908) I assumed 
that the basal portion of the afferent mandibular artery 
represented an anterior prolongation of the truncus arteriosus. 
The cavity designated as the hyal cephalic coelom in Edge- 
worfhks Text-fig. 1, showing a transverse section of a 7 mm. 
embryo of Scy Ilium, is then certainly a part of the hyal stalk, 
and the fact that the interhyoideus muscle, developed from 
this part of the stalk, is said to be at first continuous with the 
hyal myotome would seem to be of greater significance than 
the further fact, to which Edgeworth gives the greater signifi- 
cance, that this part of the stalk does not separate from the 
wall of the coelomic cavity before developing into muscle 
fibres, as the stalks of the myotomes in the branchial region 
are said to do. The so-called mandibular cephalic coelom 
of this same Text-figure of Kdgewortlds might, however, be 
considered to be a part of the cephalic coelom, for, as in the 
case of the afferent mandibular artery, there is no line of 
demarcation to indicate where the cephalic coelom ends and 
the mandibular stalk begins. But if the musculus inter- 
hyoideus is developed from the ventral portion of the hyal 
