250 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
form like S. acanthias, where the ear capsule shifts backward, the exit of 
the root of the glossopharyngeus lies behind encephalomere VII, whereas 
in such forms as the chick and swine, where the ear capsule does not 
similarly shift backward, the exit of its root is from the expansion of 
encephalomere VII. In all Vertebrates, the roots of the glossopharyn- 
geus and the Urvagus lie close to each other, but in S. acanthias, where 
there is a greater amount of posterior displacement than in any other 
Vertebrate that I have studied, these roots are more crowded together 
than in other forms. These facts seem to me to warrant the conclusion 
that the roots of the glossopharyngeus and the Urvagus primitively made 
their exit from those encephalomeres which give rise to t! vir ganglionic 
Anlagen. And we may likewise assume that the local thickenings of 
these encephalomeres have their significance in this primitive relation, 
i. e. they contained the “Kerne” of these roots. I am able to find no | 
facts which render this assumption untenable. | 
On the other hand, encephalomere IV never has nervous connection 
with a visceral arch. From it few neural-crest cells are proliferated, 
and in consequence it never forms the ganglionic Anlage of a nerve, 
nor does it ever in ontogeny have a motor nerve in connection with it. 
Since the other four encephalomeres are related to visceral arches, I 
incline to think that this encephalomere was once related to a visceral 
arch of its own. Otherwise, so far as I can see, its existence is in- 
explicable. In this condition, then, I find additional evidence of a 
lost visceral arch, which van Wijhe (82), Miss Platt (91°), and Hoff- 
mann (94) believe once existed in the region of this neuromere. These 
investigators have found a want of exact correspondence between the | 
somites and the visceral arches in the region of the spiracular cleft. 
Van Wijhe was led to believe that the Ayoid (2d visceral) arch is 
double, —i. e. represents two arches, the fusion of which has resulted in 
the obliteration of the visceral cleft between them, — while Miss Platt 
and Hoffmann have held that the mandibular arch is double, and that 
an anterior gill cleft has disappeared. ‘The disappearance of a visceral 
cleft is rendered plausible, if we assume that such a loss would greatly | 
strengthen the mandibular arch when it came to function as a lower | 
jaw. The evidence from a study of mesomerism and neuromerism there- 
fore seems mutually confirmatory. 
If encephalomere IV was related to a lost visceral arch, it follows that 
the lost arch must have been situated posterior to the mandibular (1st 
visceral) arch, for the musculature of this arch is innervated from en- 
cephalomere III. It also follows, because of the relation of the nerve 
