+ 
334 The American Naturalist. 
RECENT STUDIES OF THE VERTEBRATE HEAD, 
BY H. W. NORRIS. 
(Continued from page r02.) 
Poon researches on Amphioxus and the Craniata Van Wite 
(89) concludes that the skull never consists of metameres; — 
that only in the occipital region behind the vagus were there at 
one time separate cartilaginous neural arches. The parietal mus- 
culature, and the peripheral nervous system, with the exception of a 
the I., IL, and III. nerves, were once segmented in the region of 
the heat as well as in the body. The number of the myotomes — 
of the head is in general nine, but in those Craniata which have — 
no hypoglossus, less. To a cranial or a body segment belong 
both a dorsal and a ventral nerve, which were originally separate. 
Wherever in the Craniata the ventral roots are wanting the cor- 
responding myotomes do not appear. The vagus is a complex + 
of two dorsal nerves. There are no grounds for assuming that 
the Craniata ever possessed more than eight branchial sacks, 
unless an additional aborted one belongs to the hyoid arch! 
Beard (’89a) states that certain portions of the cranial and spi. a 
nerves arise not as outgrowths from the central nervous system 
but from the ectoderm just outside the neural tube. This mode 
of E oo agrees with that described by Kleinenberg uS i 
ectodermal si cculisitons jist above the lateral limit 
ventral cord, and like the ganglia of vertebrates arise bs 
tally. It is to be noted that Rabl and Dohrn both con 
two limbs of the neural plate the neuro-epithelium of one Ii 
separated from that of the other by a ciliated groove. Two! 
of neuro-epithelium separated by a ciliated groove are chata 
tic of Annelids. This ciliated groove in vertebrates’ later 
most, if not all, of the ciliated epithelium of the permanent € 
ace : 
* Iti the above statements of Van Wijhe with the more 
O CoO aM par © 
‘headin of D Duties 
