160 THE DEVELOPMENT OF THE CHICK 
The acustico-facial periaxial cord attains definiteness some 
time before the trigeminal (ef. Fig. 71), and indeed appears almost 
from the first as a specially strong part of the periaxial layer: 
whereas in the region of the trigeminus the cells of this layer are 
first widely dispersed and secondarily aggregate, between the 
stages of 14 and 18 somites. Both cords are attached to the 
brain, the trigeminus to the first neuromere of the myelencepha- 
lon, and the acustico-facialis to the third (Fig. 83 E). 
The trigeminal and facial periaxial cords are supplemented, 
as we have seen, by proliferations of the ectoderm on each side 
of the first visceral pouch; the trigeminal cord then enters the 
mandibular arch, and the facial the hyoid arch, and in the stages 
between 20 and 27 somites form at least part of the mesenchyme 
of these arches. The axial mesoblast likewise contributes to the 
mesenchyme of these arches, and it becomes impossible in later 
stages to separate these two mesenchymal components. The 
gangha proper differentiate from the upper portions of the cords. 
The trigeminal periaxial cord divides over the angle of the mouth 
and sends out a process into the rudimentary manillary process. 
A third projection of the same cord towards the eye forms the 
path of the ophthalmic division of the trigeminus (Fig. 117). 
At the stage of about 27s the trigeminus forms a connection 
with a thickening of the ectoderm (placode of the trigeminus) 
situated in front of and above the first visceral cleft; and the 
facial connects similarly with a larger ectodermal thickening 
(placode of the facialis) situated on the posterior margin of the 
uppermost part of the first visceral furrow. These ectodermal 
thickenings are rudimentary structures of very brief duration, 
representing parts of the sensory canal system of the head of 
aquatic vertebrates. Their occurrence in the chick is an interest- 
ing example of phylogenetic recurrence. <A third and fourth 
like organ arises in connection with the post-otie ganglia. 
At the stage of 72 hours there are two ectodermal thicken- 
ings (placodes) in connection with the trigeminus, one in front 
of the other, derived probably by division of the original first. 
The facialis placode is more fully developed. 
(2) The post-otie ganglionic crest is a direct continuation of 
the pre-otic behind the ear, and it is at first difficult to make an 
exact boundary between them. At the stage of 13s the pre-otic 
crest extends beneath the auditory epithelium nearly to its middle 
