160 THE DEVELOPMENT OF THE CHICK 



The acustico-facial periaxial cord attains definiteness some 

 time before the trigeminal (cf. 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 myelenoepha- 

 Ion, 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 

 ganglia 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 maxillary 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 27 s 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-otic 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-otic ganglionic crest is a direct continuation ot 

 the pre -otic behind the ear, and it is at first difficult to make an 

 exact boundary between them. At the stage of 13 s the pre-otic 

 crest extends beneath the auditory epithelium nearly to its middle 



