ELEMENTS OF THE CENTRAL NERVOUS SYSTEM. 



501 



neural plate 



crista neuralis 

 x skin 



/ganglion 

 r^ 



n a way that seems at first sight utterly abnormal, until it is remembered that 

 j[ .hey afford examples of very 

 .Primitive methods of nerve- 

 Ibrmation. 



The essential part of the 



>rgan of hearing is an ecto- 



lermal sac (otic vesicle) that 



ievelops as a diverticulum on 

 ,,he side of the head, from a 



,hickened patch of ectoderm, 



vhich in the lower vertebrates 



brms part of a more exten- 



dve area, known as the dorso- 



ateral placode. Some of the 



;ells of this area seem to be- 



iome transformed into nerve- 

 jells, which migrate into the 



space between the otic vesicle 



ind the neural tube (Fig. 443) 



ind form the acoustic ganglion. 

 At the upper margins of 



}he branchial clefts a series of 

 :ictodermal (and possibly also 



mtodermal) thickenings develop, 

 ,, vhich are known as the epi- 

 oranchial placodes. Com- 

 oarison with the process of 



levelopment in fish embryos, 



;vhich has been elucidated by 



Landacre (Journal of Compara- 

 tive Neurology and Psychology, 



L910-1912), suggests that the 



lerve-cells may arise from these placodes, from which the nerves of taste originate 



peripheral sensory nerve 

 sensory nerve root 



vesicula o 



'ptica 



FIG. 442. DIAGRAMS OF TRANSVERSE SECTIONS REPRESENTING 

 THREE STAGES (A, B, AND C) IN THE DEVELOPMENT OF A 

 SENSORY GANGLION FROM THE NEURAL CREST ; AND Two 

 DIAGRAMS (D AND E) SUGGESTING A POSSIBLE HOMOLOGY 

 OF THE OLFACTORY (D) AND VISUAL (E) EPITHELIUM WITH 

 THE NEURAL CREST. 



Ganglion geniculi 



Nervus facialis 



Ganglion acusticum 



Vesicula otica 



,.- Ganglion petrosum 



Epibranchial placode 

 ,. of glosso-pharyngeal 

 nerve 



Ganglion 

 nodosum 



Epibranchial 

 placode of 

 vagus nerve 







Area olfactoria 



443. RECONSTRUCTION OF THE GANGLIA OF THE FACIAL, ACOUSTIC, GLOSSO-PHARYNGEAL, AND VAGUS 

 NERVES OF A HUMAN EMBRYO 5 MILLIMETRES LONG (ABOUT THREE WEEKS OLD). 



'he epithelium of three branchial clefts and the otic vesicle is represented diagrammatically ; and the supposed mode of 

 rigm of the gustatory nerve-cells (and their fibres) from the epibranchial placodes is indicated in blue, and of the 

 istic nerve-cells from the otic vesicle in purple. 



