THE NERVOUS SYSTEM. 



459 



tube, either evaginations (optic vesicles, olfactory bulbs), or ganglia, to fuse 

 with ectodermal thickenings (placodes) at the site of the future sense organs. 

 There appear to be often two series of ganglionic placodes in the head, a 

 dorsal (suprabranchial) series and a ventral (epibranchial) series, the latter 

 being often known as gill cleft organs. The former appear to be especially 

 connected with the development of the acustico-lateral system, the latter prob- 

 ably with the gustatory (see p. 469). (Fig. 399). The bodies of the efferent 



Neural crest cells --^Sji^f-f.* * 



/ $ - " ' a 



, -.i*' 



jj,'" 

 Suprabranchial placode --^^ 



c- 

 '^ d- Notochord 



Preoral gut 



FIG. 399. Transverse section through the head of a 7 day Ammocoetes in the region 

 of the trigeminal ganglion, -von Kupfier. 



neurones (except the sympathetic) remain in the neural tube, lying . in its 

 ventral half, and send their axones out as the efferent peripheral nerve fibers to 

 the effectors. 



The formation of the neural plate and its closure into a tube are the em- 

 bryological expression of the above noted tendency of highly specialized neural 

 structures to concentrate and withdraw from the surface (p. 455). The same 

 is true of the less highly specialized placodes, in which this process is not carried 

 so far. The neural plate may thus be regarded as the oldest placode. The 

 afferent peripheral neurones would naturally originate from the borders of this 

 plate, such portions being the last to separate from the non-neural ectoderm 

 or outer surface. They may be regarded as the youngest portions, phylo- 

 genetically, of the plate, and there seems to be some variation among Chordates 

 as to the degree of inclusion of the afferent peripheral neurones in the plate. 



In the neural tube thus formed, there can be distinguished four longitudinal 



