im: ORGAJra Of mi: 01 



fnxpiently ( I I or.-,-) i- r\-n in connection with tin- anterior OQAQ 

 mean- of :i narrow canal in tin- tractn- olfact orin-. 



Tlif olfactory lull,- ( /.,,/ f '/',-,,) attains an < \tr.i< 'nlinary develop- 

 ment Mi-, L':>7i in tli.- S.-lachia, in \\lnVh it exceeds in size the 

 bet weon-brain (XII) and mid brain (Mil . In tin- Selachian-* two 

 long hollow j MI- olfach.rin.-, '/Vo) extend out I 



ant. -Ho r end .f the li til. developed cerebrum and terminate at a 

 considerable distance from the fore-brain in tuo largo hollow lobes, 

 that are sometime.- provided with furrow- (/,/). 



/!. The Development of the /V/'/y. roua 



Although it is easy to follow the de\elo|.ment of the hrain and 

 spinal cord, the investigation of the origin of the peripheral nervon- 

 sy.-tem is \t-rv difficult, for it requires the -tmly f hi>toloL r i<-al jiroc-esses 

 of the most subtle nature- tin- tirst appearance of non-medullated 

 nei-ve-fibres and the method of their termination in embryos 

 composed of more or less undillerentiateil cells. One who knows 

 how difficult it is even in the adult animal to follow non-ine.lnllated 

 nerve-fibril he in epithelial layers or in non->triate inn . and 



to get a clear idea of their method of termination, will understand 

 that many, and indeed the most inteie>t ing, qu< I 



to the development of the peripheral nerves are not yet ripe for 

 disru-Mon, berati-o the observation- DeceSSary ftr their 

 are >till wanting. There is only one point whiih is i-ntirely clear. 

 That i-oiut -n is the development of the spinal ganglia, \\hii-h His and 

 BALFOIK independently of each other wen- the liist to recognise, th- 

 one in the Chick, the other in Selac-hians. Since then numerous 

 investigations embracing dill. -ivnf group- of \".it 

 publislied on this subject by HIV-IN, M i IHALL, !\i.i: 



\IKHI.. VAN Wl.lIIK, IlKlioT, O.Ni'IH. K. IiABL, BEARD, 



1\ \-r- i IK NK i) ? and others. 



(a) The Der< 



The development of the spinal ganglia in the -pina! 

 ea-ily followed. It begin- jn.-t at the time the medullary groove 

 doeee to form a tul^e (fig. l'-"'^ -1 and time a thin 



ridge of c-ells (</./', fj/) one or two layer- d.--p u r ro\\ - out of the 

 neural tube on either side of the line of fusion, and, passing outv 



20 



