164 



loose areolar tissue. The enormous development of this nerve layer 

 may be seen from Symington's Fig. 1 (loc. cit.). Inside the posterior 

 part of the capsule of the organ a compact rounded bundle is formed 

 from these nerves. Escaping from under cover of the dorsal lip of 

 the cartilaginous capsule, the bundle turns upwards and backwards in 

 the submucosa on the lateral aspect of the septum nasi. Reaching 

 the roof it bends backwards and, just in front of the cranium, is 

 joined by the olfactory nerves proper, which run a much shorter 

 course. Together they form a rounded bundle, in which however the 

 nerves from Jacobson's organ are easily distinguished (in the foetus) 

 as a rounded strand on the dorsal and lateral aspect of the olfactory 

 nerves proper. 



The cephalic pole of each cerebral hemisphere early becomes 

 distinguished in the foetal Platypus as the olfactory bulb by the 

 attachment of the olfactory neural ganglion to it. This ganglion con- 



G. oif 



o J. 



Tn ed. . 



Fig. 4. Transverse section of ol- 

 factory bulb of foetal Platypus. )>^ 30. 

 G. olf. olfactory ganglion; G. J. "Jacob- 

 son's" ganglion ; | med, medullary layer ; 

 ■pros, prosencephalon. 



hifii. 



sists of two quite distinct parts, which are represented in transverse 

 section in Fig. 4. Covering the whole of the mesial side and extend- 

 ing on to the dorsal and ventral aspect there is a large ganglionic 

 mass {ß. olf)^ which is connected anteriorly with the olfactory nerves 

 proper. Situated on the lateral aspect of the prosen cephalic part of 

 the organ {pros) there is a smaller ganglionic mass {G. J.\ which 

 is associated with the nerves proceeding from Jacobson's organ. Each 

 ganglionic mass is separated from the prosencephalon by a clear nerve 

 layer (med.), which is the source of the olfactory radiations. Hitherto 

 doubts have been expressed as to what parts of the adult bulb are 

 developed from the two elements — ganglionic and prosencephalic — 

 of the foetal organ. The examination of the condition in the foetal 

 Platypus and comparison with that of highly macrosmatic brains, such 

 as that of Perameles, clearly indicates the medullary layer in the 

 bulb (which is immediately connected with the internal and external 



