165 



olfactory radiations) as the superficial medullary layer of the pros- 

 encephalon and therefore the line of demarcation between the two parts. 

 It is evident from this that the prosen cephalic part forms an altogether 

 subordinate — largely ependymal — part of the bulb in the adult, the 

 whole of the mitral-cells and the glomerular layer being ganglionic. From 

 the few neuroblasts, which develop in the prosencephalic part, root 

 fibres of the pars olfactoria of the anterior commissure arise. It is 

 to be noted however that the endings of the same series of commis- 

 sural fibres extend into the ganglionic portion of the bulb. 



In the adult Platypus the olfactory bulb forms a small oval 

 mass into whose anterior pole the rounded bundle of olfactory nerves 

 is inserted. But for its relatively larger size, it is not at all unlike 

 the corresponding region in the duck. This resemblance however is 

 merely superficial. In this disposition of the olfactory nerves and 

 absence of a cribriform plate Ornithorhynchus differs from all 

 other Mammals and resembles most Submammalia ^). Wiedersheim 2) 

 wrongly includes Echidna in this generalisation. The bulb of Platy- 

 pus is completely invaginated from the upper part of its lateral 

 and anterior aspects, so that it forms a very deep and complete 

 cup, whose opening is directed upwards and outwards {Olf. c. Figs. 5 

 and 6). The arrangement of the olfactory ganglion in the foetus 

 (Fig. 4), in which the cupping has not commenced, clearly moulds 

 the prosencephalic part of the bulb, so that the olfactory ventricle 

 becomes modified into a cupshaped slit; but is not wholly oblitera- 



Fig. 5. 



Pig. 6. 



■3^ 



-ihr 3 ^■ 



"^W^ 



Fig. 5. Transverse section, olfactory bulb, adult Platypus. )«( 4. men. meninges; 

 Olf. c. olfactory cup; v. ventricle; glom. glomerular layer. 



Fig. 6. Sagittal section through bulb. X ^- Adult Platypus. Olf. c. olfactory 

 cup; ». ventricle; p. pallium; g. v. c. great cerebral vein; o. r. olfactory radiation. 



plate. 



1) According to Owen Apteryx and Dinornis have a cribriform 

 I. 



2) Grundriss der vergleichenden Anatomie der Wirbeltiere. 



