236 J. B. JOHNSTON, 
responds to the cerebral cortex and is the fundament in which the 
cortex is developed. According to the AHLBORN-STUDNIGKA hypo- 
thesis, the membraneous pallium never can give rise to a massive 
cortex and the substance from which the cortex is developed is found 
in the corpus striatum. 
In order to discuss these views it is necessary first to define 
what is meant by cortex. We can at once eliminate from the dis- 
cussion all parts of the cerebral hemisphere of Mammals which are 
not connected with the olfactory function. Since the part of the 
cortex which serves the olfactory sense is the first to be developed 
it is the only part of interest in the present connection. To define 
the cortex by its position — dorsal to the corpus striatum in the 
situation of the membraneous pallium of fishes — is not sufficient. 
The cortex does not all lie dorsal to the corpus striatum. The cor- 
tex is distinguished by its connections, and solely by its afferent 
or sensory tracts. The cortex receives sensory (olfactory) fibres 
of the third order, not of the second order. This is made clear by 
setting down the steps in the path by which olfactory impulses reach 
the cortex: 
lower Vertebrates: fila olf. — lobus olf. — area olf. — cortex 
Mammals: fila olf. — bulbus olf. — lobus olf. — cortex. 
The cortex is sharply distinguished from the olfactory area by the 
fact that the latter receives fibres from the mitral and other cells of 
the olfactory lobe and sends fibres to the cortex. The neurites of 
mitral cells (or other cells in direct relation with the fila olfactoria) 
never reach the cortex. The cortex is not so sharply distinguished 
from the olfactory area by its efferent (motor) tracts. The area ol- 
factoria sends fibres to the ganglia habenulae, lobi inferiores, prob- 
ably to the central grey of the thalamus, and possibly to the corpus 
mammillare. The cortex also sends fibres to the ganglia habenulae 
and the corpus mammillare, these fibres running in a definite system 
known as the fornix. In addition, the cortex gives rise to commissural 
fibres crossing in the anterior commissure. 
F. Mayer’s description of the Petromyzon brain is so brief that 
it is impossible to know, in the absence of figures, on what facts 
he bases his statement that the dorsal and lateral parts of the striatum 
represent the cortex and that the cells in this region are to be con- 
sidered cortex cells in consequence of their relations. This is not 
supported by his description. His “cortex cells” receive the neurites 
of mitral cells in his “tractus olfacto-corticalis” and send their neu- 
