529 



For the closest analogy to this condition of aflfairs I shall have 

 to go far afield — to the brain of a foetal Ornithorhynchus, which I 

 described twelve years ago i). [Much closer resemblances can be found 

 among the Reptilia, but I know of no suitable published illustration 

 that lends itself to my purpose.] There we find a similar pallial column 

 of cells, thickened at each edge, just as in the brain of Lepidosiren — 

 the mesial edge being the hippocampus and the lateral edge the pyri- 

 form lobe, the surface of which is covered with fibres of the external 

 olfactory tract (e. o. t). 



There can be no doubt that the mesial and lateral portions re- 

 spectively of the pallial formation in Lepidosiren are homologous to 

 the hippocampus and pyriform lobe respectively. I have not deter- 

 mined the position of the lateral olfactory tract in Lepidosiren, but 

 by analogy with the condition found in Rana'^) and other Amphibia 

 (compare also Burckhardt's account of Protopterus) there can be no 

 doubt that this tract occupies the position marked A in Figure 1, i. e. 

 as in Ornithorhynchus, on the surface of the pyriform lobe. 



How much of the pallial formation in Lepidosiren is hippocampus 

 and how much is pyriform lobe or whether, as in the Mammalia, there 

 is any representative of the neopallium interposed between them are 

 all questions which it is impossible to answer. We ought rather to 

 look upon the pallium of Lepidosiren as an area, which is yet un- 

 specialised, the rudiment of that more extensive cortical field which 

 in the Mammalia becomes differentiated into distinct formations. 



It is in the highest degree improbable that there should be any- 

 thing definitely representing the neopallium of the Mammalia in this 

 simple brain. The cortical area is very narrow, and olfactory im- 

 pressions are poured into both its edges — into the pyriform lobe 

 by the lateral olfactory tract (Tractus bulbo - corticalis) and into the 

 hippocampus by the Fasciculus marginalise^) — so it is certain that 



1) The Brain of a Foetal Ornithorhynchus. Quart. Journ. of Microsc. 

 Science, N. S. Vol. 39, 1896, PI. 2, Figs. 6 and 7. 



2) In CxAUPp's Edition of Ecker's and Wiedersheim's "Anatomie 

 des Frosches", 2. Abt., 1899, this olfactory tract is indicated on the 

 left hemisphere in Fig. 29, p. 107, but not labelled. On p. 113, how- 

 ever, Gaupp describes it, employing the name "Fasciculus bulbo-corticalis", 

 given to it by Pedro Ramon y Cajal. 



3) The Fasciculus marginalis, which I described in Ornithorhynchus, 

 represents the bundle described by recent writers as the "Tractus ol- 

 facto-corticalis" in fishes and Amphibia. It is well shown in a diagram 

 (Fig. 150) by Johnston ("The Nervous System of Vertebrates", 1906), 



Anat. Anz. XXXIII. AufsStze. 34 



