466 



PEOr. G. ELLIOT SMITH OX THE MOEPHOLOGT OF THE 



many of them assume a longitudinal direction and associate themselves either with the 

 post-commissural group (which forms the columnar for nicis) or the precommissural group 

 of fihres \vkle Journal of Anatomy and Physiology, vol. xxxii. pp. 36 et seqJ]. 



Fig. 13. 



fta.sc. 



Part of a coronal section through the cerehral hemispheres of Ornithorhynclnis, to show the relation of the 



parncommissural hod}- to the hippocampus. 



The recesstis superior in this section rests upon the upper surface of the " commissure- 

 bed," and presents an oblong shape. It is lined with simple cubical epithelium: its roof 

 consists of a simple epithelial layer, and its characters resemble those of the rest of the' 

 roof {tela) of the third ventricle of which it forms the cephalic extremity ; it is 

 invaginated into the cavity of the superior recess to form the anterior extremity of the 

 mesial choroid plexus — the " dlaplexus " of Biirt Wilder : the lateral walls of the recess 

 are thickened by a small quantity of grey substance derived from the paraterminal bodies, 

 and in this grey substance longitudinal libres belonging to a peculiar set of fornix-tibres 

 which are found at the ventral margin of the fascia deutata are found. These fibres 

 belong, in all probability exclusively, to the group which I have called "fasciculus 

 marginaUs" (Journ. of Anat. and Phys., vol. xxxii. p. 38). In the next figiu*e these 

 features are shown upon a much larger scale (fig. 14). 



The recessus superior exists and presents similar features in the brain of Echidna, and 

 therefore it will be legitimate to institute comparisons between the adult Oriiithorhynchus 

 and certain foetal stages of Echidna, Avith a view to the explanation of certain earlier 

 fcEial stages of Ornithorhyiichus. Such comparative studies are necessary, because it has 

 been impossible to obtain a comjDlete series of foetal stages of Platypus. 



If sections be made through the brain of a foetal Echidna (at the age when the spines 

 are just making their appearance in the skin), it will be found that a section passing 

 through the two cerebral commissures will present features (fig. 15) which, on the 

 whole, so closely resemble those of the adult Ornithorhynchus just considered, that it is 



