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ated and apparently ciliated columnar cells, and evidently formed 

 by specialisation of the ependymal epithelium which lines the cavity 

 of the brain. Previous to this time there appear to have been, at 

 the most, only a few scattered references to the occurrence of any 

 such structure in the vertebrate brain. We now know, however, that 

 it occurs throughout the whole vertebrate series, from the lampreys 

 to the primates, it having recently been figured, in the case of Macacus, 

 by Sir Victor Horsley (1908). 



Sargent (1903) has given to this structure the name "ependymal 

 groove", and has described it in a number of Ichthyopsidan types. 

 He says, however (1903, 1904), that it is inconspicuous in mammals. 

 We ourselves have recently observed it in a number of difierent forms. 

 It is very well developed, for example, in Sphenodon, and we find it 

 also strongly developed in the mouse and the cat. Sargent showed, 

 further, that the "ependymal groove" is intimately connected with the 

 anterior end of Reissner's fibre, and in fact regarded it merely as a 

 kind of attachment plate for the latter. We also have been able to 

 demonstrate quite clearly its connection with Reissner's fibre in a 

 large number of types, e. g. Geotria (Dendy, 1907), and Rana 

 (NiCHOLLS, 1908). We shall discuss its possible function later on in 

 the present communication ; but in the meantime, we may state that 

 we do not consider that the therm "ependymal groove" is sufficiently 

 distinctive for so remarkable and constant a feature of the verte- 

 brate brain, for it is, of course, not the only ependymal groove present. 

 Inasmuch as it lies beneath the posterior commissure, we propose to 

 speak of it in future as the "sub-commissural organ." It appears 

 primarily to be made up of two bands of columnar epithelium, usually 

 more or less completely united together in the form of a groove; 

 but, whereas it remains throughout life in a well developed condition 

 in all the lower vertebrates, in man, as we shall endeavour to show 

 in the present communication, it becomes reduced in the adult to a 

 mere vestige sunk in the brain tissue at the back of the posterior 

 commissure, but unmistakably recognisable as the homologue of the 

 sub-commissural organ of lower types. 



II. The sub-commissural Organ in the Mouse, Cat and 



Chimpanzee. 



As the sub-commissural organ is as yet very little known, we 

 propose, for purposes of comparison, to give a short description of 

 it as it occurs in the mouse, the cat and the chimpanzee, which form 



Anat. Anz. XXXVII. Aufsätze. qo 



