516 Prof. A. Dendy and Mr. G. E. Nicholls. [May 24, 



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 different 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 Eeissner'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 

 Eeissner's fibre in a large number of types, e.g. Geotria (Dendy, 1907) and 

 Bana (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 term " ependymal groove " is sufficiently distinctive for so 

 remarkable and constant a feature of the vertebrate 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 Suh-Comviissural 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, tlie cat, and the chimpanzee, which form a series leading up to the 

 vestigial condition in the human brain. We believe the organ in question 

 lias never yet been deH(;ril)ed in any of tliese typos, nor, indeed, has a complete 

 description of it been given for any mammal. 



Mouse. — In a sagittal section of the brain of tlie common house-mouse, such 

 as is represented in tcxt-lig. 1, we see the infra-pineal rcc(!ss (iqi.r.) i)r()jecting 

 upwards and l)ackwar(ls between the posterior (p.r.) and superior (s.c.) 

 commissures, and leading to the elongated pineal gland (p.g.). In front of 

 the 8ui)eri()r commissui(! the roof of the tliird ventricle is produced upwanis 

 and ba(;kwar(ls to form tlie supra-i)incal recess (n.p.r.), a narrow diverticulum 

 which lies paralU-l to the infra-pineal recess mid pineal gland, and 

 which is doubtless liomologous with the dorsal sac. of lower types. Tlie 



