498 



a series leading up to the vestigial condition in the humain brain. 

 We believe the organ in question has never yet been described in 

 any of these types, nor, indeed, has a complete description of it been 

 given for any mammal. 



The Mouse. — In a sagittal section of the brain of the common 

 house-mouse, such as is represented in text-fig. 1, we see the infra- 

 pineal recess (i.p.r.) projecting upwards and backwards between the 

 posterior (p-c.) and superior {s.c.) commissures, and leading to the 

 elongated pineal gland (j).g.). In front of the superior commissure 



Fig. 1. Mouse. Sagittal section through the region of the posterior commissure, &c. 

 X 36. (The lines ab, cd, indicate approximately the planes of the transverse sections 

 represented in text-figs. 2 and 3.) 



c.c. corpus callosum ; c.p. choroid plexus ; i.p.r. infra-pineal recess ; o.l. optic 

 lobe; p.c. posterior commissure; p.g. pineal gland; pit isolated invaginated patches 

 of the epithelium of the sub-commissural organ; r.f. Reissner's fibre; s.c. superior 

 commissure : s.c.o. sub-commissural organ ; s.p.r. supra-pineal recess ; v. veins, 



the roof of the third ventricle is produced upwards and backwards 

 to form the supra-pineal recess (s.jy.r.), a narrow diverticulum which 

 lies parallel to the infra-pineal recess and pineal gland, and which 

 is doubtless homologous with the dorsal sac of lower types. The wall 

 of the supra-pineal recess is greatly folded to form a choroid plexus 



