501 



of the sub-commissural organ. Text-fig. 5 represents another section 

 from the same series, but near the anterior end of the sub-commissural 

 organ. The epithelium is seen to be losing its characteristic columnar 

 appearance, especially in the middle line, but Reissner's fibre is still 

 present, and, indeed, in this and neighbouring sections may be seen 

 breaking up into branches, which actually run to the epithelium. 



The Chimpanzee. — Here, again, we have studied the sub-com- 

 missural organ by means of serial transverse sections. The entire brain 

 had been hardened in situ by means of formalin and alcohol for 

 ordinary anatomical purposes. It is naturally not in a good condition 



fie 



frc ~. 



rf 



■' »CO 



( 



Fig. 4. Fig. 5. 



Fig. 4. Cat. Transverse section through the sub-commissural organ in the region 

 of the posterior commissure. ^ 192. 



p.c. posterior commissure ; r.f. Reissner's fibre ; s.c.o. sub-commissural organ. 



Fig. 5. Cat. Transverse section through the region of the posterior commissure 

 considerably antei-ior to the section shown in text-fig. 4. X 75. 



p.c. posterior commissure; r.f. Reissner's fibre; s.c.o. sub-commissural organ. 



for the study of minute histological details, but shows very clearly 

 the general form and relations of the parts under discussion. 



The sub-commissural organ commences, as in the mouse, on the 

 anterodorsal aspect of the posterior commissure , where the latter 

 forms the floor of the infra-pineal recess (compare text-fig. 1), where 

 however, it is very feebly developed. It can be followed backwards 

 beneath the posterior commissure as a broad and very shallow groove 

 (v. Plate, fig. 1 s.c.o.)^ the epithelium of which shows the nuclei arranged 

 in several layers, but does not exhibit the typical differentiation until 

 we come to about the hinder limit of the posterior commissure (text- 

 fig. 6). Here the sub-commissural organ takes the form of two 



