1910.] On a Mesocodic Recess in the Hainan Brain, etc. 523 



throughout the vertebrate series. It is possible that in all cases the invagina- 

 tion may be apparent rather than real, and due to overgrowth of the sub- 

 commissural organ by the surrounding tissues. 



We have found no trace of Reissner's fibre in the chimpanzee, but it 

 was hardly to be expected in material preserved merely for anatomical 

 pui'poses. 



III. The Siih-Commissural Organ in Man. 



A mesoeoelic recess, evidently homologous with that of the chimpanzee, 

 but very much smaller, is readily recognisable in transverse sections of 

 the adult human brain, where it appears to constitute the last vestige of the 

 sub-commissural organ, while in the five months' foetus this organ is still 

 fairly well developed. 



Fig. 7.- — Human foetus of about five months. Transverse -section through the Region of 

 the Posterior Commissure, x 60. 



e.p. e., ependj'mal epithelium of iter ; m. r., mesoeoelic recess ; p. c, posterior com- 

 missure ; s.c. o., sub-commissural organ. 



In the foetus the sub-commissural organ is, indeed, rather better developed 

 than in the chimpanzee. As in that animal, it begins anteriorly in the infra- 

 pineal recess, where the characteristic high columnar epithelium, with 

 deeply-seated nuclei, is well marked. As it is traced backwards around 

 the curve of the posterior commissm-e its paired nature becomes evident. 

 The two halves diverge, and come to lie widely separated in the roof of the 



