PINEAL REGION IN MAMMALIA 101 



covered anteriorly by the arched suprapineal recess. In rodents 

 it undergoes a much more extensive development and resembles 

 somewhat the large epiphysis of reptiles. In the cat and rabbit 

 it is characterized by a long hollow stalk with fairly thick walls 

 and an extended distal end, which shows distinct tubule forma- 

 tion and inclines backward over the posterior commissure. 



7. The superior commissure is well developed and present in 

 all forms studied. It is deeply placed between the epiphysis 

 and the suprapineal recess. The posterior commissure makes a 

 very early appearance, being present in almost all forms in the 

 stages where the primary arches are present in the roof of the 

 forebrain. Its first traces can be seen in both the forebrain and 

 the mid brain and it soon occupies all of the pars intercalaris, 

 extending close upto the dorsal wall of the epiphysis. 



8. The diencephalic and lateral telencephalic plexuses are 

 well developed in mammalian embryos but there is no trace of 

 the median telencephalic plexus which is so distinctive in 

 Amphibia. 



