82 JOHN WARREN 



eye and an epiphysis, a condition which seems to be pecuhar 

 to Reptiha, and the structure is relatively much suppressed as 

 compared with its development in birds, Reptilia and Amphibia. 

 As it is intended to indicate only the early appearance and topo- 

 graphical position of the epiphysis in this paper, no attempt will 

 be made to consider its histological structure. For histological 

 details see Jordan (13) on the epiphysis of the sheep and Jordan 

 (14) on the structure of the same organ in the opposum. For a 

 review of the appearance of the epiphysis in other vertebrates 

 see Warren (24), 



6. The commissures 



The posterior commissure appears relatively early in sheep 

 embryos and at first begins partly in the forebrain, in the hinder 

 end of the pars intercalaris. In this respect it resembles the 

 early form of the posterior commissure in Reptilia but is even 

 more precocious in its development. It apparently must be 

 regarded as belonging partly to the forebrain and is not wholly 

 confined to the midbrain as is usually stated. It is often diffi- 

 cult in the younger stages to determine accurately its limits, 

 which tend to gradually fade out into the outer layer of the brain 

 wall, but in all the younger stages it had approximately the 

 extent as shown in figures 1 and 2. In embryos of about 20 mm. 

 the commissure has completely filled up the pars intercalaris 

 and its cephalic end lies directly against the dorsal wall of the 

 epiphysis. This same condition is also found in young human 

 embryos of from 10 to 20 mm. The extent of the pars inter- 

 calaris in the roof of the early mammalian brain, where it really 

 should be considered as one of the primary arches or segments 

 in the roof of the brain, is one of the most striking features in 

 these embryos. It is constant in all vertebrates and always 

 precedes the superior commissure except in ammocoetes. 



The superior commissure can first be observed in sheep em- 

 bryos of 24 to 26 mm. and lies directly against the anterior wall 

 of the epiphysis. It occurs in practically all vertebrates in- 

 cluding man and its development has been thoroughly described 

 by Minot (16), Neumeyer (17), Cameron (3), Osborn (18), and 

 Dexter (4) . 



