PINEAL REGION IN MAMMALIA 97 



enlarged, contains a well marked cavity, with its tip directed 

 forward and the whole structure is of somewhat irregular out- 

 line. The posterior commissure has completely filled the pars 

 intercalaris and has developed backward into the midbrain. 



The oldest stage at my disposal was an embryo of 22 days 

 about 36 mm. in length, which was not very well cut or pre- 

 served, but an embryo of 19 days, 30 mm. (fig. 30) shows essen- 

 tially the same conditions. This was from a special series not 

 catalogued in the collection. In the median plane the velum 

 forms a distinct angle with a low fold or groove in the paraphysal 

 arch which passes immediately over into the lamina terminalis. 

 On either side of the median line the velum is much obscured 

 by the plexus which has become much increased, leaving clear 

 however a small suprapineal recess. The superior commissure 

 is of good size, while the posterior commissure and the pars 

 intercalaris seem somewhat reduced in length by the pressure 

 from the midbrain behind. The epiphysis consists of a long 

 tubular body, which ends in an enlarged tip and the central 

 cavity in the body is prolonged into numerous smaller tubules 

 in the extremity of the organ. The whole resembles somewhat 

 the epiphysis of birds. The striking feature in rat and rabbit 

 embryos is the extreme development of the epiphysis which 

 differentiates them from the other mammalian specimens in 

 the Harvard Collection, where the organ remains in a more 

 rudimentary condition. 



3. Ungulates 



No additional description is needed here to any extent, as the 

 development in the sheep has been already discussed. As re- 

 gards pig embryos Heuser's (9) work on the shape of the ven- 

 tricals covers the main points in the development of this part of 

 the brain. See also Johnston (12) on the morphology of the 

 forebrain in vertebrates for additional details, especially the 

 velum. The formation of the primary arches and the three 

 main subdivisions of the forebrain correspond to the account 

 already given for sheep embryos. Heuser found no sign of a 

 paraphysis but there is a deep fold running the whole width of 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 28, NO. 1 



