PARAPHYSIS AND PINEAL REGION IN REPTILIA 343 



Kupffer (57), fig. 185; Nectums, 244, Anguis fragilis; and Hill (42) 

 figs. 40 and 41, chick. It is best marked near the dorsal side of 

 the brain while below its limits become less distinct. Its caudal 

 limit is the groove separating it from the mid brain, M. This 

 segment has been described by von Kupffer as the 'Schalthirn,' 

 synencephalon, synencephalic neuromere and pars intercalaris. 

 It forms a narrow though a well marked segment immediately 

 cephalad to the mid brain and above the habenular flexure. It 

 is a sort of isthmus between mesencephalon and diencephalon but 

 belongs to the latter. Its roof is occupied by the anlage of the 

 posterior commissure which appears here before spreading back 

 into the mid brain and finally covers in all this part. As the mid 

 brain enlarges on its dorsal aspect this part becomes much com- 

 pressed and in later stages its identity seems to become in many 

 cases practically lost. It is very constant in the vertebrate 

 series (Burckhardt), but is not found in Accipenser according to 

 Kupffer. Its early appearance in the lizard and turtle is very 

 striking, being here differentiated almost before the velum or any 

 of the other arches in the roof of the fore brain. In the chick it 

 also makes its appearance at an early stage with the velum, Kupf- 

 fer (57), fig. 277, chick of 30 somites. In the pig I have seen it as 

 a trace only at 7.5 mm., but well marked from 8-10 mm., figs. 34 

 and 35. This is true also of the sheep, it being seen first here at 

 about 8-9 mm., figs 36 and 37. In these mammalian forms it 

 appears relatively late after the velum, paraphysal and post velar 

 arches are well formed and the boundary between telencephalon 

 and diencephalon clearly defined. In amphibia, according to 

 Kupffer, it is best marked, persists in the adult brain as a distinct 

 interval and is not wholly taken up by the posterior commissure. 

 Burckhardt (13) gives diagrams of various forms of vertebrate 

 brains. These diagrams represent sagittal sections of Am- 

 phioxus, Petromyzon, sturgeon, trout, Notidanus, Protopterus, 

 Ichthyophis, Anguis, crow and man. In all of these the roof 

 of the ' Schaltsttick' or synencephalon is clearly shown though 

 varying in length in different cases (see also Terry (102) for 

 teleosts). 



Figs. 28 and 30 are parasagittal sections of the lizard of the 

 same stage as figs. 3 and 4, and of the turtle of the same stage as 



