PINEAL REGION IN MAMMALIA 77 



PART I. SHEEP EMBRYOS 



1 . The primary arches and the subdivisions of the forebrain 



The primary arches were first described by Minot (16) in 

 Acanthias and have since been demonstrated in practically all 

 vertebrates below Mammalia. In a previous article, Warren 

 (24), the primary arches and the three main subdivisions or 

 segments of the forebrain in vertebrates were considered and 

 the previous work on these subjects was reviewed at length. 

 The primary subdivisions of the forebrain consist of a telen- 

 cephalic segment whose roof is formed by the paraphysal arch. 

 This is separated by the velum from the first diencephalic seg- 

 ment (Kupffer's parencephalon) mth the postvelar arch and 

 later the epiphysal arch in its roof. This in turn is followed by 

 the second diencephalic segment bounded above by the pars 

 intercalaris (Kupffer's synencephalon) which contains a portion 

 of the posterior commissure and is limited caudally by the groove 

 and ridge between the fore- and midbrain. These subdivisions 

 were demonstrated in Reptilia and also in the pig and sheep, 

 Warren (24), figures 28 to 37. For further details on this subject 

 and on the question of neuromeres readers are referred to the 

 review and discussion of the pre\dous investigations on the sub- 

 divisions of the forebrain in the above article. 



Figure 1 is from a model of a sheep embryo of 9.9 mm. sagittal 

 series which shows the subdivisions of the forebrain as outlined 

 in the pre^dous paragraph into the telencephalic segment an- 

 terior to the velum, V, with the paraphysal arch, P. A., forming 

 its roof, the first diencephalic segment the roof of which is made 

 up of the postvelar arch only, P.V.A. (the epiphysal arch is 

 not as yet formed) and separated by an internal ridge from the 

 second diencephalic segment. The roof of the latter, the pars 

 intercalaris, P. I., is practically filled by the posterior commis- 

 sure, P.C. The midbrain is composed of two segments sepa- 

 rated from the forebrain and from each other by distinct in- 

 ternal ridges. Compare this model with those for Reptilia, 

 Warren (24), figures 3, 4, 17. All of the primary arches and the 

 velum are shown in figure 2 taken from an embryo of 14 mm. 



