78 JOHN WARREN 



sagittal series and they correspond to the conditions abeady 

 described in other vertebrates, Minot (16), Terry (22), Neu- 

 mayer (17), Warren (24). The extension of the posterior com- 

 missure into the forebrain is very striking and it is evident that 

 this belongs partly to the forebrain and partly to the midbrain. 

 The primary arches, including the pars intercalaris, and the three 

 primary subdivisions of the forebrain can therefore be said to 

 be definitely established in sheep embryos. 



2. Paraphysal arch and paraphysis 



The paraphysal arch is clearly seen in the last two figures 

 and passes over into a relatively low velum. The earliest signs 

 of a paraphysis could be made out in an embryo of 21 mm. 

 (fig. 3). It is a tiny conical structure. P., springing from the 

 telencephalic roof plate immediately in front of the velum. 

 Figure 4, an embryo of 26 mm., shows a good sized paraphysis, 

 P., with a relatively wide opemng lying between the foramina 

 of Monro and the anlagen of the lateral telencephalic plexuses 

 and inclined backwards towards the velum. 



The next stage appears in figure 5 and is from an embryo of 

 29 mm. In the last stages the wall of the paraphysis consisted 

 of cells similar to those in the roof plate of the forebrain. Here 

 the wall of the organ has become much thicker, the distal end 

 more solid and the cavity much reduced. The outline is quite 

 irregular and this feature is shown clearly in figure 6, which 

 represents a model of the paraphysis of the same embryo at 

 100 diameters The view in the picture is from the front and 

 outside of the brain looking in between the walls of the hemi- 

 spheres in the direction of the guide line from the letter P in 

 figure 5. The base of the paraphysis where it is attached to the 

 telencephalic roof plate, is noticeably wide and thick and is 

 prolonged into a narrow tapering extremity, which contains a 

 partial ca\dty and is apparently the main portion of the origi- 

 nal outgrowth. On each side are two lateral prolongations 

 from the base showing a striking uniformity which are prac- 

 tically solid. Directly behind the paraphysis in the mid-line 



