620 PERCIVAL BAILEY 



The forebrains of four embryos — 1 a, greatest length 5.1 mm.; 

 H. E. C. no. 1433, greatest length 8.8 mm. ; 5 b, carapace 8.6 

 mm.; and 4 b, carapace 10.6 mm. — were reconstructed by the 

 Born method at a magnification of 100 diameters. Millimeter 

 plates were used and every section drawn. It was necessary 

 to dissect the models rather extensively to expose- the fissura 

 chorioidea. The models were stacked from a side view of the 

 head drawn from a photograph, with the exception of the Har- 

 vard embryo. This latter embryo was cut sagittally and the 

 stacking was guided by the epiphysis and paraphysis. Since 

 the embryo was not cut in an exactly sagittal plane, after the 

 paraphysis and epiphysis had passed out of the plane of section, 

 most of the lateral telencephalic plexus had been stacked and 

 the remaining sections were added with no other guide except 

 comparison with other models. 



DESCRIPTION 



This description will be confined to the lateral telencephalic 

 plexus, since concerning the other plexuses I find no reason to 

 differ from Warren's account, with the exception noted in the 

 history. 



The main landmarks of the region in which the lateral telen- 

 cephalic plexus develops are already laid down in an embryo of 

 5.1 mm. greatest length. Figure 26 shows the roof plate of the 

 forebrain in such an embryo and figure 10 shows the same region 

 schematically represented. The roof plate of the telencephalon, 

 back of the lamina terminalis, appears as a triangular area (only 

 half of it shown in figure 26) its base formed by the velum trans- 

 versum and its apex lying at the posterior end of the lamina 

 terminalis while from its center arises the paraphysis. The 

 lateral sides of the triangle are formed by the taeniae fornices. 

 At the lateral angles of the triangle, taenia fornicis, velum trans- 

 versum, taenia thalami and di-telencephalic groove meet. It 

 is not possible accurately to determine this point in the embryo 

 under consideration because the roof-plate has been but imper- 

 fectly differentiated histologically, but in later stages these 

 angles of the triangle may be easily located. 



