100 PERCIVAL BAILEY 



the 19 mm. embryo to those of the 10 mm. cat and 20 hr. chick 

 figured by Tilney ('15) and of a 4 mm. embryo of Platydactylus 

 mauritanicus, figured by Tandler and Kantor C07), is rather 

 striking. 



d. Recessus neuroporicus. An identification of this point is 

 absolutely essential to a final definition of the boundary be- 

 tween lamina terminalis and lamina supraneuroporica. By 

 recessus neuroporicus is meant the most caudal point, i.e., last 

 point, of closure of the neuropore. In many vertebrates at 

 this point a recess appears on the ventricular surface of the roof 

 plate (Johnston, '09). 



It was found impossible to identify with certainty this point 

 in the embryos examined. In the 19 mm. embryo, and the 28 

 mm. embryo, no such recess is apparent. In the 32 mm. em- 

 bryo, there has been noted just in front of the pit in the roof 

 plate, a shallow notch on the ventricular surface (fig. 25, r.n.f). 

 But there is no evidence that this is the recessus neuroporicus, 

 since no such notch appears in either younger embryo. 



e. Lamina terminalis. The upper end of the lamina terminalis, 

 as defined by Johnston, has not been determined because the 

 recessus neuroporicus is not apparent. Concerning the major 

 portion of the lamina, however, there can be no doubt. The 

 thick lamina above the recessus preopticus is unmistakable 

 (figs. 18, 20 and 22, l.L). 



f. Tela chorioidea telencephali medii. Just in front of the 

 paraphysal arch in the 28 mm. embryo, and the 32 mm. embryo, 

 the roof becomes a single layer of flattened cells (fig. 5, t.c.t.m.). 

 This is certainly tela chorioidea telencephali medii. The iden- 

 tification of this tela in the 19 mm. embryo is not so easy. How- 

 ever, I am inclined to identify the summit of the greater arch 

 of the roof plate (fig. 18, r.s) as the recessus superior. This 

 would make the roof between this point (fig. 18, r.s.) and the 

 paraphysal arch, tela chorioidea telencephali medii (fig. 18, 

 t.c.t.m.). The roof plate here is somewhat thinner than the 

 lower limb of the greater arch (fig. 18, l.s.f) and the adjacent 

 hemisphere wall is considerabl}^ thinner. The angulus terminafis 



