DEVELOPMENT OF THE CHOROID PLEXUS 91 



Below the sulcus limitans lie the subthalamus (fig. 18, s-t.) 

 and hypothalamus (fig. 18, hy.). The elevation which marks 

 this region is very long. The pars optica is only indistinctly 

 marked off (fig. 18, p.o.hy.). At its posterior end, the elevation 

 divides into two portions, one of which, subthalamus, continues 

 upward into the tegmentum, the other, hypothalanms, backward 

 into the mammillary recess. 



The floor plate is thin and somewhat widened (fig. 3). The 

 infundibulum (fig. 18, inf.) lies a considerable distance back of 

 the optic chiasm. The postoptic recess is but jioorly marked 

 (fig. 18, r. post.). 



2. The 28 mm. embryo (H. 91) 



The entire telencephalon is not modeled in this embryo. The 

 model was made primarily to show the lateral choroid plexus. 

 Those portions not modeled differ in no essential respect from the 

 corresponding j^ortions of the 32 mm. embryo. 



Immediately ccphalad of the velum transversum (fig. 20, 

 v.t.), which is clearly indicated, the roof plate forms a small 

 arch (fig. 20, p.a.) to the sides of which are attached the lateral 

 choroid plexuses. In front of the arch, the roof plate becomes 

 very thin for a few sections (fig. 5, t.c.t.m.). Then as we pass 

 cephalad of this thin lamina, we come to a region where the roof 

 plate thickens in a peculiar manner (fig. 0, t.r-p.). 



The hemisphere wall on each side of the roof plate is thin and 

 in it two distinct zones are discernable, a broader zone next the 

 ependymal surface, where the nuclei are very numerous, mantle 

 layer, and a narrower zone next the pial surface which is relatively 

 free from nuclei, marginal layer. In the roof plate, however, 

 these two. zones are not discernable, the nuclei are equally 

 numerous throughout from the pial to the ependymal surface, 

 and at the ependymal surface are loosely arranged, so that the 

 outHne is indefinite and irregular. This is the characteristic 

 arrangement, and such an arrangement I have found in this region 

 in five human embryos of about this age in the collection of the 

 Department of Anatomy and in a 25 mm. pig embryo in the 



