DEVELOPMENT OF THE CHOROID PLEXUS 87 



entire structure, when viewed from above, is somewhat trumpet- 

 shaped, with the bell at the anterior end. There is no indication 

 of plexus formation. The entire roof plate of the diencephalon 

 is perfectly smooth. The commissura superior is clearly indi- 

 cated (fig. 18, com.s.). 



The epiphyseal evagination is a hollow outgrowth (figs.* IS 

 and 3, e.e.). The top of the evagination is cupped and in this 

 cup lies a ball of cells (figs. 3, 10 and 18, e.v.). This ball of cells 

 has an irregular lumen in its center (fig. 10, e.v.). The cells of 

 the ball stain more lightly than the cells of the cup, and the line 

 of separation is fairly distinct. It will be seen that this ball 

 of cells, while not in actual contact with the epidermis, approaches 

 it very closely (fig. 10, e.v.). The epiphyseal evagination lies 

 some distance cephalad of the commissura posterior (fig. 18, c.p.). 



The extent and morphology of the lateral choroitl plexus is 

 shown in figure 19. It is clearly divisible into two parts, an 

 anterior part (figs. 2 and 19, p.c.v.L, p.o.) attached to the lateral 

 margin of the paraphysal arch along its entire length, and by the 

 taenia 'fornicis (fig. 2, /./.) to the medial hemisphere wall im- 

 mediately above and lateral to it; and a posterior part (figs. 1 

 and 19, p.c.v.L, p:p.) attached by the taenia chorioidea (fig. 1. 

 t.c.) to the lateral thalamic wall and by the taenia fornicis (fig. 

 1, t.J.) to the medial hemisphere wall immediately below the 

 hippocampus. The fissura chorioidea is very wide throughout 

 most of the extent of the posterior part of the lateral choroid 

 plexus. 



If the angle between the taenia chorioidea and the thalamic 

 wall be followed anteriorly, it is found to be continuous with the 

 velum transversum; if the angle is followed posteriorly, it con- 

 tinues backward between the lateral thalamic wall and the 

 medial hemisphere wall, turns downward between them and 

 passes anteriorly and downward behind the optic nerve on the 

 lateral wall of the hypothalamus (fig. 19, d-t. gr.). This groove 

 is more clearly marked on the lateral wall of the 32 mm. embryo. 



The ependymal portion of the plexus is still relatively thick 

 (fig. 27). The mesenchymal tissue of the plexus is typical em- 



