86 PERCIVAL BAILEY 



ether-alcohol, then through 0.5, 1, 2 and 3 per cent celloidin, 

 hardened in chloroform-alcohol, cleared in benzol and imbedded 

 in paraffin under the air pump. 



The plane of section in each case is shown in figures 29; 30 and 

 31. 



Several other human embryos were studied, the most helpful 

 being embryo H 44 of the Chicago collection, which measured 

 60.4 mm. after fixation in formalin. A transverse series from 

 a 25 mm. pig in the collection of Dr. F. R. Lillie was also used. 



DESCRIPTION 



1. The 19 mm. embnjo {H. 173) 



The recessus preopticus is well marked (fig. 18, r.pre.). The 

 roof plate stretches dorsad from this recess as a thickened lam- 

 ina (fig. 18, l.t.) to about the level of the sulcus separating the 

 medial and intermediate roots of the corpus striatum. Above 

 this point the roof plate narrows and extends cephalad and dor- 

 sad (fig. 18, Z.s.?), forms a broad arch (fig. 18, r.s.), and then 

 passes caudad and dorsad (fig. 18, tx.t.m.) as a still thinner mem- 

 brane toward the velum transversum. Just in front of the 

 velum transversum, the roof plate forms a small narrow arch 

 (fig. 18, p.a. and fig. 2, p. a.), from the sides of which arise the 

 lateral choroid plexuses. 



(Throughout these descriptions, narrow and wide are used of 

 dimensions tangential to the ventricular surface, and thick and 

 thin of dimensions perpendicular to the ventricular surface. 

 For example, in figure 12, the tela chorioidea diencephali is thin 

 and wide.) 



The velum transversum is well marked (fig. 18, v.L), indicat- 

 ing the boundary in the roof plate between the diencephalon and 

 telencephalon. 



The roof of the diencephalon (fig. 18, tx.d.) is still narrow 

 throughout most of its extent. It is also relatively thick, with 

 several rows of nuclei in cross-section. It is narrowest at its 

 posterior end and remains narrow almost to the anterior end of 

 the thalamus, where it suddenly widens (fig. 1, t.c.d.). The 



