108 Walter E. Dandy. 
EXPLANATION OF PLATES. 
PLATEVIVE 
Fie. 13. Dorsol-lateral view of model of the embryo, with the amnion 
removed. Umbilical vein with possible vitelline branch (?) seen on right side. 
(The vein is projected on exterior for purpose of clearness.) Large um- 
bilical venous and arterial sinuses posteriorly, with allantois lying between. 
Arterial sinus is partially cut away, venous is intact. Neural canal is open 
throughout, and shows the three primary vesicles of the brain. 
AWE Vis 
I'ic. 14. Same view as Plate IV: window of ectoderm removed, exposing 
the somites and mesoderm. 
PLATE VI. 
Itc. 15. Same view as in Plates IV and V. Mesoderm is removed. 
I, II, I11-Three primary brain vesicles; I-First aortic arch; studs of arches 
2, 3, 4, 5 are seen just posterior; U-Umbilical arterial sinus; V-Umbilical 
veins (umbilical veins and umbilical venous sinus have been cut away to 
show the arterial system). Vi, Branch of umbilical vein, only suggestion of 
possible vitelline vein; Fg-Foregut; Hg-Hindgut; All-Allantois; Ch-Chorda ; 
Ht-Heart; Coe-Ceelom; P.C.-Pericardial coelom; Ca-Capillary, but no connec- 
tion; Pl-Plexus of lateral aortic branches (shown only in this place). Mes- 
Mesoderm in primitive streak region separated from the ectoderm by a 
shrinkage space, which is only present laterally, in this region. (See also 
LS ee) 
