102 Walter E. Dandy. 
EXPLANATION OF PLATES. 
PLATE ] (Fics. 1-7). 
Fie. 1. Section 22, x 50. Bi, First primary brain vesicle; Oo, Aorta; A1, 
Complete view of longitudinal section of first aortic arch; A2, Cross section 
of stub of second aortic arch; En, Endothelial heart; Ht, Mesodermal wall of 
heart; P.C., Pericardial cavity; Fg, foregut; Am, Amnion; Mes, Mesoderm: 
Ent, Entoderm. 
Fic. 2. Section 538, «x 50. Ch, Chorda; Ao, Dorsal aorta; V, Umbilical 
vein; Fg, Foregut; P.C., Pericardial cavity; B2, Second primary vesicle of 
the brain. 
Fic. 3. Section 76, x 50. Pr, Pronephros; Coe, Celom; Pl, Pleural Ceelom ; 
V, Umbilical vein; Br V, Branch of umbilical vein, only suggestion of a vitel- 
line vein, it unites with umbilical vein two sections below; Ch, Chorda; M1, 
First Somite in each side. 
Fic. 4. Section 93, x 50. Br, Cross section of lateral branch of dorsal 
aorta; V, Umbilical vein; M4, Somite IV, with cavity (anterior tip of somite 
IV on opposite side) ; Coe, Celom; B.C., Communication of ceelom with ex- 
terior; Ch, Chorda. 
Fie. 5. Section 101, x 50. Br. Lateral branch of Aorta; Ao, Aorta; 
V, Umbilical vein; M4, Somite IV with cavity; M5, Tip of Somite V; Coe, 
Ceelom. 
Fig. 6. Section 187, & 50. Ao, Dorsal aorta; Br, Lateral branch of aorta 
(branch on left side terminates in a mass of mesodermal cells which may be 
a small blood island); Ch, Chorda at point of greatest development; Coe, 
Celom; S.M, Slit in mesoderm. (See also Fig. 11.) Pr. S, Beginning of 
primitive streak. U. V, Umbilical vesicle. 
Fig. 7. . Section 152, « 50. Shows primitive streak region (Pr. S.) from 
fusion of ectoderm, mesoderm and entoderm. Hg, Hindgut; All, Allantois, 
near point of origin from hindgut; U, Umbilical artery; V, Umbilical vein; 
S. M, Slit in mesoderm. Ect, Hctoderm; Hnt, Entoderm; Mes, Mesoderm. 
