212 ADAM SEDGWICK. 
area, constituting the primitive streak, which is marked by a groove. 
p. g- Primitive groove. 
Fic. 26, a—d.—Series of transverse sections through an embryo of Stage 
A (Pt. 1, Pl. XXXT, fig. 22). Mesoblastic bands (m. 6.) have begun to grow 
forward from front end of primitive streak. Zeiss’s D, oc. 2, camera. 
a. Through the posterior end of blastopore, and two sections behind the 
front end of the mesoblastic band. 
6. Hindermost section through the blastopore. 
c. Through the primitive streak three sections behind the blastopore. 
d. Through the primitive streak ten sections behind the blastopore. 
This section shows a portion of the polar area lying unaltered in the 
deeper part of the primitive streak. 4/. Blastopore. p.g. Primitive 
groove. p. st. Primitive streak. m. 6. Mesoblastic band. 
