DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 455 
Fig. 18 B.—Transverse section of a portion of a rat?s ovum immediately 
in front of the embryonic area at a period corresponding to the latter part 
of the ninth day in the mouse. x 260. 
Fics. 18 C—#.—Transverse sections through the anterior portion of the 
embryonic area of a rat’s ovum at a period corresponding to the latter end 
of the ninth day in the mouse. Sections 18C and 18D are taken in the 
directions of the lines C and D respectively in Fig. 18.4. Section 18 Z is 
situated more posteriorly. x 260. 
PLATE XXVII— 
Fic. 18 #—Transverse section through the embryonic area represented 
in Figs. 18 A—Z, but in the region posterior to that represented in Fig. 18 #. 
x 260. 
Fic. 19.—Longitudinal section of the ovum of a rat at a period corre- 
sponding to the tenth day in the mouse. x 58. 
Fic. 19.4.—Mesial longitudinal section of the anterior portion of the 
embryonic area of the ovum represented in Fig. 19. x 566. 
Figs. 20 and 20. 4.—Transverse sections through the central portion of 
the embryonic area of a mouse ovum at the tenth day. The sections have 
passed through the area a short distance in front of the anterior end of the 
primitive streak. Section 20 is the more anterior, and Section 20a the 
more posterior of the two. x 283. 
Fic. 20 B.—A transverse section through the anterior end of the primi- 
tive streak of a mouse ovum at the tenth day. x 283. 
Fic. 21.—A mesial longitudinal section through the posterior portion of 
the embryonic area of a mouse ovum at the eleventh day. x 185. 
Fic. 214.—A mesial longitudinal section through the posterior end of 
the chorda dorsalis and the anterior end of the primitive streak of a mouse 
ovum at the eleventh day. x 283. 
Fic. 21B.—A slightly oblique transverse section through the posterior 
part of a mouse embryo at the eleventh day. The section passes through 
the posterior end of the chorda dorsalis. x 285. 
