4.54, ARTHUR ROBINSON. 
Fic. 15. 4.—Longitudinal section [twenty-ninth of forty-eight] of the 
embryonic area of a mouse ovum at the ninth day. x 50. The ovum was 
taken from the same uterus as the ovum represented in Fig. 15, but it has 
attained a slightly more advanced stage of development. 
Fic. 15 B.—Longitudinal section [slightly oblique, twenty-ninth of fifty- 
two] of the embryonic area of a mouse ovum a little older than the ovum 
represented in Fig 15a. x 150. 
Fic. 15 C.—Longitudinal section of the anterior portion of the embryonic 
area of the mouse ovum represented in Fig. 15. x 566. 
Fic. 15D.—Mesial longitudinal section of the centre of the germinal 
area of the mouse ovum represented in Fig. 15. x 283. 
Fic. 15 #.—Longitudinal section [twenty-ninth of fifty-two] of the 
anterior portion of the embryonic area of the mouse ovum represented in 
Fig. 15. x 283. 
PLATE XXV— 
Fies. 15 F—H.—Oblique transverse sections of the embryonic area of a 
mouse ovum at the ninth day. This ovum isa little more developed than 
that represented in Fig. 15D. The positions of the sections are indicated 
by the lines £ g, and Zin Fig. 15B. x 198. 
Fic. 16.—Oblique transverse section of the distal end of the embryonic 
cylinder of a rat’s ovum at a period corresponding to the ninth day in the 
mouse. X 320. 
Fics. 16 4—#.—Slightly oblique transverse sections of the germinal area 
of a rat’s ovum at a period corresponding to the ninth day in the mouse. 
Each section is five micromillimetres thick. Section 16 is the sixth, 16B 
the eleventh, 16C the seventy-third, 16D the eighty-fourth, and 167 the 
ninety-first section from the centre of the germinal area. X 320. 
Fie. 16 #.—Section through a portion of the extra-embryonic area of a 
rat’s ovum at a period corresponding to the ninth day in the mouse, 
showing the formation of the mesoblast from the extra-embryonic hypo- 
blast. x 566. 
PLATE XXVI— 
Figs. 16 G, H.—The ninety-seventh and hundred and first sections respec- 
tively of the embryonic area of the ovum represented in Figs. 16 4—Z#, 
Plate III. Fig. 16Gis x 85, and Fig. 16H is x 320. 
Figs. 17 and 17.4.—Mesial longitudinal sections of the central portion 
and the anterior end of the embryonic area of a mouse ovum at the ninth 
day. x 378. 
Fies. 18 and 18 4.—Mesial longitudinal sections of the central portion 
and the anterior end of the embryonic area of a rat’s ovum at a period 
corresponding to the latter end of the ninth day in the mouse. x 566. 
