DEVELOPMENT OF THE ALBINO RAT 355 
node, the antimesometrial portion of the proamniotic cavity. 
A little later a second cavity develops in the extraembryonic 
ectoderm, the mesometrial portion of the proamniotic cavity, 
the two cavities fusing by the end of the eighth day to form a 
single proamniotic cavity, lined in its antimesometrial portion 
by primary embryonic ectoderm, and in its mesometrial portion 
by extraembryonic ectoderm, the two types of ectoderm forming 
a continuous layer with the line of junction readily distinguish- 
able. No evidence of bilateral symmetry is at this stage ob- 
served in the egg-cylinder. 
During the ninth day after insemination there is observed 
the anlage and the early developmental stage of the mesoderm 
and the anlage of the primitive streak and groove. The meso- 
derm has its anlage in the caudal portion of the primary em- 
bryonic ectoderm in the sagittal region and is of the nature of 
prostomial mesoderm, extending laterally in wing-like exten- 
sions between the ectoderm and entoderm. 
