DEVELOPMENT OF THE ALBINO RAT 111 



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. 



