DEVELOPMENT OF THE ALBINO RAT 341 
region of the junction of the primary embryonic and extraem- 
bryonic ectoderm; D, a little above the middle of the antimeso- 
metrial portion of the proamniotic cavity. The levels of the 
Fig. 30° Four figures from a series of cross sections of an egg-cylinder of the 
albino rat in the stage of development shown in figure 29. > 200. Rat No. 42, 
8 days, 16 hours after insemination. 
The levels at which the several sections drawn were taken is approximately 
indicated by the several crosses found to the left of figure 29. A, middle of 
ectoplacental cone; B, ectoplacental end of the proamniotic cavity; C, just above 
level of junction of the primary embryonic and extraembryonic ectoderm; a 
little above the middle of primary embryonic ectoderm. The want of any definite 
bilateral symmetry of albino rat egg-cylinders of this stage of development is 
shown by this series of sections; p.ect., parietal or transitory ectoderm; ez.ect., 
extraembryonic ectoderm, surrounding mesometrial portion of proamniotic cav- 
ity; pr.emb.ect., primary embryonic ectoderm; v.ent., visceral entoderm; p.emb. 
ent., primary embryonic entoderm; pr.c., proamniotic cavity. 
