DEVELOPMENT OF THE ALBINO RAT 349 
Pr. © --} --Nilgaes 
Fig. 32. Three cross sections from egg-cylinders of the albino rat, show- 
ing early stages in the development of the mesoderm. X 200. A and B, rat 
No. 34, 8 days, 17 hours; C, rat No. 41, 8 days, 16 hours, after insemination. 
These sections taken from three egg-cylinders are through the primary embry- 
onic ectoderm, near its junction with the extraembryonic ectoderm, thus through 
the antimesometrial portion of the proamniotic cavity. <A, early stage, in 
anlage of the mesoderm; B, anlage of the primitive streak and groove; C, well 
developed primitive streak and groove, with lateral wings of mesoderm; pr.emb. 
ect., primary embryonic ectoderm; pr.emb.ent., primary embryonic entoderm; 
mes., mesoderm; pr.str., primitive streak; pr.gr., primitive groove; p.ect., parie- 
tal or transitory ectoderm; pr.c., proanmiotic cavity. 
cell proliferation. The cells of this region have not the form 
of tall columnar cells, such as seen in the greater part of the 
remaining primary embryonic ectoderm, but are of polyhedral 
