STUDY OF RABBIT BLASTODERM 1C VESICLES IN ALCOHOL. 



173 



(Rauber's) layer has disappeared. The 

 entoderm is very thin, but is thickened a 

 little where each nucleus is lodged. The 

 nuclei are smaller than those of the ecto- 

 derm, more darkly stained, and the gran- 

 ules in them less coarse than those in the 

 nuclei of the ectoderm. Between the two 

 layers is a narrow space; whether an 

 artefact or not is difficult to say. Figure 

 128 represents a transverse section through 

 the posterior part of the embryonic 

 shield where the primitive streak, pr.s, is 

 just forming. The position of the median 

 plane is approximately indicated by the 

 vertical line M. About this plane there 

 is a considerable accumulation of cells 

 which merges without boundary into the 

 superficial cells of the shield. A short 

 distance from the median line the outer 

 layer of the shield becomes a distinct epi- 

 thelium, EC, consisting of a single layer of 

 cells. The edge of the shield is marked 

 by a rather abrupt transition to the thin 

 outer layer of the extra-embryonic region. 

 On the under side of the section extends 

 the thin entoderm as a continuous layer, 

 which is only loosely connected with the 

 central mass of cells overlying it near the 

 median plane. Finally, from the median 

 mass of cells extends laterally the sheet 

 of mesoderm, Mes, between the outer 

 and inner germ-layers. The mesodermic 

 cells are somewhat loosely distributed, and 

 have round nuclei with distinct chroma- 

 tin granules and well-marked protoplasmic 

 bodies, which give off strands by which 

 the cells are united to one another. The 

 middle germ-layer is the least compact of 

 the three. 



