RABBIT VESICLES OF SEVEN DAYS. 



311 



where each nucleus is lodged. The 

 nuclei are smaller than those of the 

 ectoderm, more darkly stained, and 

 the granules 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 177 represents 

 a transverse section through the poste- 

 rior part of the embryonic shield; the 

 position of the median plane is ap- 

 proximately indicated by the vertical 

 line M. About this plane there is a 

 considerable accumulation of cells which 

 merges without boundary into the sup- 

 erficial cells of the shield. A short dis- 

 tance from the median line the outer 

 layer of the shield becomes a distinct 

 epithelium, 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 ento- 

 derm as a continuous layer, which is 

 only loosely connected with the central 

 mass of cells overlying it near the me- 

 dian plane. Finally, from the median 

 mass of cells extends laterally the sheet 

 of mesoderm, Mes, between the outer 

 and inner germ-layers. The mesoder- 

 mic cells are somewhat loosely dis- 

 tributed, and have round nuclei with 

 distinct chromatin 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. 



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