RABBIT VESICLES IN ALCOHOL. 307 



upon the ovum of the dog (pages 59, and 60), but are smaller in all of their dimen- 

 sions. Immediately underneath the external membrane there is a continuous 

 layer of cells belonging to the ectoderm and extending completely around the 

 ovum. The layer is sometimes designated specifically as the " outer layer" or as 

 the ' ' subzonal layer. ' ' It also extends over the embryonic shield ; the portion upon 

 the shield is often termed Rauber's layer, it having been first observed by that 

 investigator. The cells of the outer layer are quite large and their boundaries 

 are easily recognizable in surface views. Their sides may number four, five, or 

 six, six being perhaps the more usual number, and are variously disposed, so that 

 the cells differ in shape and size. During the next two days of development the 

 cells become, if anything, more irregular in outline and somewhat smaller. The 

 boundaries between the cells are very fine lines ; the nuclei are rather large and 

 oval in form, and contain from three to four or five highly refringent granules. 

 Each nucleus is surrounded by a denser court of protoplasm in which there are 

 many granules, some of which are highly refringent. The peripheral portion of 

 the cell is of a loosely reticulate structure with comparatively wide meshes be- 

 tween the threads of the protoplasm. Occasionally there appear in the proto- 

 plasm of these cells narrow, elongated, highly refringent bodies somewhat re- 

 sembling bacilli in appearance, and therefore they are termed the bacillijorm 

 bodies. Their nature is unknown ; they are more apt to be found in older vesi- 

 cles. The outer or subzonal layer can be made out over the embryonic shield 

 only by very careful observation. In the shield the cells are several layers 

 thick. The inner cells are very much smaller in size than the cells of the outer 

 layer, are more granular, and contain smaller nuclei which take up a relatively 

 large place in the cell in proportion to its apparent area. Closer observation, 

 utilizing the fine adjustment of the microscope, will show that there are two 

 kinds of cells in the inner part: first, those which, like the cells of the subzonal 

 layer, belong to the ectoderm; and, second, an inner layer of cells, which appar- 

 ently belongs entirely to the entoderm. In the region of the embryonic shield 

 the ectoderm is, therefore, made up of two distinct layers of cells. The outer or 

 subzonal (Rauber's layer) disappears during the sixth day of development as a 

 distinct layer. The cells of the entoderm form a very thin continuous layer on 

 the under side of the embryonic shield. They may be recognized by the very 

 granular, and therefore dark,* appearance of their protoplasm, and by the 

 rounded form and small size of their nuclei. Similar cells may be observed also 

 extending beyond the limits of the embryonic shield, though not there forming a 

 continuous layer, except perhaps for a very short distance, but rather lying 



* As seen by transmitted light. 



