168 STUDY OF THE SEGMENTATION OF THE OVUM. 



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 ir- 

 regular 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 between the threads of the protoplasm. Occasion- 

 ally there appear in the protoplasm of these cells narrow, elongated, highly refrin- 

 gent bodies somewhat resembling bacilli in appearance, and therefore they are 

 termed the bacilliform bodies. Their nature is unknown; they are more apt to be 

 found in older vesicles. The outer or subzonal layer can be made out over the 

 embryonic shield only by very careful observation. In the shield the cells are sev- 

 eral 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 observa- 

 tion, 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 sub- 

 zonal (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 scattered about in patches 

 or isolated. As the cuboidal cells of the ectoderm are confined to the region of 

 the embryonic shield, the cells of the entoderm outside of the shield lie close 

 against the subzonal layer. Here they may be more easily studied than in the 

 shield itself. They are very much smaller than the cells of the outer layer and 

 contain each a nucleus with highly refringent granules, which are now numerous 

 and smaller than the somewhat similar granules in the overlying nuclei of the ecto- 

 derm. The farther away we proceed from the edge of the embryonic shield, the 

 fewer we find the entodermal cells. The extent of their distribution varies greatly, 

 and apparently more or less in relation to the size of the blastodermic vesicle, since 



* As seen by transmitted light 



