DEVELOPMENT OF THE ALBINO RAT 55 



mass into ectodermal and entodermal cells; the outer layer, 

 covering layer, Rauber's cells, or trophoblast, is clearly differ- 

 entiated from the inner mass by a distinct space. Duval has 

 recognized in early stages of blastodermic vesicle formation of 

 the mouse and rat, in the thicker part of the vesicle, entodermal 

 and ectodermal cells. The former are of irregular form, pos- 

 sess granular protoplasm and are said to possess the property 

 of ameboid movement. The remaining cells are recognized 

 as ectoderm; a distinct covering layer is not recognized. In 

 Christiani's figures (rat), which are, however, so diagrammatic 

 as to be of little value and are evidently drawn from poorly 

 fixed material, entodermal cells, ectodermal cells, and covering 

 cells may be recognized as per legends. Melissinos (mouse), 

 while not describing definitely a peripheral or covering layer, 

 states that outer cells of the thicker pole, like the cells enclosing 

 the segmentation cavity, stain less deeply than do the more 

 centrally placed cells. In earlier stages of vesicle formation, 

 neither in figures nor in text as given by this observer, do I find 

 reference to a differentiation into ectodermal and entodermal 

 cells. The observations of Selenka, Duval, Robinson, Jenkin- 

 son, and others, bearing on the structure of the blastodermic 

 vesicle of the mouse and the rat in early stages of development 

 have been so thoroughly and critically reviewed by Sobotta 

 that an extended discussion has here been deemed unnecessary. 

 It may here suffice to say that while Sobotta' s observations were 

 made and his discussions based on ova obtained from the mouse, 

 my own observations made on the albino rat are in agreement 

 with his and support the contention that in early stages of blas- 

 todermic vesicle formation a differentiation of the thicker part 

 or the floor of the vesicle into a covering, Rauber's cell, or tropho- 

 blast layer, and a further differentiation into ectodermal and 

 entodermal cells, is not to be made: we differ in our accounts of 

 the beginning of the formation of the segmentation cavity. 

 An outer or covering layer is suggested in certain of my own prep- 

 arations, most clearly in that sketched in E of figure 20. How- 

 ever, a uniform difference in structure and reaction to staining 

 reagents of the outer layer of cells is not present in my own 

 preparations. None of my own preparations gives evidence of 



