EARLY STAGES OF DEVELOPMENT OF THE MOUSE. 63 



hypoblast and an outer layer of epiblast " (25, p. 377), 

 while the other end, the " roof," and sides of the vesicle 

 are composed of a single layer of cells (see his figs. 3 and 4). 

 The '' floor " he terms hypoblast, the sides and " roof " epi- 

 blast, the '^cavity " segmentation cavity (Plate 5, fig. A). The 

 long axis of the blastocyst is parallel to that of the uterus 

 lumen, the "floor" is distal or anti-mesometric, and the 

 embryo is as yet free. The " roof " and sides of the blastocyst 

 now increase very considerably in extent (see his fig. 5), and 

 in the next stage (his fig. 6) the embryo is found, still not fixed, 

 in a cylindrical anti-mesometric diverticulum of the uterus 

 lumen, which is lined not by the ordinary columnar cells, but 

 by a rather low cubical epithelium. The "floor," as before, 

 stains more faintly than the sides and "roof," which latter 

 is (25, p. 879) " continuous with a mass of nucleated 

 protoplasm which occupies the interior of the vesicle," and 

 which agrees in staining characters with the walls, though 

 its nuclei are small. " In the interior of the inner mass is a 

 small cavity, from which a dark line extends to the surface of 

 the proximal pole of the ovum." "It seems probable that 

 during the sixth day the thin roof of the blastodermic vesicle 

 is invaginated into the blastodermic cavity. ... It does 

 not become divided into an outer layer and an inner mass, for 

 the inner mass is undoubtedly formed by invagination of 

 the thin roof of the vesicle " (the spacing is mine) (fig. B). 



The segmentation cavity thus becomes obliterated, and 

 a cavity is formed in the thickened floor by vacuolation, the 

 cavity of the yolk-sac (his fig. 7). By the development 

 of this a stage is reached in which the epiblastic knob (the 

 invaginated inner mass) is found resting on the concave 

 proximal end of an elongated yolk-sac, and covered by a cap 

 of trophoblast derived from the sides of the original segmenta- 

 tion cavity (his figs. 8 and 9) (Plate 5, fig. C). By the growth 

 of the trophoblast is formed the "Trager," which subsequently 

 invaginates the epiblastic knob into the cavity of the yolk- sac 

 (his figs. 10, 11, and 12) (Plate 5, fig. D). 



Such is Robinson's description. In the last-mentioned 



