DEVELOPMENT OF THE ALBINO RAT 77 



with a consequent infolding of the layer of visceral entoderm is 

 not as a rule evident, if so, only very slightly, as to the left in B ; 

 such infolding of the visceral entoderm is not regarded as having 

 special significance. These structures, ectodermal node and extra- 

 embryonic ectoderm, are appropriately referred to as ectodermal 

 cylinder by Widakowich, and with the visceral entoderm, as 

 constituting the egg-cylinder of Sobotta. 



Under A of figure 26 (rat No. 17, 8 days, 17 hours), there is 

 shown a representative section of a vesicle which is only very 

 slightly older than that shown under C, figure 25. This vesicle 

 was exposed, by teasing away, after fixation, the decidual tis- 

 tue forming one side of the decidual crypt; this being done before 

 embedding, so as to admit of orientation of its long axis. This 

 accounts for the collapsed state of the thin wall of the vesicle 

 and its slight folding, also for the fact that the ectoplacental cone 

 is reflected upon itself. The egg-cylinder is cut in a very favor- 

 able longitudinal plane. In its antimesometrial portion, lower 

 part of the figure, the cells of the ectodermal node now show 

 definite arrangement in practically a single layer, with alter- 

 nating nuclei. The beginning of a central cavity is evident with 

 reference to which the cells are arranged. This cavity is the 

 anlage of the 'Markamnionhohle' of Selenka, more appropriately 

 known as the antimesometrial portion of the proamniotic cavity. 

 The cells forming the wall of the ectodermal vesicle (Ektoderm- 

 blase, Selenka), derived from the ectodermal node, may now be 

 known as the primary embryonic ectoderm (Widakowich). 

 The extraembryonic ectoderm in the mesometrial portion of the 

 egg cylinder has differentiated to form a relatively long irregu- 

 larly cylindric structure, continuous with the base of the ecto- 

 placental cone, composed of irregular polyhedral cells, com- 

 pactly arranged and showing as yet no definite orientation. In 

 these cells active proliferation is evidenced by numerous mitoses. 

 The egg-cylinder is covered by a single layer of cells of the 

 visceral entoderm. Over the antimesometrial end of the egg- 

 cylinder, the entodermal cells now present a cubic or thick pave- 

 ment form, while along the sides of the egg-cylinder they are of 

 columnar form, especially long in the region where the primary 



