DEVELOPMENT OF THE ALBINO RAT 79 



embryonic ectoderm and the extraembryonic ectoderm meet. 

 The special cytomorphosis undergone by the columnar cells 

 of the sides of the egg-cylinder, in contradistinction to those of 

 the antimesometrial end, will be considered in later pages. The 

 visceral layer of the entoderm extends to the base of the ecto- 

 placental cone, in part passing over onto the layer of parietal 

 ectoderm. In the section figured, cells of the parietal layer of 

 the entoderm are not evident. The ectoplacental cone has 

 grown in length in the direction of the lumen of the uterus or the 

 mesometrial border. In the great majority of my preparations 

 this structure is slightly compressed from side to side, so as to 

 be broader in a plane parallel to the long axis of the uterus. In 

 vesicle A, it is cut at right angles to the long axis of the uterus, 

 thus appears as much narrower than in the other two vesicles 

 of figure 26, which were cut in a plane parallel to the plane of 

 the mesometrium. The increase in size of the ectoplacental 

 cone is the result of active cell proliferation. Mitotic figures 

 to the number of one, two or three, may now be observed in 

 nearly every section of this structure. The parietal or transitory 

 ectoderm, continuous with the base of the ectoplacental cone, 

 has been reduced by this stage to a thin, practically homogeneous 

 membrane, presenting scattered, flattened nucleated cells on its 

 inner surface. This thin membrane is now quite firmly adherent 

 to the wall of the decidual crypt, throughout nearly its whole 

 extent. 



Under B of figure 26 (rat No. 81, 7 days, 22 hours) there is 

 shown a representative section of a vesicle which is slightly more 

 advanced in development than that shown in A of this figure. 

 The antimesometrial portion of the proamniotic cavity, the 

 anlage of which was shown in the preceding stage, is well estab- 

 lished. Its wall, consisting of primary embryonic ectoderm 

 is composed of a single layer of cells with nuclei in essentially 

 the same plane. The primary embryonic ectoderm forms a 

 closed vesicle (Ectodermblase, Selenka) distinctly demarked 

 from the extraembryonic ectoderm. In this as in the preceding 

 stage the extraembryonic ectoderm forms a long cylindrical 

 structure continuous at its mesometrial end with the base of the 



