DEVELOPMENT OF THE ALBINO RAT 67 



considered. The antimesometrial portion of this vesicle, its roof, 

 consists of a single layer of somewhat flattened cells, the parietal 

 or transitory ectoderm. The parietal ectoderm presents on its 

 inner surface a few four in the section figured entodermal 

 cells of irregular outline. These may be designated, after So- 

 botta, as cells of the parietal entoderm. 



Vesicle B, of figure 24, taken from the same rat as was vesicle 

 A (rat No. 46, 6 days, 14 hours) presents a very favorably cut 

 vesicle, which, however, is slightly compressed from side to side, 

 so that its form appears more nearly circular in the sections cut 

 in the plane of the figure, than were they cut at right angles to 

 this plane. This is especially true of the ectoplacental cone, 

 which for the greater part appears in only two sections of 10 /j. 

 thickness, while in the plane of the figure it measures nearly 90 n. 

 Cognizance of this is to be taken in considering the relative 

 size of the ectoplacental cone as shown in this figure. This 

 vesicle is only very slightly older than that shown in A of this 

 figure. Its ectoplacental cone is made up of a core of relatively 

 large cells, bordered by more flattened cells, which in this prepara- 

 tion stain somewhat more deeply than do the more centrally 

 placed cells. These covering cells are continuous with the cells 

 of the parietal ectoderm. The cell mass projecting into the 

 blastocele is more definitely circumscribed than in the slightly 

 younger stage shown in A of this figure. The ectodermal node 

 appears as an oval mass composed of compactly arranged cells, 

 and is separable on all sides from the surrounding cells. The 

 yolk entoderm, which may now be known as the visceral layer 

 of the entoderm (Sobotta) passes as a single layer of cells of quite 

 regularly cubic or short columnar form, nearly about the ecto- 

 dermal node to reach the base of the ectoplacental cone, extend- 

 ing over on the parietal ectoderm at one side (see right side of 

 figure). A few of the cells of the parietal entoderm, three in 

 the figure, are evident. The parietal ectoderm forming the roof 

 or antimesometrial portion of this vesicle consists of a single layer 

 of flattened cells, which rest on, and are adherent to the decidual 

 tissue; the uterine epithelium lining the decidual crypt in which 

 the vesicle is lodged having in part disappeared in the immediate 

 region of the vesicle. 



