DEVELOPMENT OF THE ALBINO RAT 89 



consideration. During the early stages of egg-cylinder differ- 

 entiation and anlage of the proamniotic cavity, the layer of viscer- 

 al entoderm differentiates into a portion which is in relation 

 with the primary embryonic ectoderm of the antimesometrial 

 portion of the egg-cylinder, in which region the cells of the en- 

 toderm are first of short cubic shape, later of the pavement type; 

 this portion may be regarded as forming the primary embryonic 

 entoderm, since it forms the greater part of the entoderm of the 

 embryo. The greater part of the visceral entoderm, that which 

 surrounds the sides of the mesometrial portions of the egg-cylin- 

 der, consisting of extraembryonic ectoderm, differentiates into 

 cells of the columnar type. In this latter portion, with the 

 formation of a continuous proamniotic cavity, the entodermal 

 cells undergo characteristic cytomorphosis. In them, as stated 

 by Sobotta, there may be recognized three main zones: (1) a 

 basal zone with denser protoplasm containing the nucleus; 

 (2) a middle zone with markedly vacuolated protoplasm; (3) 

 an outer zone in which hemoglobin granules are recognized, the 

 latter zone staining deeply in eosin. These three zones in the 

 cells of the visceral entoderm in the region of the extraembryonic 

 ectoderm of the egg-cylinder may be recognized in figures 26 and 

 27, not so clearly as in Sobotta's colored figures, particularly his 

 figure 17 ('03) and figure 8 ('11). However, I am able to follow 

 closely his description in my own preparations of a somewhat older 

 stage than thus far figured. It is Sobotta's contention that in 

 the extravasated blood surrounding the egg vesicle, in close 

 apposition to its thin outer wall, there may be observed many 

 red blood cells which, though presenting normal form, show 

 a distinctly granular content. These granules stain deeply in 

 eosin and are in shape, size, and reaction to stain very similar 

 to granules found in the peripheral part of the cells of the visceral 

 entoderm. On the outer surface of the thin wall of the vesicle; 

 on its inner surface; in the cells lining this; in the yolk sac cavity; 

 and on the outer surface of the cells of the visceral entoderm, 

 similar granules are found. These appearances are interpreted 

 as showing an absorption of maternal hemoglobin by the ento- 

 dermal cells of the mesometrial portion of the egg-cylinder. 



