DEVELOPMENT OF THE ALBINO RAT 335 
cells. However, I am disposed to regard these flattened cells 
as derived from the parietal ectoderm. The parietal entodermal 
cells are never numerous in the rat, and mitotic figures are sel- 
dom observed in them. With the extension of the vesicle with 
the enlargement of the blastocele, the cells of the parietal or 
transitory ectoderm become attenuated until they appear for 
the greater part as a thin cuticular membrane, and I am dis- 
posed to regard the flattened nucleated masses of protoplasm 
lining the inner surface of this membrane as derived from the 
cells of the parietal ectoderm. 
Much attention has been given to certain large cells which are 
found in close relation with the outer surface of this thin mem- 
brane. These cells, generally referred to as giant cells (Riesen- 
zellen) were, by Duval, Sobotta (earlier publications) and Gros- 
ser thought to be of embryonic origin and derived from the 
cells of the parietal ectoderm. Selenka, Disse, Kolster, Melis- 
sinos, Pujiula, Widakowich, and later Sobotta (11) regard them 
as derived from the maternal tissue and as representing differ- 
entiated decidual cells. It is not my purpose to consider more 
fully these cells in the present communication, since they are 
by me not regarded as of embryonic origin. My own observa- 
tions as concerns them agree in the main with those of Wida- 
kowich, who, in the albino rat has followed their origin from 
decidual cells. Since not of embryonic origin, they have been 
disregarded in making the figures. 
I have previously, in connection with a discussion of the 
structure of vesicle C, figure 24, alluded to the fact that the 
cells of the ectoplacental cene as also the cells of the parietal 
or transitory ectoderm have a phagocytic action for maternal 
blood cells. This Sobotta has also observed for the mouse, in 
which he is confirmed by Kolster who has further shown that the 
cells of the ectoplacental cone also take up fat particles. Withthe 
ingestion of maternal blood cells by the cells of the ectoplacental 
cone, more particularly, with the absorption of hemoglobin by 
the entodermal cells of the mesometrial portion of the egg- 
eylinder, a period of rapid growth of the egg vesicle is initiated. 
To this Sobotta has called attention for the mouse; the same 
