TRANSITORY CAVITIES IN THE CORPUS STRIATUM. 99 



Several characteristics, easily recognized, permit the inclusion in the same tissue class 

 of the large mononuclear cells found so extensively distributed in the membranes of the 

 embryo, in the umbilical cord, in the mesenchymal spaces, and in the cavities in the corpus 

 striatum. 



The average cell-body of the younger embryo measures from 10 to 13/t, while in the 

 older specimens it ranges from 10 to 19/x. This increase in size accompanies roughly the 

 growth of the embryo, although there are always smaller cells belonging to this group 

 (figs. 19, 25, 30). Thus it may be seen that the largest red cell is several times smaller 

 than the largest macrophage, and the former may be easily accommodated inside the latter. 

 Comparison of the measurement of many cells from chorion and cavum corporis striati 

 shows them to be of the same size, when one is fortunate enough to get the membranes and 

 embryo mounted on the same slide; even when mounted separately there is very little 

 discrepancy in favor of cells in either position. 



The shape of the cell depends largely on its environment. When free it tends to assume 

 a spherical form (figs. 13, 17, 18, 20, 21), but in places where it can find support it sends 

 out long processes which are undoubtedly pseudopodia (figs. 15, 22). To obtain speci- 

 mens of cells with extended pseudopodia the fixation must be prompt; failure to procure 

 such pictures of moving cells is best explained on this basis. The shape is often irregularly 

 oval, due to the ingestion of foreign bodies, particularly erythrocytes (figs. 26, 27). At 

 times the macrophages grow in clumps of from 20 to 50, forming a mulberry-like mass. 

 This may occur in either medial or lateral cavity of the corpus striatum, and is well illus- 

 trated in figure 12 for the cavum laterale. Here the cells have becope flattened by the 

 pressure of their neighbors and in section seem to be forming a loose membrane the elements 

 of which are polygonal in outline. 



The cells have a limiting membrane, very thin but definite; although it is almost 

 impossible to defend the view that the phenomenon illustrated in figure 12 is not an instance 

 of protoplasmic bridges connecting several cell-bodies. Large numbers of cells are extremely 

 vermcose (figs. 23, 25, 26). Examination of the cavities where these cells are found 

 reveals a great deal of debris simulating protoplasm. This is undoubtedly coagulated 

 proteid which has accepted the counterstain and has precipitated on the surface of the 

 macrophages. Figures 13, 18, 19, and 20 represent more faithfully the delicate cell-mem- 

 brane when the cell has withdrawn all of its pseudopodia. 



The protoplasm is finely granular and is delicately arranged throughout the cell-body 

 in anastomosing strands which give an appearance of lacework. These trabecular vary 

 greatly in thickness and surround vacuoles within the cytoplasm. In the younger cells 

 these vacuoles are usually small, but at times they may be larger than the cell-nucleus 

 itself. Where the protoplasmic strands reach the cell-membrane they reinforce it in an 

 irregularly shaped mesh, such as may be seen in the illustrations of cells from chorion or 

 brain. This appearance is brought about by the extreme vacuolization which extends 

 throughout the whole cell, and as a result the finely granular protoplasm is heaped up in the 

 interstices between the vacuoles and the cell-wall. Great numbers of macrophages may be 

 found with foreign material included within their cell-bodies; the power of phagocytosis is 

 one of the most interesting phenomena exhibited by these cells. Although cells containing 

 ingested matter are rarely found in the chorionic villi, the cells everywhere in the body of 

 the embryo may contain phagocytized matter. The erythrocyte forms the foreign material 

 most frequently recognized within the phagocytes. An extravasation of red corpuscles 

 in the young embryo has occurred in most of the material studied; this is probably due to 

 the fact that it was subjected to high pressure in abortion. The presence of large numbers 



