A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. 129 



The syncytium, or plasmoditrophoblast, over the vesicle wall and the bases 

 of the villi consists of a thin layer of slightly varying thickness, but as a rule thinner 

 than the cellular layer beneath it. Both its cytoplasm and nuclei stain very densely. 

 Traced outward upon the villi, the syncytium rapidly thins out on the cell-columns 

 and soon disappears. The largest syncytial masses are found in the equatorial 

 zone just outside the cell-islands. Here it forms large, often extensively vacuolated 

 or spongy masses which can not always be definitely separated from the cell-islands. 

 Scattered through the intervillous spaces, some of them close to the wall of the vesi- 

 cle, are free syncytial masses of every possible size and shape. The nuclei vary 

 widely in number; they may be small and stain quite densely, or large and pale, 

 and this in the same bit of syncytium. "Prickle processes" are seen quite distinctly 

 on some of these masses and their protoplasm is often very finely vacuolated. 

 Smaller fragments of syncytium often lie in shallow pits or excavations in the cell- 

 islands or trophoblastic columns. These masses are often very small, with one or 

 more nuclei, and are only very lightly stained. Here again there seems to be a direct 

 transformation of cytotrophoblast into plasmoditrophoblast. If there are evidences 

 of cell-division in the chorion they have so far eluded us. 



The amount of maternal blood in the intervillous spaces varies considerably in 

 different localities. In a few places it is very abundant, in others almost wanting. 

 It is most plentiful on the flattened poles of the ovum, where the villi are fewer and 

 shorter and where the cell columns and islands and syncytial masses are least in 

 evidence. It would appear as if the anastomosing cell-columns around the equator 

 of the ovum had prevented the entrance of maternal blood, except very indirectly 

 through the more distant intervillous spaces. That the blood should have drained 

 out more readily from these deeper spaces, many of which are closed externally 

 by the remains of the trophoblastic shell, seems quite improbable. Over much of 

 the ovum externally is a layer of clotted blood in which leucocytes are more numer- 

 ous than in the blood in the intervillous spaces. 



In concluding this account of the chorion mention may be made of a small 

 cyst-like structure faintly seen on plate 1, figure 3. It is composed of tissue to all 

 appearances like the mesoderm of the chorion and lies close to, but seemingly not 

 in connection with, the vesicle wall. No indications of a chorionic duct have 

 been encountered. 



Concerning the magma in the exocoelom, it will be recalled that upon gross 

 examination of the ovum a few fine strands were observed connecting the yolk-sac 

 and chorion. At that time it could be seen that traction upon these strands was 

 not without effect upon yolk-sac. In the sections there can be found only some 

 ragged wisps of a finely fibrillar nature, which at various points grade insensibly 

 into irregular clumps of a finely granular or fibrous character extensively present in 

 the cavity of the vesicle. In a few places where the larger strands have an attach- 

 ment to the chorion there occur very intensely staining nuclei. Where best devel- 

 oped the fibrils are very conspicuous; they form loose bundles and stain very dark 

 with hematoxylin. Over the amnion and the yolk-sac near it is a very thick, con- 

 densed layer of a finely granular texture (plate 2, figs. 5 and 6). 



