A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 



401 



Where the villi project from the surface of the chorionic membrane the same 

 general structure is maintained. The villi present a rather uniform calibre and some 

 indication of their shape and manner of branching may be obtained by an examina- 

 tion of text-figures 3 and 4, and figures 6 and 8, plate 1. In general their tips merge 

 directly into the incrusting trophoderm, and where this occurs it is no longer possi- 

 ble to differentiate sharply between the Langhans layer and the syncytial layer. 

 One gains the impression that the former merges into the latter tissue, where the 

 tips of the villi come in contact with the trophoderm, the syncytial layer con- 

 tinuing along its margins. 



FIG. 3. Profile reconstruction showing the Mateer ovum in the median sagittal plane. Enlarged 15 diameters. 



The trophoderm completely incrusts the ovum as a trabeculated shell. 

 Although it seems to consist of a uniform tissue, there is considerable variation in 

 its detailed structure in certain areas, these areas merging gradually into each other. 

 This variation applies to the size of the cells, the distinctness of their outline, and 

 the compactness of their cytoplasm. As has been noted in some areas, the tropho- 

 derm seems to merge directly with the Langhans layer of the villi and histologically 

 to be a continuation of it. In many places along the periphery it merges into 

 decidua, and much of the trophoderm along the margins gives the appearance of 

 rapid transition into syncytial tissue like that found on the villi. In some cases 

 this transition occurs directly within the substance of the trophodermic mass, and 

 we therefore find small syncytial masses completely imbedded in trophoderm. One 

 also finds more or less detached masses of syncytium scattered everywhere through 

 the irregular spaces of the trophoderm and in the intervillous spaces. These masses 

 present the greatest variety of size and form; some of them seem to be entirely 

 detached, others project into the spaces and are attached only at one end. In 

 some cases they form an enveloping coat for the adjacent trophoderm. They fre- 

 quently show vacuolization and present a wide variety in the form, number, and 

 character of their nuclei. As a rule the margins of the trophoblastic spaces show a 

 tendency toward the formation of a cell-border which merges into the adjoining 

 trophoderm. In some places this marginal arrangement resembles a thickened 

 endothelium. Along other margins one finds all varieties of transition into syn- 

 cytial masses. In many respects the conversion of the trophoblastic margins into 



