356 



HUMAN UTERUS AND FETAL APPENDAGES. 



cellular layers. To summarize, we may say that the ectoderm of the chorionic 

 membrane undergoes patchwise manifold changes. It exists in three general forms: 

 the nucleated protoplasm or syncytium, the cellular layer, and the canalized fibrin. 

 A patch of the ectoderm may consist of any one of these modifications or any two, 

 or of all three. But they have fixed relative positions, for when the syncytium is 

 present, it always covers the free surface of the chorion; when the cellular layer is 

 present, it always lies next the mesoderm; and when all three forms are present 

 over the same part, the fibrin is always the middle stratum. 



The mesoderm of the chorion in early stages has a homogeneous matrix, which 

 about the ninth week begins to change its appearance. In the frondosum, in our 

 specimen, the matrix has acquired a distinctly fibrous structure. Usually the pro- 

 duction of fibers is much greater in the immediate neighborhood of the ectoderm, 



FIG. 237. ADENOID TISSUE FROM A VILLUS or A HUMAN PLACENTA or FOUR MONTHS. 



/, /, /, Degenerating blood-cells, v, v, Capillary blood-vessels, d, Finer meshwork surrounding a capillary. 



X 352 diams. 



and this may go so far as to mark out a more or less distinct subectodermal 

 fibrillar layer (Fig. 235, Fib). There appears to be no mesothelial layer upon the 

 chorion at this stage, but it seems possible that its presence might be revealed by 

 the application of proper special methods. 



In the mill the ectoderm differs from that of the chorionic membrane in 

 several respects: (i) The cellular layer after the first month becomes less and less 

 conspicuous, and after the fourth month is present only in a few isolated patches, 

 which have been termed the cell-knots. (2) For the most part the villi remain 

 covered by the syncytial layer, which in many places is thickened. In later stages 

 these thickenings are small and numerous, constituting the so-called proliferation 

 islands with many nuclei. Many of the little thickenings appear in sections of 



