THE PL A CENTA IN SITU. 333 



chorion. The fibrin layer consists of a very refringent substance permeated 

 by numerous channels (Fig. 192, /5). The substance has a violent affinity for 

 carmine and hematoxylin, and is always the most deeply colored part of a section 

 thus stained. The channels tend to run more or less parallel, to the surface of 

 the chorion, and are connected by numerous short cross-channels. Some of the 

 channels contain cells or nuclei. The appearances, however, are very variable; 

 the fibrin often sends long outshoots into the 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 ecto- 

 derm 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 production of fibers is much greater in the immediate neighborhood of the 

 ectoderm, and this may go so far as to mark out a more or less distinct subecto- 

 dermal fibrillar layer (Fig. 191, 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 villi 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 the villi, and here and there are converted into canalized 

 fibrin. (3) The proliferation islands are converted into canalized fibrin and 

 at the same time grow and fuse, forming larger patches, particularly on the 

 larger stems. In this manner are produced the large areas and columns of 

 fibrin such as appear in our section. (4) Over the tips of the villi, where they 

 are imbedded in the decidua serotina, the epithelium apparently degenerates 

 and becomes hyaline tissue, but without canalization. The mesoderm exists 

 in two principal forms, adenoid tissue and fibrillar tissue around the blood- 

 vessels. The adenoid tissue (Fig. 193) may be considered as the proper tissue of 



