342 HUMAN UTERUS AND FCETAL APPENDAGES. 



the same time prolongations of the chorionic mesoderm extend into the tropho- 

 blast. The ectodermal cells arrange themselves as a covering for these mesoder- 

 mic outgrowths and so complete a villus. The trophoblast between the develop- 

 ing villi entirely disappears. The ectoderm, which covers both the villi and the 

 chorionic membrane proper, consists of two layers, an inner cellular and an outer 

 syncytial layer. Much of the trophoblast may still remain for awhile around 

 and beyond the tips of the villi, but it disappears rapidly, probably during the 

 third week, so that the villi alone are left. The two-layered stage of the ecto- 

 derm is only partially preserved during the later development. Many parts of it 

 become thinned out so as to contain only one layer of cells, while other parts 

 thicken and degenerate. These changes may be studied in sections of older 

 placentas (see Fig. 190). 



The mesoderm of the chorion consists at first of mesenchymal cells with a 

 homogeneous matrix and a layer of mesothelium. In later stages the mesen- 

 chymal tissue becomes partly fibrillar, and it is doubtful whether the mesothe- 

 lium persists or not. During the third week we find the chorion vascular. 

 Around the larger blood-vessels the mesoderm forms a more or less distinct 

 coat in which the cells are somewhat more crowded together in laminae. After 

 the perivascular coats have acquired a certain thickness the cells of their inner 

 portions become more elongated, more regularly spindle-shaped, and more 

 closely packed than those of the outer layer. The transition from the denser to 

 the looser tissue is gradual. We are perhaps entitled to call the' denser, inner 

 layer the media, and the outer, looser layer the adventitia, although neither of the 

 layers has by any means the full histological differentiation characteristic of the 

 like-named layers of the blood-vessels of the adult. The histogenetic changes in 

 the chorionic frondosum go further than in the chorion laeve, which may be said 

 to be, as it were, arrested in its development. 



The Chorion with Trophoblast. 



When the chorionic vesicle has an internal diameter of from 3 to 6 or 7 mm., 

 it will be found to exhibit well-developed trophoblastic layers. Such a vesicle 

 may be hardened in Zenker's fluid, or, better, in Flemming's or Hermann's fluid, 

 as these produce at the same time a differential color (Fig. 198). The chorionic 

 membrane is quite thin, and consists chiefly of mesoderm, m,es, with a covering 

 of ectoderm, Ec, consisting of two layers of cells. The mesoderm extends down 

 to form the core of the villi shown. These villi are much branched and are also 

 covered by a layer of ectoderm. At the denser ends of the villi the ectoderm is 

 very much thickened, forming a great mass of cells, so that the ectoderm con- 

 nected with one villus is fused with that of adjacent villi, the whole constituting 



