364 ' HUMAN UTERUS AND FETAL APPENDAGES. 



The ectoderm undergoes a very precocious growth producing a very large 

 number of cells, which form the thick trophodermic layer as described on page 365. 

 Then follows the stage in which, by degeneration, spaces are produced in the 

 trophoderm into which the blood of the mother enters and circulates; and at the 

 same time prolongations of the chorionic mesoderm extend into the trophoderm. 

 The ectodermal cells arrange themselves as a covering for these mesodermic 

 outgrowths and so complete a villus. The trophoderm between the developing 

 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 trophoderm 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 ectoderm 

 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. 234). 



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 mesothelium 

 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 chorion 

 frondosum go further than in the chorion Iseve, which may be said to be, as it 

 were, arrested in its development. 



The Chorion with Trophoderm. 



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

 it will be found to exhibit well-developed trophodermic 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. 242). The chorionic 

 membrane is quite thin, and consists chiefly of mesoderm, mes, 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- 



