THE PL A CENTA IN SITU. 329 



modified, and the original structure of the mucous membrane is more or less, but 

 not perfectly, preserved in the deep portion of the decidua. But the majority 

 of the cells, especially in the compact layer, have grown in size and become trans- 

 ' formed into true decidual cells. In the ectoderm of the chorion, c, the cells lie 

 two or tjiree deep. They have distinct walls, a very coarsely granular proto- 

 plasm, and nuclei which stain darkly. By these characteristics they are easily 

 distinguished from the neighboring decidual cells, to which, however, they offer 

 a slight superficial resemblance.* 



The Placenta in Situ. 



The placenta in its natural position in the uterus follows the curvature of the 

 uterine walls, hence its free or amniotic surface is slightly concave. Its decidual 

 surface is strongly convex. It is thickest in the center and thins out gradually 

 toward its edge. The uterus should be obtained in the freshest possible condi- 

 tion and be opened by a crucial incision on the ventral side. The embryo should 

 then be removed, the umbilical cord cut through, care being taken to bring as 

 little pressure as possible on the uterus or the placenta. The whole organ is then 

 placed in the preseryative, which should be either Telly esnicky's or Muller's 

 fluid. In view of the large size of the organ, it is very necessary to use large quan- 

 tities of the preserving fluid, and this fluid must be changed several times in order 

 to insure good histological preservation. When the hardening is completed, 

 columns about one-half inch square may be cut out so as to pass vertically 

 from the inner to the outer surface of the placenta, preserving the amniotic and 

 chorionic membranes in place. The blocks are to be imbedded in celloidin and 

 ought to remain at least three days in thin and three days in thick celloidin, so as 

 to insure a thorough penetration of the imbedding material into the intervillous 

 spaces. Make the sections so that they pass vertically through the placenta. 

 Stain with hematoxylin and eosin or with picro-carmine. 



Placenta at Seven Months. A section made according to the method just 

 described is represented in figure 190. The thin amnion, Am, covers the upper 

 (or inner) surface of the chorionic membrane, Cho. This membrane is separated 

 from the decidua, D, by a dense forest of villi, of which innumerable sections 

 appear. In younger placentas the distance between the chorion and the de- 

 cidua is considerably less, and the number of sections of villi is smaller, but the 

 average size of those sections larger. In the present specimen the distance be- 

 tween the chorion and the decidua is nearly twice as great as the diameter of the 



* It should perhaps be noted that in some comparatively recent text-books the chorionic ectoderm has been 

 described as the decidua reflexa, an error which is much to be regretted. , 



