352 HUMAN UTERUS AND FETAL APPENDAGES. 



some of the larger blood-vessels there is connective tissue only slightly modified, 

 and the original structure of the mucous membrane is more or less, but not 

 perfectly, preserved in the deep portion of the decidua. The majority of the 

 cells, especially in the compact layer, have grown in size and become transformed 

 into true decidual cells. In the ectoderm of the chorion, c, the cells lie two or 

 three deep. They have distinct walls, a very coarsely granular protoplasm, and 

 nuclei which stain darkly. By these characteristics they are easily distinguished 

 from the neighboring decidual cells, to which, however, they offer a slight super- 

 ficial 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 condition 

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

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

 little pressure as possible on the uterus or the placenta, and the whole organ 

 placed in the preservative, which should be either Tellyesnicky's or Miiller's fluid. 

 In view of the large size of the organ, it is very necessary to use large quantities 

 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 mem- 

 branes 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 

 alum hematoxylin and eosin. 



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

 described is 'represented in figure 234. 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 decidua 

 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 between the 

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

 coat, Me, of the uterus. The ends of some of the villi touch the decidual tissue, 

 and are imbedded in it. Their imbedded ends are without , covering epithelium, 

 but their connective tissue is immediately surrounded by hyaline substance which 



* 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. 



