DISTRIBUTION OF MITOCHONDRIA IN THE PLACENTA. 109 



in shape, vary from 15 to 40 // in diameter, and contain from one to three nuclei. 

 The average giant cell is about 20 M in diameter and contains only one nucleus, 

 which is large, round, and stains light green. Near its center is a single large 

 fuchsinophilic nucleolus from which delicate strands of chromatin radiate to the 

 nuclear membrane. The protoplasm stains faint green and may contain numerous 

 minute vacuoles, especially in the older cells. The mitochondria are fairly abund- 

 ant in the form of minute granules and rods arranged in an eccentric zone around 

 the nucleus. The peripheral zone of the proptoplasm is entirely free from mito- 

 chondria or other granules. In some cells an eccentric ring of clear protoplasm is 

 present between the nucleus and the zone of mitochondria. In cells containing 

 two nuclei the zone of mitochondria may surround both and not invade the proto- 

 plasm lying between them; or, there may be a separate mitochondrial zone around 

 each nucleus. In the giant cells of younger placentae the mitochondria are slightly 

 larger and more numerous in proportion to the amount of protoplasm and are not 

 so definitely concentrated in the zone around the nucleus. 



The small ectodermal cells form a layer which surrounds the giant cells and 

 maternal vessels and separates them from the connective-tissue stroma containing 

 the fetal vessels. At first these cells have quite distinct boundaries, but during 

 the latter part of gestation then 1 cytoplasm coalesces to form a syncytium which 

 closely invests the giant cells and the maternal vessels. The smaller ectodermal 

 cells are low columnar, about 10 ^ in height, and are quite uniform in size. In the 

 central portion of each cell is a single round nucleus which stains deep green and 

 contains a single deep green nucleolus and several heavy masses of chromatin. The 

 protoplasm contains a large number of vacuoles of various sizes, more abundant at 

 the pole of the cells nearest the maternal vessels. In osmic preparations these 

 vacuoles are seen to be filled with fat. The mitochondria, in the form of small 

 granules and short rods, are fairly numerous and lie in the protoplasm between 

 the fat vacuoles (fig. 3). They are not concentrated in any particular part of 

 the cell. In the younger placentae transition can be traced between the small ecto- 

 dermal cells and the giant cells. 



Lying between and over the columns of fetal ectoderm is a delicate connective- 

 tissue stroma which contains the fetal blood-vessels. In the young placenta the 

 cells of the stroma are quite rich in mitochondria, but in the mature placenta they 

 possess no more mitochondria than do ordinary connective-tissue cells. The 

 endothelial cells of the fetal vessels possess a few, but they are in no way remarkable. 



The chorionic membrane of the cat consists of a single layer of cuboidal to 

 columnar cells resting on a basement membrane supported by a very vascular 

 connective-tissue stroma. On the opposite side this stroma is covered by the 

 allantoic membrane. Numerous folds project from the chorionic surface toward 

 the uterine mucosa. 



The chorionic epithelium near the placental margin is composed of high 

 columnar cells, but as the distance from the placenta is increased the cells become 

 lower and more cuboidal. They are crowded together, their outlines being distorted 

 by pressure from the surrounding cells. Many cells are apparently in process of 



