48 



HUMAN ANATOMY. 



one centimetre to about two millimetres, the disappearance of the ducts and open- 

 ings of the uterine glands, and the con\ersion of the compact layer into a dense 

 homogeneous stratum, in which the tightly compressed glands later entirely disap- 

 pear. The spongy layer, on the contrary, retains the dilated gland-lumina, which, 

 however, in consequence of pressure, are converted into irregular spaces arranged 

 with their longest dimensions parallel to the uterine surface. The clefts next the 



Fig. 55. 



Blood-space- 

 Giant cell 



Enlarged lumen 

 of glands 



Degenerating- 

 glandular epi- 

 thelium 



Amnion 

 Chorion 



Decidua reflexa 



Blood-space of 

 compact layer 



Spongy layer 



Muscle 



Section through foetal membranes and uterus at margin of the placenta ; sixth month of pregnancy. [After Leopold.) 



muscular tissue are clothed with well-preserved epithelium ; the lining cells of those 

 towards the compact layer, on the contrary, early atrophy and disappear. 



The Decidua Placentalis. — The decidua placentalis, or decidua serotma, being 

 destined to contribute the maternal portion of the placenta, undergoes profound 

 changes which particularly affect the blood-vessels of the mucosa. In addition to 

 the initial general hypertrophy of the mucous membrane, which the placental decidua 

 shares in common with other parts of the uterine lining, peculiar polynucleated ele- 

 ments, the giarit cells, make 



Fig. 56. 



their appearance during the fifth 

 month ; by the end of preg- 

 nancy they are found in large 

 numbers within the basal plate 

 and the septa of the placenta, 

 although they are not wanting 

 within the remains of the spongy 

 layer. The giant cells are par- 

 ticularly numerous in the im- 

 mediate vicinity of the large 

 blood-vessels. The relations 

 between the ingrowing foetal 

 trophoblastic tissue and the ma- 

 ternal structures early become 

 so intimate within the placental 

 area that especial modifications 

 are instituted destined for the production of the vascular arrangement by which the 

 maternal and foetal blood-streams are brought into close relations. 



The proliferating trophoblastic tissue invades the stroma of the mucous mem- 

 brane and encroaches upon the capillaries until the latter in places become ruptured, 

 allowing the escape of the maternal blood, which thus is brought into direct contact 

 with the trophoblast. The erosion effected by the blood, on the one hand, and the 

 encroachment of the foetal mesoblast, on the other, gradually reduces the tropho- 

 blastic stratum, which is broken up into narrow epithelial trabeculae separating the 

 rapidly enlarging vascular lacunae, the primary representatives of the intervillous 



Sections of chorionic villi from placenta. X 170. a, b, small 

 branches of umbilical artery and vein ; 7^ capillary vessels ; c, cell- 

 aggregations of syncytium (rf ) ; m, mesoblastic stroma of villi. 



