No. 3.] UTERUS AND EMBRYO. 437 
the multinucleate decidual cells are found chiefly in the serotina; 
at the edge of the placenta decidual cells are found in the 
chorion. 
§ 20. The decidual cells arise by direct enlargement of the 
connective tissue cells of the mucosa. All parts of the decidua 
and placenta arise in place by metamorphosis of the tissue; the 
mucosa is preserved, and there is no production of placental 
tissues by new formation. 
§ 21. The changes of the uterus during menstruation and 
gestation are homologous, the menstrual cycle being prolonged 
and modified by pregnancy; hence it is that conception takes 
place only at the menstrual period, for the ovum can only 
modify the menstrual change, not initiate the formation of a 
decidua. No satisfactory explanation of the origin of the am- 
nion has yet been offered. The placenta is an organ of the 
chorion; its evolution cannot be traced to modifications of 
either the allantois or the yolk sack; the allantois is originally 
the intestinal canal of the Bauchstiel, which serves as the means 
of vascular communication between the chorion and embryo; 
the enlargement of the allantois is secondary. We possess no 
positive information as to how the placenta performs its nutri- 
tive functions. 
Boston, Aug. 3, 1888. 
