942 GENERATION". 



to assume that there were originally two sets of membranes, which had become fused 

 into one. The instances on record, one of which we have given, of twins, one white and 

 the other black, show conclusively that two ova may be developed in the uterus at the 

 same time. While there can be no doubt upon this point, the question of the possibility 

 of the development of two beings from a single ovum remains unsettled. It is thought 

 to be more difficult to understand how two conjoined monsters, like the celebrated 

 Siamese twins, who died in 1874 at the age of sixty-three years, could be developed 



ENG. CHANG. 



FIG. 312. The Siamese twins. 



V V, vena cava ; V P, V P, portal vein ; a, upper hepatic pouch of Chang ; &, peritoneal, or umbilical pouch of Eng ; <, 

 lower peritoneal, or umbilical pouch of Chang ; D, D, connecting liver-band ; e, lower border of the band ; /, upper 

 border of the band. 



from two ova which became fused, than to imagine the development of two beings from 

 a single ovum. This question, however, belongs to teratology and could be settled only 

 by observations of conjoined monsters very early in their development, which do not 

 exist in literature. 



As pathological conditions, we have extra-uterine pregnancies, in which the fecun- 

 dated ovum, forming its attachments in the Fallopian tube (Fallopian pregnancy) or within 

 the abdominal cavity (abdominal pregnancy), undergoes a certain degree of development. 

 The uterus usually enlarges, in these instances, and forms an imperfect decidua. 



Cause of the First Contractions of the Uterus in Normal Parturition. The cause 

 of the first contraction of the uterus in normal parturition is undoubtedly referable to 

 some change in the attachment of its contents, which causes the foetus and its membranes 

 to act as a foreign body. When, for any reason, it is advisable to cause the uterus to 

 expel its contents before the full term of pregnancy, the most physiological method of 

 bringing on the contractions of this organ is to cautiously separate a portion of the mem- 

 branes, as is often done by introducing an elastic catheter between the ovum and the 

 uterine wall. A certain time after this operation, the uterus contracts to expel the ovum, 

 which then acts as a foreign body. 



In the normal state, toward the end of pregnancy, the cells of the decidua vera and 

 of that portion of the placenta which is attached to the uterus undergo fatty degenera- 

 tion, and, in this way, there is a gradual separation of the outer membrane, so that the 



