TERATOGENESIS. 595 



hand, the nutrition of one embryo may be so seriously impaired that it dies. 

 When death occurs during the earlier months of pregnancy the dead embryo 

 is subjected to pressure by the living one and is sometimes distorted and 

 flattened into a thin mass known as a fcetus papyraceus. 



Equal growth of monochorionic twins implies an almost perfect nutritive 

 balance, since both embryos derive their nourishment from the same placenta. 

 Any condition that disturbs the nutritive balance tends to affect adversely the 

 less favored embryo, so that development does not proceed equally (unequal 

 monochorionic twins). One of the first consequences of such a disturbance 

 may be an impaired or arrested development of the heart, in which case the 

 weaker embryo may become an acardiac monster. 



This condition does not necessarily imply the absence of the heart in the 

 affected twin; for it may possess a functionating heart, or it may become an 

 amorphous mass of tissue which derives its total blood supply through the 

 action of the heart of the stronger twin, or there may be any form between the 

 two extremes. In any case the acardiac monster acardiacus receives its 

 blood wholly or in part through the agency of the stronger heart. 



Acardia is always accompanied by a so-called "reversal of the circulation;" 

 and there are three periods at which it may originate, (i) It may originate 

 before the heart develops. As is well known, the heart appears independently 

 of the area vasculosa and joins the vessels secondarily (p. 222). If, for any 

 reason, the heart of one of the embryos fails to develop, anastomoses may 

 occur between the vascular anlagen of the two vascular areas and consequently 

 the normal embryo assumes the duty of nourishing the other. The latter be- 

 comes an acardiac monster. (2) It may originate after the heart has begun to 

 develop, but before the placental circulation is established. If, for any reason, 

 the heart of one embryo should cease to develop further, there would probably 

 be sufficient anastomoses between the vitelline vessels of the two embryos to 

 enable the affected embryo to live and become an acardiac parasite. In this 

 case no placental circulation would develop in the parasite. (3) Acardia may 

 originate after the establishment of the placental circulation. Since there is 

 but one chorion or placenta for both embryos there is naturally a communica- 

 tion between the two circulations in the chorionic villi. There are also likely 

 to be anastomoses between the umbilical arteries and veins. If, for any reason, 

 the heart action of one of the embryos should become impaired, there would be 

 an influx of blood into the vessels of that embryo owing to diminished pressure. 

 Thus the blood from the stronger heart would be pumped into the affected 

 embryo as well as into the placenta. This blood, being impure, fails to nourish 

 the weaker embryo properly and the result is atrophy or degeneration. 



Upon the basis of other malformations that naturally accompany impaired 

 development of the heart, acardiac monsters are divided into four classes. 



