The condition is called twin reversed arterial perfusion sequence, or TRAP sequence, and the blob is known as an acardiac twin while the healthy baby is called the pump twin. It only happens in identical twins who share one placenta, and estimates range from about 1 in 9,500 to 1 in 35,000 pregnancies depending on the study. A direct artery to artery connection inside the placenta lets the pump twin’s heart push blood backwards through the umbilical arteries into its twin, which stays alive only on that borrowed flow. Older reports put the pump twin’s death rate without treatment at 50 to 75 percent, mostly from heart failure and very early birth, so doctors watch the pump twin’s heart closely and, if needed, stop the flow with radiofrequency ablation, a laser or by tying off the cord. The shapeless type is called acardius amorphus in human medicine, and amorphous globosus mostly in veterinary medicine, where it is described as a round mass covered in hairy skin. The acardiac twin cannot survive after birth. Music composed by Bert Cole (bitbybitsound.com)