The partial twinning that led to the births of conjoined twins Abby and Brittany Hensel must have happened during the first two weeks of gestation,because the girls
A) have two heads.
B) share tissues that descend from ectoderm, endoderm, and mesoderm.
C) share only tissues derived from mesoderm.
D) were born on time.
E) have two separate nervous systems.

Question

Quizplus · Accepted Answer

Conjoined twins occur when the separation of the developing embryo into twins is incomplete. Abby and Brittany Hensel are dicephalic parapagus twins, meaning they have two heads and share a single torso. They have separate spinal cords, but share many of their internal organs and other structures, because they developed from a single fertilized egg that didn't completely split. The fact that they share tissues derived from all three primary germ layers (ectoderm, endoderm, and mesoderm) indicates that the division occurred after the layers had already formed, which is typically within the first two weeks of gestation. Therefore, the best choice is B.

The Partial Twinning That Led to the Births of Conjoined