Final answer:
The small tissue band on the heart is called the ligamentum arteriosum but was known as ductus arteriosus during fetal life. It is part of the fetal circulatory system, which originates from the mesoderm around 18 to 19 days post-fertilization and involves critical shunts for fetal circulation.
Step-by-step explanation:
The small band of tissue on the heart is known as the ligamentum arteriosum. During fetal life, its name was the ductus arteriosus. This ductus arteriosus is an essential part of the fetal circulatory system and acts as a shunt to direct blood away from the lungs, which are not yet in use, and towards the placenta for oxygenation. The heart begins its development from mesoderm tissue around days 18 to 19 after fertilization. As the primitive heart tube elongates and folds within the pericardium, it eventually forms an S shape which places the chambers and major vessels into alignment similar to an adult's heart. The fetal circulatory system includes shunts like the ductus arteriosus to bypass certain organs. After birth, the ductus arteriosus closes and eventually forms the ligamentum arteriosum.