As the folds grow towards the midline, carrying the phrenic nerve, the root of the folds migrate ventrally

pleuropericardial membranes

pleuropericardial membrane

uberon

In hagfishes a transverse septum extends upward from the ventral body wall posterior to the heart, partly separating an anterior pericardial cavity from a larger peritoneal cavity. (...) These basic relationships have not been modified by urodeles. The small pericardial cavity remains far forward where it is separated by a transverse septum from the principal coelom, which may now be called a pleuroperitoneal cavity because slender lungs are present. (...) The heart [of other tetrapods] is separated from the lungs (and liver if present) by more or less horizontal partitions that have their origin in the embryo as folds on the serous membrane of the right and left lateral body walls. These grow out to join in the midline of the body. They are called lateral mesocardia (birds) or pleuropericardial membranes. Posteriorly they join the transverse septum to form the adult pericardial membrane, or pericardium. (...) In their partitioning of their coelom, embryonic mammals resemble first early fishes (incomplete partition, posterior to heart, consisting of the transverse septum) and then reptiles (pericardium derived from transverse septum and pleuropericardial membranes) Mammals then separate paired pleural cavities from the peritoneal cavity by a diaphragm. The ventral portion of this organ comes from the transverse septum. The dorsal portion is derived from the dorsal mesentery and from still another pair of outgrowths from the lateral body wall, the pleuroperitoneal membranes.[well established][VHOG]

UBERON:0006278