The nature and origin of the cells lining the pleuroperitoneal cavity have attracted much attention; the opinions on these points are as yet far from uniform. In order to solve this question it seemed only natural that the investigators should turn to embryology, in order to determine from which layer of the embryo the pleuroperitoneal cells are developed, but great difficulties were soon met with.
In the first place, it is very hard to determine the exact moment when the law of specificity of cells goes into force; hence the different opinions among embryologists. The majority of them were of the opinion that the pleuroperitoneal cavity constitutes a large lymph space, and that the lining cells were equivalent to the endothelial cells of vessels. Waldeyer, on the other hand, assumes that histogenetic differentiation is completed with the formation of the archiblast and the parablast. All muscles, nerves, and epithelia are