the gastrula in the development was interpreted as a “repetition” of this ancestral adult stage. Thus the two primary layers are supposed to have an historical meaning.[100] Embryologists soon began a search for a similar mode of interpreting the middle germ-layer, or layers, which led, amongst other views, to the formulation of the “gut-pouch hypothesis.” From this point of view the body cavities, or cœlomes, are supposed to have been originally sac-like outgrowths from the digestive tract of an ancestral adult animal. Later, these cœlome sacs are supposed to have been shut off from connection with the digestive tract--their cavities becoming the body cavities, and their walls giving rise to the mesodermal organs. The formation of pouches from the walls of the archenteron of the embryo in several groups of animals has been interpreted as a repetition of the ancestral adult animal. A comparison of the germ-layers in different forms very soon led to an attempt to “homologize” the layers in different animals. If the layers have had historically the same origin, or appear in the same way in the embryos, or give rise to the same organs, they are said to be homologous. In the absence of a knowledge of the first two of these conditions it is generally considered sufficient, if it can be shown that similar organs arise from a layer, to “homologize” that layer in the two forms. The study of embryology soon became a search for homologies. The results led to inextricable difficulties and innumerable contradictions until, a reaction setting in, many embryologists became sceptical in regard to the value of this entire method of study.