At the seventh International Zoological Congres at Boston, HUBRECHT (1907) delivered an address “On the foetal membranes of the Vertebrates”. In this lecture he expounded his revolutionary ideas about the origin of the foetal membranes and the phylogenetic history of the vertebrates in general. HUBRECHT was of the opinion that in the ancestors of the Mammalia there was only one embryonic membrane, and that has been retained as the the trophoblast. Its original function was to protect the larva, but when, in the early mammals, intra-uterine development was established, this membrane assumed the function of feeding the embryo as we see in our present day mammals. In order to function properly a connection was then established with the embryonic vascular system, this connection became the allantois. According to HUBRECHT the ancestors of the mammals had not the large, yolk-laden eggs as found in reptiles and birds, but they were viviparous animals with small, holoblastic eggs. From such an early viviparous ancestor, the reptiles and birds might have developed as a side branch, when the small vesicular blastocyst accumulated more and more yolk in its yolk-sac, which made it possible for viviparity to be replaced by oviparity. HUBRECHT appeals to the palaeontologists to find these hypothetical Promammalia.