He joined his wife and took her to San Remo and to Villafranca. Her health had improved and she was even able to take part in his work. He was engaged in studying Medusæ and Siphonophora, animals which interested him, not only from the point of view of the origin of embryonic layers, but also from that of general morphology, for he was still pursuing the problem of genetic links between animals. He had already been able to prove the presence of embryonic layers in many inferior animals; moreover, he had found, while studying the metamorphoses of Echinodermata, the proof that the _structural plan_, hitherto considered immutable, could become transformed in course of development. Thus the bilateral plan of the larva of Echinoderma becomes a radial plan in the adult. The structural plan therefore is not an absolutely differentiating character, since specimens of the same type can show a different plan according to their stage of development. One of the genetic questions still unsolved was that of the body cavity. Always present in higher animals, it is totally absent in certain lower groups, such as Sponges, Polypi, and Medusæ. It was being questioned whether their dissimilar morphological characters did not correspond with a duality of origin separating animals which possessed a body cavity (Coelomata) from those which did not (Acoelomata). Kovalevsky, it is true, had observed that the body cavity of many animals (Amphioxus, Sagitta, Brachiopoda) took its origin in the _lateral sacs_ of the digestive cavity, sacs which detach themselves from it in order to form the body cavity. But, in order to establish a genetic connection between those animals that have a body cavity and those which are devoid of it, it was necessary to show the homology of corresponding organs in both groups. Through his researches on the development of Coelomata (Echinodermata) on the one hand and Acoelomata (Ctenophora and Medusæ) on the other, Metchnikoff succeeded in proving that the lateral sacs of the digestive cavity which give birth to the body cavity of the Coelomata (Echinodermata) correspond to the canals and vaso-digestive sacs of the Acoelomata (Ctenophora and Medusæ). The difference consists in that the latter do not detach themselves in order to form a body cavity, which is therefore lacking. The result of his researches satisfied Metchnikoff; moreover, he began to feel again hopeful of his wife's recovery. The only dark spot was that Setchénoff's efforts had failed. Metchnikoff was not appointed by the Faculty of Medicine, for it was found advisable to replace the Chair of Zoology by one on Venereal Diseases. On the other hand, he was nominated for the Odessa University, supported by Cienkovsky and unanimously elected.