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.
As he only had to go to his new post in the autumn, he went for the summer to St. Vaast in Normandy to study Lucernaria; unfortunately the stay was not a success; the weather was cold and the sea very rough, which made the Lucernaria impossible to find. Life conditions were very difficult, all the male population being at sea and the women being in the fields. In order not to waste this journey he studied Ascidians, and found that he had previously been mistaken at Naples when he thought that the nervous system of those animals originated from the lower embryonic layer. Kovalevsky had been right in affirming the contrary, and Elie hastened to write to tell him so.
St. Vaast, open to every wind, was not favourable to the patient, and Metchnikoff had to take her away. They went to Russia to stay with her parents and then to Panassovka. The doctors having advised a course of treatment by "koumiss," or fermented mare's milk prepared in a special way by the Tartars, Elie engaged a Tartar servant specially for that purpose, but in vain. In spite of every treatment, his wife's health was steadily growing worse. The cold at St. Vaast had been followed by such a dry heat in Russia that, in order to procure a little coolness for the patient, they had to spread wet sheets around her. She constantly had high temperatures and frequent attacks of hæmorrhage. It was obvious that she must leave Russia, and Metchnikoff, obliged to rejoin his post at Odessa, asked Mlle. Fédorovitch to go with her to Montreux.