Youth is elastic: the young couple started full of joy, gay as children, and ready to forget all their trials. Alas, it was not for long: having halted at Vilna in order that the patient should have a rest, she had an attack of hæmorrhage of the lungs, to the great alarm of her husband, who nevertheless did his best to reassure her. They continued the journey as soon as her condition allowed it, only to be interrupted by another relapse. At last they reached Spezzia, chosen on account of the climate and the marine fauna.
Little by little, Ludmilla Metchnikoff’s health improved and her husband was able to resume work. He studied aquatic animals in view of the genealogy of inferior groups, and, amongst others, studied the Tornaria, which was believed to be the larva of the star-fish. However, to his astonishment, he ascertained that, in spite of great similarity, it was not the larva of an Echinoderm, but that of one of the Balanoglossi, of the worm type. This fact established a link between the Echinodermata and worms, a very important result from the point of view of the continuity of animal types.
Metchnikoff felt his courage returning and also his natural high spirits. His wife, who was a clever draughtswoman, helped him with the drawings for his memoir, and both felt happy and contented; this stay at Spezzia was a real oasis in their life.
When the heat became excessive they went to Reichenhall, a summer resort prescribed by the doctor. There, Metchnikoff completed his previous researches on the development of the scorpion, and finally established the fact that this animal possesses the three embryonic layers which correspond to those of the Vertebrates.
As his young wife’s health was still too precarious to allow her to spend the winter in Russia, Metchnikoff, obliged to return to Petersburg, installed her at Montreux and asked his sister-in-law, Mlle. Fédorovitch, to stay with her. The enforced separation deeply grieved the young couple, whose only consolation was daily correspondence.
Metchnikoff resumed a life of hard work; he was now an agrégé at the Petersburg University and had to leave the School of Mines; this diminished his resources, but at the same time he obtained an extra salary of 800 roubles as Extraordinary Professor. His position in the University was nevertheless very difficult, for his situation was coveted by different parties with which he had nothing to do. They wanted it for one of their adherents. His devoted friend Setchénoff, Professor of Physiology, then thought of proposing him to the Faculty of Medicine as a Lecturer in Zoology, and whilst Metchnikoff awaited the result of his efforts, he obtained leave to go to the seaside to do research work.
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 (Cœlomata) from those which did not (Acœlomata).
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 Cœlomata (Echinodermata) on the one hand and Acœlomata (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 Cœlomata (Echinodermata) correspond to the canals and vaso-digestive sacs of the Acœlomata (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.