72 LIFE OF ELIE METCHNIKOFF 



she was even able to take part in his work. He was 

 engaged in studying Medusae 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 meta- 

 morphoses of Echinodermata, the proof that the striLC- 

 tural 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 there- 

 fore 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 Medusae. It was being ques- 

 tioned whether their dissimilar morphological char- 

 acters 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 homo- 

 logy of corresponding organs in both groups. 



