108 LIFE OF lilB METCHNIKOFF 



We know that tte ovule or primitive genital cell 

 of every animal may be compared to a unicellular 

 organism. After fertilisation the egg undergoes con- 

 secutive divisions or segmentation ; each Segment 

 constitutes a new cell, and their aggregation forms a 

 hollow sphere called a blastula, which ig similar to a 

 colony of unicellular beings. The blastula differenti- 

 ates itself into embryonic layters, the ectoderm, endo- 

 derm, and mesoderm already mfentioned. 



In the majority of animals iihe origin of the first 

 two layers, ectoderm and endoderm, is due to the 

 invagination of one of the poles of the blastula ; the 

 invaguiated part of the walls forms the internal layer, 

 the endoderm, and lines the cavity produced by 

 invagiaation ; this cavity thus becomes a digestive 

 cavity. This stage of development, called gastrula, 

 is similar to a cup with a double wall, of which 

 the outer is the ectoderm and the imier the endo- 

 derm. 



This stage, discovered by Kovalevsky, is to be 

 found in the evolution of most animals and corre- 

 sponds to the adult stage of some of them. It was 

 consequently considered as the primitive type of multi- 

 cellular beings. 



Haeckel founded thereupon his theory of the 

 gastrcea, according to which the common ancestor 

 of animals was a lower animal, now disappeared, 

 and similar to that stage of development. He 

 therefore gave to this hypothetical animal the name 

 of gastrcea. 



MetchnikofE, however, discovered among primitive 

 multicellular animals, such as sponges, hydroids, and 

 lower medusae, a stage of development still more 

 simple than the gastrula ; this stage is without a 



