OF MICRO-ORGANISMS. 59 



organism which likewise springs from a single cell 

 called the egg, and the resultant divisions of which 

 do not separate. 



The colony constitutes in a way a first step towards 

 the physiological constitution of a pluricellular organ- 

 ism; it serves to fix a stage of transition in the animal 

 kingdom, between Protozoa and Metazoa. A fact which 

 strengthens this analogy is, that certain colonies, as the 

 Synura uvella and the Uroglena volvox, can divide into 

 two other colonies; strangulation acts upon the mass 

 just as if upon a pluricellular organism. This curious 

 observation was made by Stein and Biitschli. 



Nevertheless, an essential difference still separates 

 the Metazoa and the Protozoan colonies, even when 

 in these colonies a division of function has been 

 established among several individual groups. The 

 physiological differentiation brought about in these 

 Protozoan colonies is the result of a mechanism which 

 differs in every respect from that by which it is 

 effected in the case of the Metazoans. In the latter 

 instance the differentiation results from the division of 

 the embryo into germinative folia each of which is the 

 origin of a separate group of organs. At a certain 

 stage of development, the superposition of these folia 

 gives rise to the formation of a gastrula; the gastrula 

 is formed by two folia joined together, representing a 

 pouch open to the outside; it is characteristic of Met- 

 azoans, the Protozoan never reaching this stage. Cer- 

 tain colonies observed by Haeckel, the Magosphcera plan- 

 it la for example, and the volvox, of which we have before 

 spoken, appear in the form of a sphere; they suggest 

 an anterior stage of development to which the name of 

 morula or of bias tula has been given; but they do not 

 get beyond this stage. 



