50 



11. E. Metschnikoff on the 



Ccelenterata and Echinodermata, and consequently belongs to 

 the domain of the middle lamella (mesoderm, Hackel). Be- 

 sides the points cited, the fact that the cells of the middle 

 lamella in both the Coelenterata and the Echinodermata (at 

 least in the young state) are exceedingly changeable, may also 

 be adduced in favour of this view. Thus, for example, we 

 see that the naked amoeboid cells of the skeleton-forming layer 

 in the larva? of Echinoderms move about in the body-cavity, 

 and, in consequence of active movements, collect in particular 

 spots, e. g. in the stone-canal. 



In order to explain my view still more clearly, I will here 

 compare with each other three objects which are at the first 

 glance very similar. If we examine the three figures here given 

 (A, B, C), we observe that all three consist of a one-layered 



Fig-. A represents an embryo of Reniera, B an embryo of Sertnlaria, 

 and C an embryo of Echinus. 



sphere, densely packed with a compact cell-mass. If it were 

 possible in the comparison to take only such data into considera- 

 tion, we should say (as Hackel actually has done with regard 

 to the planulge of the Sponges and Hydroida) that all the three 

 embryos are perfectly homologous, and especially that all three 

 have originated in a similar manner. It is only the close ex- 

 amination of subsequent stages that shows us that the homo- 



