38 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



different with respect to others of its parts. If development 

 were to go on in this direction alone, high organisatory 

 complications might occur : but there would always be 

 only one sort of cells, arranged in a sphere ; there would 

 be only one kind of what is called " tissue." 



FIG. 2. EARLY DEVELOPMENT OF ECHINUS, THE COMMON SEA-URCHIN. 



a. Two cells. &. Four cells, c. Eight cells, arranged in two rings of four, above one 

 another. 



d. Sixteen cells, four "micromeres" formed at the " vegetative" pole. 



e. Optical section of the "blastula,"a hollow sphere consisting of about one thousand 



cells, each of them with a small cilium. 



But in fact development very soon leads to true 

 differences of the parts of the germ with respect to one 

 another, and the next step of the process will enable us 

 to apply different denominations to the different parts of 

 the embryo. 



At one pole of the swimming blastula, exactly at the 

 point where the descendants of the micromeres are situated, 



