exposure, and so on. We may, then, suppose that its metaboHsm is modified 

 in a distinct way. It may produce distinct substances or different proportions 

 of by-products. Respiration goes on more rapidly in some cells than in 

 others. The by-products of each cell will in turn influence neighboring cells 

 somewhat differently. Differentiation, having once started, continues in all 

 directions. 



Twins and Quintuplets In a sense, differentiation begins with the very 

 first cell division. The single cell has in it all the "makings" of a complete 

 and complex individual which it in time becomes and which contains 

 perhaps trillions of cells. But so has each of the two daughter cells into 

 which the egg divides. This we know from the fact of twins. Experi- 

 mentally, the two cells in the two-celled stage of a frog or fish or sea urchin 

 or some other species can be separated. Each cell then rounds up and starts 

 to divide again. Under suitable conditions, each develops into a complete 

 individual — the two as much alike as true twins are known to be. In other 

 words, a single egg has the makings of a complete individual; and half the 

 egg also has the makings of a complete individual. If the two halves re- 

 main together, however, each produces only half an individual! Something 

 must make the two-together different from the two-separated. 



More water 

 (or less salt) 



Artemia 

 aiietina 



From Gnicnhers. T/ic stori) uf Eroluliun. After Abonyi 



RELATION OF SALT TO DEVELOPMENT OF THE BRINE SHRIMP 



The brine shrimp, Artemia arietina, lives in brackish water. A Russian experimenter, 

 Schmankewitsch, diluted the water slowly, and in other cases let the water evaporate 

 so that the salts became more concentrated. The forms that appeared in the course 

 of a few generations had been recognized previously as different "species". Other 

 experimenters have repeated this process, which seems to be "reversible". 



359 



