264 



BIOLOGY 



sex union. Only one more step is needed to have a typical 

 sexual reproduction. In Eudorina all of the cells of the colony 

 share in the reproductive process. If only a few of the cells 

 of the colony should thus develop into sex cells, leaving the 

 colony to live an independent life, even after the sex cells have 

 been extruded, there would be a typical sexual reproduction. 



B 



FIG. 123. EUDORINA 



D 



A, showing the asexual reproduction by division. C, D, and E show some cells which are 

 dividing into numerous flagellate gametes. These unite with the larger cells in B, which 

 are thus also gametes. 



Such a condition is found in the multicellular plants and ani- 

 mals generally. 



Prom such data as these it is evident that the probable 

 origin of sexual reproduction has been something as follows: 

 The first method of reproduction was by simple division, but 

 the independent individuals acquired the habit of fusing with 

 each other, as we have seen in the case of the Paramecium, 

 this fusion reinvigorating the life power of the fused individual. 

 Next there was probably a tendency for the cells to break up 

 into many parts which subsequently united with each other, 

 the parts being at first all alike. The next step seemed to be 

 for some of these cells to contain more food than the others 

 and become larger; this led to the larger cells having less power 

 of motion, while the smaller ones retained the power. Next 



