THE PROBLEM 



35 



exclusive mode of reproduction, in some or all forms of 

 the three lowest groups of multicellular organisms,' the 

 sponges, flatworms, and coelenterates. More rarely it 

 may occur in other of the lower invertebrate 

 groups. It may occur in the form of budding 

 or of fission comparable to that of the Proto- 

 zoa. The agamic reproduction of one of the 

 flatworms, Planaria dorotocephala, studied 

 by the writer many years ago, as shown in 

 Figure 6, may serve as an illustration. 



This simply organized Worm, which lives 

 under stones in sluggish streams and ponds, 

 after attaining a certain size, will under the 

 appropriate environmental conditions exhibit 

 a constriction towards the posterior end of 

 the body, as shown in Figure 7. 



For a time the animal moves about as a 

 rather ungainly double individual. It finally 

 separates into two. The larger anterior part 

 forms a new tail, and the smaller posterior 

 fission product forms a new head and rapidly 

 grows to full size. The process is, in princi- 

 ple, exactly the same as the multiplication of 

 Paramecium by fission. In another member 

 I of the same general group of animals as 

 \ / Planaria, named Stenostomum, several fis- 

 \J sion planes may form and the process start 

 anew before the products delimited by the 

 first plane have separated. As a result, we 

 get frequently in this form chains of individ- 

 uals attached in a long string to each other, 

 as shown in Figure 8. 



It is obvious that so long as reproduction goes on in 



Fia. — 7. Begin- 

 ning of process of 

 agamic repro- 

 duction by fis- 

 sion in planaria. 

 (From Child) 



