Heredity in Protozoa 625 



When this happened at successive fissions, the animal bore such 

 a point at each end. 



It is evident that these points are due to the same cause that pro- 

 duced the inherited tendency to remain united in chains (as in Fig. 

 20). They result from the ridge of new material along the aboral 

 side of the animal, shown in Fig. 4, ^. Now, it is easily conceiv- 

 able that this new material might be of such texture and thickness 

 that it would always be drawn out at fission in such a way as to 

 produce points of a definite form and size. These would then 

 appear regularly after fission; a race of Paramecia with this as 



Fig. 22 Examples from the race a, of individuals having a point at the posterior end, due to the draw- 

 ing out of the connecting band at the time of fission. 



a new characteristic would have been produced. The spine would 

 be hereditary, because produced anew in each generation, just 

 as are the setae of the Hypotricha, or the organs of the Metazoa.* 



SUMMARY AND GENERAL DISCUSSION 



The following general statements of the laws and principles 

 bearing on heredity^ that result from our investigation are made 

 with direct reference to the Protozoa, and will best be grasped 

 by keeping in mind concrete cases, such for example as those 

 shown in Fig. 4, Fig. 20 and Fig. 22. 



*It is of course possible that the origin of new permanently inherited characters is not normally through 

 mere modifications of the external parts of the cell, such as we see in our illustrative cases. Possibly 

 there must be originally some modification of more recondite parts — nucleus, chromosomes, or the hke — 

 and that these then secondarily act upon and change the outer parts. This would add farther compli- 

 cation, but would not change the essential point, which is, that in order that a characteristic may be 

 inherited, it must be due to some modification that causes a change in the processes of reproduction. 



fFor a summary of results on other matters than heredity (on the changes during fission and growth, 

 etc.), see pp. 599-604. 



