SELF-DIVISION AND REGENERATION 145 



ration takes place in the shortest axis, without regard to the structure 

 of the animal. A law similar to that enunciated in connection with 

 the division of the cell seems to hold for the organism as a whole : 

 namely, division takes place, as a rule, in the shortest diameter of the 

 form. The protozoa are, in a sense, excluded, since being unicellular 

 forms they come under the rule for the division of the cell. In the 

 ccelenterates we find the actinians and corals, that have short, cylin- 

 drical bodies, dividing from the oral to the aboral end, while the longer 

 scyphistoma divides transversely. The flat, bell-shaped medusa, gas- 

 troblasta, divides in an oral-aboral plane. The flat-worms and an- 

 nelids divide transversely, and, therefore, in the plane of least resist- 

 ance. The most important illustrations of this principle are furnished 

 by the echinoderms. Those brittle-stars that divide through the disk 

 do so in the shortest direction, that is, from the oral to the aboral side, 

 whilst the holothurians that are long, cylindrical forms divide across 

 the body and, therefore, in a structural plane at right angles to that of 

 the brittle-stars. It may be claimed that in all these cases the plane 

 of division is that in which the animal is most likely to be broken 

 in two by external agents, but this is, I think, only a coincidence, 

 and the result is really due to internal conditions. The division is 

 brought about in most cases, and perhaps in all, by the contraction 

 of the muscles ; and the arrangement of the muscles in connection 

 with the form of the body is the real cause of the phenomenon. 



Returning to the general question of the occurrence of the process 

 of division in the different groups, we find that in nearly all of them 

 in which self-division occurs it is found in a number of different 

 forms in the same group. The process seems to be characteristic 

 of whole groups rather than of species, and so far as evidence of this 

 sort has any value it points to the conclusion that the process is not 

 necessarily a special case of adaptation to the surroundings, because 

 the species that divide may live under very diverse conditions. 



A further examination of the facts throws a certain amount of 

 light on the relation between the processes of self-division and of 

 regeneration. The following questions may serve to guide us in our 

 examination : 



(i) Is regeneration found only in those groups in which self-divi- 

 sion takes place as a means of propagation ; or, conversely, does 

 self-division only occur in those groups that have the power of 

 regeneration ? 



(ii) Is regeneration confined, in the groups that make use of self- 

 division as a means of propagation, to those regions of the body 

 where the self-division takes place ? 



(iii) Is regeneration as extensive in the groups that do not propa- 

 gate by self-division as in those that do ? 



