PROBLEMS OF MODERN SCIENCE 



the nature of these cells. Are the bone-building 

 cells proficient in bone-building alone and incapable 

 of any other activity, or are they cells which build 

 bone under certain circumstances, but which, 

 under other circumstances, might set to work to 

 build some other tissue ? This is an interesting 

 problem which is yet to be solved. 



If we may believe that the bone-builders are 

 bona-fide trade unionists who would not lay a finger 

 on any other job, is there any particular part of the 

 body whence they come ? One investigator, who 

 sacrificed the promise of a brilliant career as an 

 anatomist eventually to become a Cabinet minister, 

 adduced striking evidence to demonstrate that the 

 bone-builders migrated to the deeply situated 

 cartilage from the surface of the body. If this be 

 true, the discovery may be one of the deepest 

 significance, for the following reasons. The zool- 

 ogist classifies animals into two great groups 

 the Vertebrates, or animals possessing a backbone, 

 and Invertebrate animals without a backbone. Of 

 the two groups, the Invertebrates are the lowlier 

 and more primitive, and it is believed that the 

 Vertebrates, which appeared at a later date in the 

 world's history, sprang from Invertebrate ancestors. 

 One of the great differences between the Verte- 

 brates and the Invertebrates is, that when the 

 latter are provided with a hard resistant skeleton 

 it is found on the surface of the body, and is built 

 224 



