76 Inside the Living Cell 



plant is thus contained in every part of it — or at least in many of the 

 parts. 



This ability is much less developed in animals and the more highly 

 developed the animal, the less is the power of reproducing lost parts. 

 The higher animals like man only possess the power to a limited de- 

 gree. If you cut your finger, the cells on either side of the cut will 

 begin to multiply and spread over the wound, reproducing the main 

 superficial features in doing so. In man, however, even the area of 

 skin which can be replaced is rather limited; but animals often have 

 a greater power of regenerating lost skin. Animals which are lower 

 in the scale of life have much more extensive powers of regenera- 

 tion. If a lizard loses its tail, it can grow a new one; so can a fish. A 

 starfish will reproduce a ray which is broken off; some species of 

 worms will grow a new individual from a small segment. In these 

 cases the pattern of the whole individual is inherent in the parts and 

 if the pattern is broken it is reformed. 



These are very difficult facts to account for. Why are the cells of 

 a mutilated starfish stimulated to growth and why does growth cease 

 when it has just replaced the missing ray? We must ask how the pat- 

 tern is carried in the organism. 



The answer to these questions is not known. Indeed, little is known 

 about how large organisms are formed, as they must be in all cases 

 of sexual reproduction from a single fertilized egg cell, by successive 

 divisions. Most of the higher organisms go back to a single fertilized 

 egg cell at one stage of their life cycle, and the complex individual is 

 formed by its successive divisions. About fifty successive divisions of 

 the fertilized human ovum make a human being, with all the tissues 

 of the human body, muscles, gut, liver, bone and brain. They are not 

 merely made in sufficient quantities but precisely fashioned and ex- 

 actly fitted together to make a functioning organism. It is obvious 

 that, when cells in the growing embryo divide, they do not always 

 give rise to two identical daughter cells. There must be many points 

 at which differentiation occurs; the two daughter cells formed at one 

 division will have different destinies; one may give rise to muscle and 

 the other to nerve. 



Very little is known about how this process of differentation 

 occurs. It is certainly one of the darkest chapters of scientific know- 

 ledge — rather, one should say ignorance — at the present time. It 

 would seem that, although in every cell division the chromosomes 

 are replicated and one pair goes into each of the new daughter cells, 

 yet there is a difference. One possibility is that the genes present in 

 the chromosomes are covered, or inhibited, until they are required 

 to carry out their function. This covering of the genes might be 



