THE MUTATION HYPOTHESIS 69 



During the series of cell-divisions, following the fertilisa- 

 tion of the ovum, which go to build up the embryo, the cells 

 produced are arranged in layers. The outer layer is called 

 the epiblast, the inner layer the hypoblast. At a later period 

 another layer the mesoblast appears between the epiblast 

 and the hypoblast. Different organs and different tissues 

 are produced from different embryonic layers, and the older 

 embryologists believed that epiblastic cells could only be- 

 come differentiated into tissues usually produced from the 

 epiblast. They believed the same with regard to the 

 mesoblast and the hypoblast. The crystalline lens of the 

 eye is produced from epiblast, but when it is removed in 

 the salamander, the new lens grows from cells that were 

 produced from the mesoblast. Under normal conditions 

 mesoblast cells in the salamander would never become 

 modified into anything at all like the crystalline lens. 

 Here the mesoblastic cells have still retained the general 

 potentiality of the fertilised ovum in a very high degree, 

 and are able to reproduce such a specialised structure as the 

 lens, in spite of the fact that under normal conditions this is 

 only produced from epiblastic cells. When we come to 

 mammals, however, we find only very limited traces of 

 the retention of common potentialities among the differen- 

 tiated cells forming the soma. 



The phenomena exhibited by the cells of the higher 

 animals is very different from what happens in plants. 

 In the latter we find the whole organism reproduced from 

 a few differentiated cells cut off from an existing individual. 

 There is no counterpart to this phenomenon among animals 

 except among the lowest. 



Very frequently we find this potentiality of growing from 

 grafts, cuttings, or runners, coexistent in the same plant with 

 bi-parental reproduction. Thus a strawberry may be grown 

 from seeds, but also from runners. Whether the plant pro- 

 duced from a runner ought to be regarded as a new indi- 

 vidual, or as an actual part of the parent, is not a material 

 point to us at the present moment. It is evident that it can 



