DIFFERENTIATION AND REJUVENATION 135 



will return to the mother dimension and be as large 

 as the parent cell from the division of which they 

 arose. There is thus, we learn, the constant fluctua- 

 tion in the size of cells, a fluctuation in their di- 

 mensions accompanying the process of cell division. 

 Presently we shall have more to say in regard to 

 this matter of the change in the cell in size. The 

 next picture (Fig. 47) which I want to recall to you 

 is one which we also had in an earlier lecture. It 

 represents three slices through a very young rabbit 

 before any of the organs of the animal have begun 

 to develop. We can see here clearly the nuclei, as I 

 pointed out to you before, nearly uniform in struc- 

 ture, and you notice that the protoplasm around each 

 nucleus is quite small in amount. If you will recall 

 the previous picture (Fig. 46) of the skin of the sala- 

 mander, upon the screen a moment ago, you will 

 realise immediately, in comparing the two, that in 

 these young cells the proportion of the protoplasm to 

 the nucleus is very small. That is again one of the 

 fundamental facts to which we shall recur in a mo- 

 ment. I wanted to show you this picture in order 

 to revive in your minds the conception which I en- 

 deavoured to give you before of the undifferentiated 

 tissue, where the cells have nuclei pretty uniform in 

 appearance and in size, each with its little mass of pro- 

 toplasm about it, and this protoplasm appearing in 

 all the cells under microscopic examination very much 

 the same. We cannot in this stage of development 

 say of a given cell that it displays structure by which, 



