i6o AGE, GROWTH, AND DEATH 



we can detect cells already far advanced in their de- 

 velopment. It is not certainly too much to say that 

 in the brief period of these nine days fully as much 

 differentiation has been accomplished as is accom- 

 plished during the entire remainder of the life of the 

 animal. We do not, at present at least, possess any 

 method of measuring differentiation which enables 

 us to state it numerically, but no one who is familiar 

 with these matters and observes the structure, as I 

 have myself observed it, would hesitate for a moment, 

 it seems to me, to decide that my assertion is perfectly 

 within the bounds of truth, that within a period of nine 

 days, half of the entire differentiation which is to oc- 

 cur in the whole life of the rabbit has been completed. 

 We must from this conclude that the rate of differ- 

 entiation is very rapid at first and afterwards declines, 

 and as we compare the different stages of develop- 

 ment we can see readily that this is the case. The 

 progress in the additional development in the rabbit 

 from sixteen and one half days up to the time of its 

 birth is far greater than the progress which occurs 

 after birth. We find, moreover, in the study of these 

 embryonic conditions, some instructive things, for in 

 certain parts of the body the process of differentiation 

 hurries along, and as the cells are differentiated their 

 power of growth, to a large extent, is stopped. On 

 the other hand, there are various provisions in the 

 developing animal for keeping back certain cells, al- 

 lowing them to remain in the young state. Such cells 

 may afterward differentiate. 



