ferentiation, 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 occur in the whole life of the rabbit has been completed. We must from this conclude that the rate of differentiation is very rapid at first and afterwards declines, and as we compare the different stages of development 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, allowing them to remain in the young state. Such cells may afterward differentiate.
From all that has been said it seems to me legitimate to conclude that there is an intimate correlation between the rate of differentiation and the rate of growth. I am inclined to go the one step farther, and bring them into the relation of cause and effect; and I present to you as the main general conclusion of this first part of our series of lectures, the conception that the growth and differentiation of the protoplasm are the cause of the loss of the power of growth. Now if cells become old as their protoplasm increases and becomes differentiated, we should expect to find that there would be a provision for the production of young cells. It is rather mortifying to reflect that the simple conception which I have now to express to you, although it lay close at hand, failed to combine itself in my mind for many years with the conception of the process of senescence as I have just described it to you. It is somewhat, it seems to me, like two acquaintances of mine who lived long side by side, seeing one another frequently until they were fairly past the period of youth, when their attachment became very close and by a sacrament they were permanently joined together. So in the minds of men often two ideas lie side by side which ought to be married to one another, and there is no one ready, so dull is the owner of the mind, to pronounce the sacramental words which shall join them, and the rite long remains unperformed, and when at last such neighbor ideas, which naturally should be united in close companionship, are brought together and made, as it w-ere, into one, we are astonished that the inevitableness of the union had not obtained our notice before, it is so very obvious. And so in regard to the conception of what constitutes the restoration of the young state, I have only this excuse to offer, which I have indicated to you, that even the
