along which the production of new cells is going on. These new cells migrate over the surface of the cerebellum without changing at all into nerve cells. The}-form a distinct layer which is well known to every investigator of brain structure, and presently after birth these cells accomplish a second migration, but in a different direction. Instead of moving in a constant current over the surface of the brain, each one takes a vertical pathway from the surface down towards the interior of the cerebellum; and arrived there it changes and becomes a nerve cell, or at least a part of them do; and with that the machinery of the cerebellum is complete. Thus, structurally, the cerebellum at birth is an uncompleted organ. Now, the cerebellum is that portion of the brain which regulates the combination of muscular movements, which secures that which the physiologists term coordination of movements, and it is not until the cerebellum has been perfected that it can perform this function. Were there not some provision of this special sort for allowing cells to be produced and added to the brain, the full complexity of the brain could not be attained, because after the cells have begun to change into nerve cells they lose their power of multiplication, and this is a device very exquisite in its working to supply to the brain the requisite number of cells to give it its full measure of complexity.
Another instance of the reservation of cells of a simple type is afforded us by the skin, al)out which I shall have something more to say in a few moments when we speak of the process of regeneration. It is not only in the period of childhood, and not only in the cerebellum, that we find cells exist such as I have just described to you, but it is in other parts of the body also and at other periods of life that we find the like phenomena; and in part I have already referred to these. You remember I told you in a previous lecture there is always in the body, even at the extreme of life, a store of cells of the young type, which is garnered in the marrow of the bones. The cells in question can multiply, and their descendants in part undergo a change in consequence of which they are converted into blood corpuscles. The undifferentiated or young cells are preserved in the marrow precisely for the purpose of making up the necessary number of blood corpuscles to replace those which are lost either by accident or in consequence of normal physiological processes. I mentioned to you that in the lining of the intestine there is a constant loss of cells and we find in every simple gland of the intestine, in every little gland of the stomach, a center for cell production, a center where there is a group of cells which are not differentiated, but retain their simple organization.
I could multiply these instances almost indefinitely, but perhaps it will be better to call your attention to an illustration of quite a different sort. We know that in order to have a very complex organization, the number of cells in the body must be very large indeed. Obviously a
