Page:Popular Science Monthly Volume 71.djvu/383

Rh of water takes place. During the segmentation of the ovum the condition of things has been reversed so far as the proportions of nucleus and protoplasm are concerned. "We have nucleus produced, so to speak, to excess. The nuclear substance is increased during this first phase of development.

Naturally, as we embryologists looked upon these things in earlier days and thought of the progress of development, we conceived of the earlier stage as younger, and of the ovum as being the youngest stage of; all, a conception which in terms, of time is obviously correct, but as regards the nature of the development, it seems to me clearly, is not correct. The ovum is a cell derived from the parent body, fertilized by the male element, and presenting the old state to us, the state in which there is an excessive amount of protoplasm in proportion to the nucleus; and in order to get anything which is young, a process of rejuvenation is necessary, and that rejuvenation is the first thing to be done in development. The nuclei multiply; they multiply at the expense of the protoplasm. They take food from the material which is stored up in the ovum, nourish themselves by it, grow and multiply until they become the dominant part in the structure. Then begins the? other change; the protoplasm slowly proceeds to grow, and as it grows, differentiation follows, and so the cycle is completed. Whether other naturalists will be inclined to accept this conception that the process of the segmentation of the ovum is that which we must call rejuvenation or not, I can not say, for the matter has as yet been very little discussed, but you will see that it hangs as a theory well-together. We have first an explanation of the process of the production of the young material, and out of that young material the fashioning of the embryo. The cycle of life has two phases, an early brief one, during which the young material is produced, then the later and prolonged one, in which the process of differentiation goes on, and that which was young, through a prolonged senescence, becomes old. I believe these are the alternating phases of life, and that as we define senescence as an increase and differentiation of the protoplasm, so we must define rejuvenation as an increase of the nuclear material. The alternation of phases is due to the alternation in the proportions of nucleus and protoplasm.

In the next lecture I shall be able to convince you, I hope, that this conception of the relation of the power of growth to the proportion of nucleus and protoplasm enables us to understand various problems of development, certain possibilities of regeneration and reconstruction of lost parts, and that it also leads us naturally forward to the consideration of the problem of death as it is now viewed by biologists, so that our next lecture will be upon the subject of regeneration and death, the natural topics to follow after to-night's discussion.