there are cells enough to allow this variety to be worked out. This we call the embryonic type of development.
We see, therefore, that nature has recognized a restriction which she herself has put upon development, the restriction which obliges development, if it is to be ample, to prolong the accumulation of the undifferentiated cells. In response to that condition, she substitutes for higher types of animal that development which we call embryonic, leaving for the lower type that which we call larval. Thus we see in the growth and formation of the higher animals and in the history of the comparative development of the animal kingdom, fresh illustrations of the great importance of the young type of cells.
We can see the same thing also in regard to regeneration. The regenerative process depends to a large extent upon partial differentiation, or even upon its total absence. Regeneration is a most interesting and wonderful process. I took pains only this afternoon to look at that famous classic by the Dutch Abbe Trembley on hydroids or polyps as he calls them. "The Fresh Water Polyps." a book published in 1744, was well printed, and on such good paper that it looks to-day
almost like a new book. He made the curious experiment of cutting one of these minute fresh water polyps—they are perhaps an eighth of an inch long—in two, and made the startling discovery that each half of the polyp would make up what the other half had deprived it of: each half, in other words, would become a new polyp. That which was lost was regenerated. After him came a series of yet more remarkable experiments by the famous Italian naturalist, Spallanzani, one of the masters of experimental research, and he discovered that regeneration was a property which was not peculiar by any means to polyps,
