embryo momentarily assumes. To give a few examples, we can infer from the fact that the human ovum is a simple cell that the first ancestor of our species was a tiny unicellular being, something like the amœba. In the same way, we know, from the fact that the human fœtus consists, at the first, of two simple cell-layers (the _ gastrula_), that the _gastræa_, a form with two such layers, was certainly in the line of our ancestry. A later human embryonic form (the _chordula_) points just as clearly to a worm-like ancestor (the _prochordonia_), the nearest living relation of which is found among the actual ascidiæ. To this succeeds a most important embryonic stage (_acrania_), in which our headless fœtus presents, in the main, the structure of the lancelet. But we can only indirectly and approximately, with the aid of comparative anatomy and ontogeny, conjecture what lower forms enter into the chain of our ancestry between the gastræa and the chordula, and between this and the lancelet. In the course of the historical development many intermediate structures have gradually fallen out, which must certainly have been represented in our ancestry. But, in spite of these many, and sometimes very appreciable, gaps, there is no contradiction between the two successions. In fact, it is the chief purpose of this work to prove the real harmony and the original parallelism of the two. I hope to show, on a substantial basis of facts, that we can draw most important conclusions as to our genealogical tree from the actual and easily-demonstrable series of embryonic changes. We shall then be in a position to form a general idea of the wealth of animal forms which have figured in the direct line of our ancestry in the lengthy history of organic life. In this evolutionary appreciation of the facts of embryology we must, of course, take particular care to distinguish sharply and clearly between the primitive, palingenetic (or ancestral) evolutionary processes and those due to cenogenesis.[2] By _palingenetic_ processes, or embryonic _recapitulations,_ we understand all those phenomena in the development of the individual which are transmitted from one generation to another by heredity, and which, on that account, allow us to draw direct inferences as to corresponding structures in the development of the species. On the other hand, we give the name of _cenogenetic_ processes, or embryonic _variations,_ to all those phenomena in the fœtal development that cannot be traced to inheritance from earlier species, but are due to the adaptation of the fœtus, or the infant-form, to certain conditions of its embryonic development. These cenogenetic phenomena are foreign or later additions; they allow us to draw no direct inference whatever as to corresponding processes in our ancestral history, but rather hinder us from doing so.