Hertwig sums up his conclusion in the statement that ontogenetic stages give us, therefore, a greatly changed picture of the phylogenetic series of adult ancestors. “The two correspond not according to their actual contents but only as to their form.” Hertwig also repeats His’s idea, that the reason that certain kinds of form repeat themselves in the development of animals with a great constancy depends principally on this, that they supply the necessary conditions under which alone the following higher stage of the ontogeny can be formed. The development, for instance, begins with the division of the egg, because this is the only way that a one-celled condition can give rise to a many-celled form. Again, the organs can be formed only when groups of cells have made a closer union with one another. Thus the gastrula must begin with the antecedent blastula, etc. Definite forms are, despite all modifying influences, held to firmly, because by their presence the complicated end-stages can be reached in the simplest and most suitable way. Thus Hertwig adopts here a little from one doctrine and there a little from another, and between his attempt to reinstate the old biogenetic law of Haeckel, and to adopt a more modern point of view, he brings together a rather curious collection of statements which are not any too well coördinated. Take, for example, his description of the relation between Haeckel’s gastræa and the embryonic gastrula stage. The latter he maintains is a repetition of the other, but only in form, not in actual contents. And in another connection we are told that the cause of this repetition is that the gastrula is the simplest way in which the later stages can be reached, and, therefore, it has been retained. It seems to me that Hertwig has undertaken an unnecessary and impossible task when he attempts to adjust the old recapitulation theory to more modern standards. His statement that the egg is entirely different from its amœba prototype is, of course, only the view generally held by all embryologists. His mystical statement that the embryonic form _repeats the ancestral adult stage in its form, but not in its contents_, will scarcely recommend itself as a model of clear thinking. Can we be asked to believe for instance that a young chick repeats the ancestral adult fish form but not the contents of the fish?