“_The ovarian eggs of all animals are perfectly identical,_ small cells with a vitellus, germinal vesicle and germinal spot” (§ 278). “_The organs of the body are formed in the sequence of their organic importance; the most essential always appear first._ Thus the organs of vegetative life, the intestine, etc., appear later than those of animal life, the nervous system, skeleton, etc., and these in turn are preceded by the more general phenomena belonging to the animal as such” (§ 318). “Thus, in Fishes, the first changes consist in the segmentation of the vitellus and the formation of a germ, processes which are common to all classes of animals. Then the dorsal furrow, characteristic of the Vertebrate, appears—the brain, the organs of the senses; at a later period are formed the intestine, the limbs, and the permanent form of the respiratory organs, from which the class is recognised with certainty. It is only after exclusion that the peculiarities of the structure of the teeth and fins indicate the genus and species” (§ 319). “_Hence the embryos of different animals resemble each other the more, the younger they are_” (§ 320). “Consequently the high importance of developmental history is indubitable. For, _if the formation of the organs takes place in the order corresponding to their importance, this sequence must of itself be a criterion of their comparative value in classification._ The peculiarities which appear earlier should be considered of higher value than those which appear subsequently” (§ 321). “_A system, in order to be true and natural, must agree with the sequence of the organs in the development of the embryo_” (§ 322). I do not know whether any one at the present day will be inclined to subscribe to this proposition in its whole extent.[4] It is certain, however, that views essentially similar are still to be met with everywhere in discussions on classification, and that even within the last few years, the very sparingly successful attempts to employ developmental history as the foundation of classification have been repeated. But how do these propositions agree with our observations on the developmental history of the Crustacea? That these observations relate for the most part to their “free metamorphosis” after their quitting the egg, cannot prejudice their application to the propositions enunciated especially with regard to “embryonal development” in the egg; for Agassiz himself points out (§ 391) that both kinds of change are of the same nature and of equal importance and that no “radical distinction” is produced by the circumstance that the former take place before and the latter after birth.