FIGURE 2.359. VASCULAR TISSUE. FIGURE 2.360. SECTION OF TRUNK OF A CHICK-EMBRYO. FIGURE 2.361. MEROCYTES. FIGURE 2.362. VASCULAR SYSTEM OF AN ANNELID. FIGURE 2.363. HEAD OF A FISH-EMBRYO. FIGURES 2.364 TO 2.370. THE FIVE ARTERIAL ARCHES. FIGURES 2.371 AND 2.372. HEART OF A RABBIT-EMBRYO. FIGURES 2.373 AND 2.374. HEART OF A DOG-EMBRYO. FIGURES 2.375 TO 2.377. HEART OF A HUMAN EMBRYO. FIGURE 2.378. HEART OF ADULT MAN. FIGURE 2.379. SECTION OF HEAD OF A CHICK-EMBRYO. FIGURE 2.380. SECTION OF A HUMAN EMBRYO. FIGURES 2.381 AND 2.382. SECTIONS OF A CHICK-EMBRYO. FIGURE 2.383. EMBRYOS OF SAGITTA. FIGURE 2.384. KIDNEYS OF BDELLOSTOMA. FIGURE 2.385. SECTION OF EMBRYONIC SHIELD. FIGURES 2.386 AND 2.387. PRIMITIVE KIDNEYS. FIGURE 2.388. PIG-EMBRYO. FIGURE 2.389. HUMAN EMBRYO. FIGURES 2.390 TO 2.392. RUDIMENTARY KIDNEYS AND SEXUAL ORGANS. FIGURES 2.393 AND 2.394. URINARY AND SEXUAL ORGANS OF SALAMANDER. FIGURE 2.395. PRIMITIVE KIDNEYS OF HUMAN EMBRYO. FIGURES 2.396 TO 2.398. URINARY ORGANS OF OX-EMBRYOS. FIGURE 2.399. SEXUAL ORGANS OF WATER-MOLE. FIGURES 2.400 AND 2.401. ORIGINAL POSITION OF SEXUAL GLANDS. FIGURE 2.402. UROGENITAL SYSTEM OF HUMAN EMBRYO. FIGURE 2.403. SECTION OF OVARY. FIGURES 2.404 TO 2.406. GRAAFIAN FOLLICLES. FIGURE 2.407. A RIPE GRAAFIAN FOLLICLE. FIGURE 2.408. THE HUMAN OVUM. CHAPTER 2.16. STRUCTURE OF THE LANCELET AND THE SEA-SQUIRT. In turning from the embryology to the phylogeny of man--from the development of the individual to that of the species--we must bear in mind the direct causal connection that exists between these two main branches of the science of human evolution. This important causal nexus finds its simplest expression in "the fundamental law of organic development," the content and purport of which we have fully considered in the first chapter. According to this biogenetic law, ontogeny is a brief and condensed recapitulation of phylogeny. If this compendious reproduction were complete in all cases, it would be very easy to construct the whole story of evolution on an embryonic basis. When we wanted to know the ancestors of any higher organism, and, therefore, of man--to know from what forms the race as a whole has been evolved we should merely have to follow the series of forms in the development of the individual from the ovum; we could then regard each of the successive forms as the representative of an extinct ancestral form. However, this direct application of ontogenetic facts to phylogenetic ideas is possible, without limitations, only in a very small section of the animal kingdom. There are, it is true, still a number of lower invertebrates (for instance, some of the Zoophyta and Vermalia) in which we are justified in recognising at once each embryonic form as the historical reproduction, or silhouette, as it were, of an extinct ancestor. But in the great majority of the animals, and in the case of man, this is impossible, because the