The essential agreement in the whole bodily form and inner structure is still visible in the embryo of man and the other mammals at the late stage of development at which the mammal-body can be recognised as such. But at a somewhat earlier stage, in which the limbs, gill-arches, sense-organs, etc., are already outlined, we cannot yet recognise the mammal embryos as such, or distinguish them from those of birds and reptiles. When we consider still earlier stages of development, we are unable to discover any essential difference in bodily structure between the embryos of these higher vertebrates and those of the lower, the amphibia and fishes. If, in fine, we go back to the construction of the body out of the four germinal layers, we are astonished to perceive that these four layers are the same in all vertebrates, and everywhere take a similar part in the building-up of the fundamental organs of the body. If we inquire as to the origin of these four secondary layers, we learn that they always arise in the same way from the two primary layers; and the latter have the same significance in all the metazoa (_i.e.,_ all animals except the unicellulars). Finally, we see that the cells which make up the primary germinal layers owe their origin in every case to the repeated cleavage of a single simple cell, the stem-cell or fertilised ovum. Fig.180. Very young human embryo of the fourth week, one-fourth of an inch long. Fig. 180—Very young human embryo of the fourth week, one-fourth of an inch long (taken from the womb of a suicide eight hours after death). (From _Rabl._) _n_ nasal pits, _ a_ eye, _u_ lower jaw, _z_ arch of hyoid bone, _ k3_ and _k4_ third and fourth gill-arch, _h_ heart; _s_ primitive segments, _vg_ fore-limb (arm), _hg_ hind-limb (leg), between the two the ventral pedicle.