One thing, however, we know with certainty, namely, that the human body, like that of other animals, has its origin in the egg. But the egg itself must be considered as the original animal, as the infusorial creature, appearing in the globular or primary form of animal life, and, like the seed of the plant, containing ideally within itself the whole animal, which, under given external circumstances, it develops really out of itself. Recent observations have shown that the first rudiment of the embryo is formed on the inner surface of the hollow globe of the egg, by folding or turning inwardly a part of the integuments of the egg; which reminds us distinctly of the purely vegetative bladder-worm (Cysticercus, see p. 244), in which the so-called head, or the absorbing orifice, is turned inwards. It is plain therefore that we must consider the egg in this first period of development also as a plant with a root turned inwards (see p. 243); and we find this moreover confirmed by the functionary attributes of its parts, since the first introversion of the integument forms the cavity of the stomach and the intestines, as the first rudiment of the embryo. On the other hand, the external covering of the egg being somewhat similar to the green (breathing) surface of the plant, performs the breathing function of the embryo, and contains also (as the latest observations of Pander have shown), in the external envelop of the cuticle, the origin of the organs of sensation. In a further stage of development we see the activity of formation concentrating itself more in the point turned upwards, and thus producing new opposites. The rudiment of the embryo repeats the above-described form, and the higher animal organs, which commence with the spinal marrow and the vertebral column, originate above the germ of the intestines of the cavity of the stomach. It is now in particular plainly seen how the animal is as it were transformed from a plant into an animal in the manner above described (p. 234). For at the very first the intestines are only attached to the fœtus, that is, the gut lies as yet in the umbilical cord, just as at first the root is attached to and not in the plant; but the intestines are soon afterwards drawn more and more inwards, and by degrees are completely enveloped by the animal organs, which grow simultaneously forwards on both sides out of the vertebral column; and here for the first time the fœtus presents the appearance of an independent animal organism. The vascular system, however, is much more slow in connecting itself perfectly with the fœtus; since it is rather a general connecting medium in the vegetative sphere, and therefore performs the function of a bond of union between the integument of the egg and the fœtus so long as it remains inclosed in the egg. Even at this period of development the relation of the parts of the egg reminds us of the plant; as the fœtus, by means of the umbilical cord, is united to the placenta and integuments of the egg, hold in organic connexion with the maternal organism, just as the flower (which it resembles in respect to the
