2997. Originally the whole chorion is replete all around with vessels; but as the process of oxydation occurs most powerfully upon the fundus uteri, so are the vessels developed most abundantly in that very situation, and form the placenta.
2998. The placenta is no peculiar organ, but only the more energetic part of the chorion.
2999. It must necessarily be placed around the insertion of the umbilical cord, because at this spot the inversion takes place, on account of the strong oxydation.
3000. The placenta is always situated upon the fundus uteri, because it originates only through its influence. It cannot therefore begin to suck, like the mouth of a leech, in a fortuitous or voluntary manner anywhere. If it is found occupying some other situation, it is a proof that the oxydizing process of the uterus has been displaced. This is consequently an abnormal situation.
3001. Opposite to the fœtal vascular system is first of all developed the general system of animal life, namely, the nervous system, and indeed the spinal chord, or what has been called the 'carina' or primary streak.
3002. The development of all other systems oscillates in this antagonism of blood and nerves.
3003. In the antagonism of the placenta is formed the liver, which in the embryo is one of the largest organs, and in its antagonism the brain is developed.
3004. In the antagonism of the amnion the integument is evolved, and in its antagonism the branchiæ and lungs.
3005. In obedience to the antagonism of the arteries and veins, the vitelline sac divides into small and large intestine. The one pursues a course towards the arterial extremity of the body, the brain, the other towards the venous or sexual parts; mouth and anus.
3006. Finally, from the antagonism of the allantois the sexual parts emerge at the very extremity of the body, which is placed opposite to the mouth.