2989. At the entrance of the inversion of the umbilical cord is situated a small vesicle, which divides and is prolonged into the two intestines. It is therefore the root or primary sac of the intestinal system, and is called in Man the 'vesicula umbilicalis,' in the Mammalia the 'tunica erythroides,' and in oviparous animals the 'vitellus.'
2990. In the same situation is placed another cyst, which is prolonged into what has been called the 'urachus' and the urinary cyst, and from which the primordial kidneys, the true kidneys, and the sexual parts are developed by sacciform eversion. This sac is called the 'allantois,' 'tunica allantoides,' and is consequently the root or primary sac of the sexual system.
2991. These cysts or sacs are consequently not envelopes that serve for the protection of the fœtus, but its developmental organs, which disappear, so soon as their prolongations into the fœtus itself enable them to exercise their functions.
2992. There are thus as many developmental sacs or cysts, as there are vegetative systems present, viz.:
a. Two General Cysts.
1. The Vascular cyst—Chorion.
2. Tegumental cyst—Amnion.
b. Two Special Cysts.
3. The Intestinal cyst—Vitellus.
4. Sexual cyst—Allantois.
2993. It is only the vegetative systems which take root in the fœtal envelopes, but not the animal systems. There is no developmental cyst for the nervous, muscular, and osseous systems.
2994. The persistent vegetative systems are the developmental organs for the animal systems; as the intestine for the bones, the veins for the muscles, the tegument or branchial sac for the nerves.
2995. The fœtus consists of three floors or stories like a house, whereof one has been based upon, or rather developed out of the other, viz.:
a. Of the Developmental cysts.
b. " Vegetative systems.
c. " Animal systems.
2996. According to time the sacs are developed in the following series. The first sac is that of the vitellus or the intestine, which is also the first that is present in the development of the animal kingdom. Upon this vitelline membrane the blood-vessels ('vasa omphalomesenterica') are developed, are prolonged into the body with the intestine, turn again outwards and form the chorion. From this the amnion next separates into the envelopes, and the integument upon the embryo. Lastly, the allantois appears, and in its prolongation the sexual parts.