486 BIOLOGY. 



' allantois,' ' tunica allantoides/ and is consequently the 

 root or primary sac of the sexual system. 



2991. These cysts or sacs are consequently not en- 

 velopes that serve for the protection of the foetus, but its 

 developmental organs, which disappear, so soon as their 

 prolongations into the foetus itself enable them to exer- 

 cise 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. c<rw< 



b. Two Special Cysts. 



3. The Intestinal cyst Vitellus. i ; c ^ *jd*Jj/i*)L v ^ 



4. Sexual cyst Allantois, 



2993. It is only the vegetative systems which take root 

 in the foetal 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 deve- 

 lopmental organs for the animal systems ; as the intes- 

 tine for the bones, the veins for the muscles, the tegu- 

 ment or branchial sac for the nerves. 



2995. The foetus 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 omphalome- 

 senterica') are developed, are prolonged into the body 

 with the intestine, turn__again ojitwards and form the 

 chorion. Prom this the amnion next separates into the 

 envelopes, and the integument upon the embryo. Lastly, 



