42 HUMAN ANATOMY. 



litres, due to pathological conditions of the fcetal envelopes ; such excessive secre- 

 tion constitutes hydra?rmion. During the later months of pregnancy the foetus swal- 

 lows the amniotic fluid, as shown by the presence of hairs, epithelial cells, etc., 

 within the stomach. In view of the composition of the fluid, consisting almost en- 

 tirely of water, it seems certain that the introduction of the liquor amnii does not 

 serve the purposes of nutrition ; on the other hand, it is probable, as held by Preyer, 

 that the unusual demands of the foetal tissues for water may be met largely in this 

 manner. 



The source of the amniotic fluid in man has been the subject of much discus- 

 sion. The investigations of Mandl and of Biondi, however, have shown that probably 

 the amniotic fluid is chiefly the result of excretory activity of the epithelium lining 

 the amniotic sac. At first these epithelial cells are flat and plate-like, but dur- 

 ino- the later months of gestation they become taller and cuboidal and exhibit indi- 

 cations of secretory phenomena. The early amniotic fluid resembles in appearance 



Fig. 49. 



Umbilical vesicle : •"''~--'.- '-< \V i'^- 





Umbilical stalk 



Inner surface 

 of chorion ~ 



V.^_ ^jA. 



Umbilicfil cord t — 



Cut edge of 

 amnion 



Masses of 

 chorionic villi 





Human embryo of about thirty-three days. X 4- Amnion and chorion have been cut and turned aside. 



and chemical composition a serous exudate; later, after the formation of the uro- 

 genital openings, the liquor amnii becomes contaminated by the addition of a small 

 amount of the fluid derived from the excretory organs of the foetus. During the later 

 weeks of gestation the contents of the digestive tube are discharged into the amniotic 

 sac as rneconium. 



The Umbilical Vesicle. — The umbilical vesicle, as the yolk-sac in man is 

 termed, presents a reversed growth-ratio to the amnion and body-stalk since it pro- 

 gressively decreases as these latter appendages become more voluminous. The early 

 human embryo is very imperfectly differentiated from the large and conspicuous 

 yolk-sac, with which its ventral surface widely communicates. With the advances 

 made during the third week in the formation of the primitive gut, the connection 

 between the latter and the vitelline sac becomes more definitely outlined in conse- 

 quence of the beginning constriction which indicates the first suggestion of the later 

 vitelline or umbilical ducf (Fig. 47). By the end of the fourth week the connection 



