632 



MAMMALIAN EMBRYOLOGY 



through the body-stalk into the villi to connect with the vascularization 

 there established. 



The two veins later fuse, so that a cross section of a mature umbili- 

 cal cord (Fig. 364), shows two arteries and a single large vein. 



Aa. umbilicales 



V. umbilicalis 



V. umbilicalis 

 A. umbilicalis | A. umbilicalis 



Whartonsche 

 Sulze 



Amnion 

 Coelom 

 Ductus omphaloentericus (vitellinus) 



Ductus omphalo- Whartonsche Amnion 

 entericus (vitellinus) Sulze" ' 



Fig. 364. 



I, Umbilical cord of human embryo at three months. 



II, Same at birth. (After Corning.) 



In addition to the umbilical vessels just mentioned, the yolk-stalk 

 (in the early stages only), and the allantoic stalk can be seen in cross 

 sections of the cord while the cord itself is filled with a mesenchymal, 

 mucous-like substance called Wharton's jelly. 



The cord is twisted and is attached to the umbilicus or navel of the 

 foetus and to the placenta. The outer covering of the umbilical cord is 

 a layer of ectoderm which is continuous with that of the amnion of the 

 embryo. 



The following table shows the relative increase in size and weight 

 of the human embryo and foetus throughout the period of gestation : 



Ovum (estimated) 



23 days 



66 days 



84 days 

 112 days 

 140 days 

 168 days 

 196 days 

 224 days 

 252 days 



Weight 

 0.000004 grm. 

 0.04 

 3.0 

 36.0 

 120.0 

 330.0 

 600.0 

 1000.0 

 1500.0 

 2200.0 



270 days 



280 days 3200.0 



C. H. 



2.5 mm. 



30.0 



98.0 

 180.0 

 250.0 

 315.0 

 371.0 

 425.0 

 470.0 

 500.0 



Length 



C. R. 



2.5 mm. 



25.0 



68.0 

 121.0 

 167.0 

 210.0 

 245.0 

 284.0 

 316.0 

 336.0 



