912 



CARE AND NOURISHMENT OF THE DEVELOPING YOUNG 



SINUSOIDS LACUNAE IN TROPHOBLAST ALLANTOIC DIVERTICULUM 



\yTROPHOBLAST_ \ ^^V^^rX'l^ BODY OF EMB 



Fig. 372. Extra-embryonic membranes in human embryo. (A) Diagrammatic rep- 

 resentation of extra-embryonic membranes in embryo of about 12 days of age, shortly 

 after enclosure within uterine endometrium. (Redrawn and modified from Hertig and 

 Rock, 1941. Carnegie Contr. to Embryology, vol. 29.) (B) Extra-embryonic membranes 

 in embryo of about 16 days. (C) Extra-embryonic membranes in embryo of about 28 

 days. (D) Extra-embryonic membranes in embryo of about 12 weeks. 



wall of the embryo and the yolk and allantoic stalk tissues is a passageway 

 for blood vessels to and from the yolk and allantoic stalk tissues. It is called 

 the umbilical ring, umbilicus or omphalos. As the embryo continues to enlarge, 

 the amnion in the mid-ventral area of the embryo is reflected downward from 

 the umbilical ring or umbilicus over the yolk-stalk and allantoic-stalk tissues 

 and thus eventually encloses the yolk and allantoic stalks (figs. 370E; 372C 

 and D). This entire structural complex composed of amnionic tissue, together 

 with enteric and allantoic diverticula and splanchnopleuric mesoderm, is called 

 the umbilical cord (fig. 372D). 



In the human embryo, that portion of the mesoderm which connects the 



