THE UMBILICAL CORD. 



353 



each of these is a well-developed muscular coat produced by differentiation of the 

 surrounding mesenchymal cells, which have assumed an elongated form and 

 contractile function. It will be remembered that the allantois in man is primi- 

 tively a very narrow tubular diverticulum which extends originally nearly to the 

 chorion (compare Fig. 71). As the umbilical cord lengthens the allantois fails 

 to lengthen equally. During the second month it increases very little in diam- 

 eter. After the second month it appears in sections as a small group of epithe- 

 lioid cells (Fig. 212) with distinct walls, irregularly granular contents, and round 



FIG. 214. CONNECTIVE TISSUE FROM THE UMBILICAL CORD OF A HUMAN EMBRYO OF THREE MONTHS, 



STAINED WITH ALUM COCHINEAL AND EOSIN. X 5 11 diams. 



f,f, Cells. /, Fibrillse. 



nuclei ; the group may or may not show a remnant of the original central cavity. 

 Around the cells, ent, there is a slight condensation of the connective tissue, mes, 

 to form, as it were, an envelope. 



The mesoderm varies in appearance according to the age of the specimen. 

 Its growth and differentiation are rapid. During the second month it consists 

 merely of numerous cells (Fig. 213) imbedded in a clear substance. The cells 

 23 



