374 



HUMAN UTERUS AND FETAL APPENDAGES. 



are still present but somewhat* modified (Fig. 255), while others have been partly 

 or wholly obliterated. As contrasted with the early stages, we find that the 

 ccelom is entirely obliterated, that the yolk-stalk has usually been completely 

 resorbed, and that only traces of the allantois can be seen, Y. The blood-vessels 

 have grown; there are two arteries, A, A', and a single vein, V. Around each of 

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

 rounding mesenchymal cells, which have assumed an elongated form and con- 



c.-,. //- '.0 WVJS' 



^ -\v , m..*"' 



FIG. 258. CONNECTIVE TISSUE FROM THE UMBILICAL CORD OF A HUMAN EMBRYO OF THREE MONTHS, STAINED 



WITH ALUM COCHINEAL AND EOSIN. X 511 diams. 



c,c, Cells. /, Fibrillae. 



tractile function. It will be remembered that the allantois in man is primitively 

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

 (compare Fig. 87). As the umbilical cord lengthens the allantois fails to lengthen 

 equally. During the second month it increases very little in diameter. After the 

 second month it appears in sections as a small group of epithelioid cells (Fig. 256) 

 with distinct walls, irregularly granular contents, and round 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. 



