No. 3.] UTERUS AND EMBRVO. 383 
ity is the duct of the allantois, A//, lined by entodermal epithe- 
lium ; and in this region are situated the two arteries and single 
vein ; the section is bounded by ectoderm.! The further devel- 
opment of the cord depends upon three factors: 1°, the growth 
of the connective tissue and blood-vessels; 2°, the abortion of 
the ccelom, yolk stalk, and allantoic duct in the order named ; 3°, 
differentiation of the connective tissue and of the ectoderm. 
Cut6.— Connective tissue of the umbilical cord of a human embryo of about 
three months, X 511 diams. Stained with alum-cochineal, and eosine. 
The growth and differentiation of the mesoderm proceeds rap- 
idly, encroaching upon the ccelom, which is obliterated (early in 
the fourth month). At first the connective tissue, Cut 5, is ¢om- 
posed merely of numerous cells embedded in a clear substance ; 
the cells form a complex network, of which the filaments and 
meshes are extremely variable in size; the nuclei are oval, gran- 
ular, and do not have always accumulations of protoplasm about 
1 The ectoderm is often wanting, owing to its frequent destruction post mortem. 
