396 H. H. Newman and J. T. Patterson. 



Even after the embryos have reached a length of 4 cm. the am- 

 niotic sacs are still quite free from one another, but a little later 

 they begin to fuse along contiguous surfaces. Not until about a 

 month before birth however do they become inseparably bound 

 together. After the fusion is complete the amnia occupy the entire 

 cavity of the vesicle and divide it into (normally) four quadrants 

 of equal size, each running from pole to pole. This nearly defini- 

 tive condition was described in detail in our preliminary account 

 and needs no further attention here. In fig. 46 the edges of the 

 amniotic partitions separating adjacent embryos may be seen at 

 "a." The umbilical cords are always attached just to the left of 

 the partitions. 



VII. HISTORY OF THE ALLANTOIS AND THE UMBILICUS 



The early history of the allantois was shown to be very inti- 

 mately bound up with that of the belly-stalk or primitive umbilicus 

 This intimate connection persists as long as the allantois retains 

 a distinguishable structure. In stages of the degree of advance- 

 ment shown in vesicle 17 and 11 (figs. 1 and 44) the entodermal 

 allantois is seen as a slender cord of cells more or less closely fused 

 with the umbilicus and showing here and there traces of a former 

 lumen. The outlines of the mesodermal allantois, however, are 

 no longer distinguishable from the tissues of the belly-stalk. The 

 allantois of the armadillos seems then to be entirely vestigeal 

 in later stages of development. 



The umbilicus arises directly from the primitive belly-stalk, 

 which was shown in the description of vesicles 10 and 18 to con- 

 sist of paired flat bands of mesoderm uniting the posterior end 

 of the embryo to the margin of the Trager or primitive placenta. 

 That the mesodermal allantois contributes some tissue to the defi- 

 nitive umbilicus has already been intimated, but at no time do 

 allantoic blood vessels function. The placental circulation is 

 carried on exclusively by the umbilical vessels, paired arteries and 

 veins. Each artery arises along the inner margin of a belly-stalk 

 band, while each vein forms in the scroll-like outer margin. In 

 later stages the two bands fuse at a short distance from the vesicle 



