THE HUMAN FCETAL MEMBRANES. 37 



In the next diagram, (Fig. 43), (T, the extra-embryonic coelom has invaded the 

 sheet of mesoblast above the amniotic cavity to such an extent that the chorion is 

 completely separated from the amnion and the body of the embryo except at one 

 point, the posterior end of the body, where a solid stalk of mesoblast connects 

 the chorion and embryo. This solid band of mesoblast is called the body-stalk. 

 It represents, therefore, a primary and permanent connection between the chorion 

 and the body of the embryo. A small diverticulum from the entoblastic sac growing 

 into the mesoblast of the body-stalk marks the beginning of the allantois. As the 

 diagram shows, the amnion is at first a comparatively small membrane overlying the 

 embryonic area. The ectoblast of the amnion is on the inner side facing the embryo, 

 the mesoblast on the outer side. In the chorion these layers are placed inversely, 

 the mesoblast on the inner side, the ectoblast (trophoblast) outside. The space 

 between amnion and chorion is seen to be a continuation of the extra-embryonic 

 coelom. 



In diagram Z?, the amnion has become considerably expanded in association 

 with the growth of the body of the embryo and the accumulation of amniotic fluid. 

 A constriction in the entoblastic sac has made its appearance, a constriction which 

 separates the gut of the embryonic body from its appendage, the yolk-sac, the 

 narrower connecting piece being known as the yo/k-stalk, or sometimes as the 

 vitello-intcstinal diid. This constricted area is brought about by the rapid growth of 

 the body of the embryo. In the early condition the entoblastic sac is attached to 

 the embryonic body practically along its entire ventral surface. The body region 

 grows very rapidly, particularly the head end, which comes to project from the 

 entoblastic sac to a marked extent ; the tail end also projects somewhat. There is a 

 corresponding growth of the gut within the body of the embryo. As a consequence 

 of this process of expansion of the body, the area of attachment of the entoblast 

 external to the body becomes relatively much reduced in size, occupying only a 

 small portion of the ventral surface of the body, and a progressively smaller portion 

 as the body increases in bulk. In other words, the narrow area of the yolk-stalk 

 makes its appearance. 



In the diagram {D, a/') the allantois projects from the posterior end of the 

 embryonic gut into the body-stalk. It will be noticed that the human allantois is 

 never a free structure as it is in many of the lower types, where it grows from the 

 body freely into the extra-embryonic coelom and only later becomes connected with 

 the chorion to form the placenta, but that in man it grows directly into the body- 

 stalk, where, outside of the body of the embryo, it is an insignificant structure. 

 Inside the body, part of the allantois persists as the bladder. The urachus, a fibrous 

 cord which in the adult passes from the top of the bladder to the umbilicus, is also a ' 

 remnant of the allantois. The thick irregular projections of the trophoblast have 

 received a core of mesoblast tissue, so forming the early chorionic villi. These villi, 

 at the point of attachment of the body-stalk, the area where the placenta is 

 developing, are increasing in size, while the villi over the remainder of the chorion 

 are diminishing in size. 



In diagram £, the amnion has become greatly expanded. It lies closer to the 

 inner surface of the chorion. In close association wath this expansion of the amnion, 

 and the accompanying growth of the body of the embryo, the structures which form 

 the umbilical cord are so closely approximated that the area of the cord is clearly 

 defined. These structures are the body-stalk containing the allantois and allantoic 

 vessels, the yolk-stalk, and, bounding the other side of this area, the fold of the 

 amnion from beneath the head. At first the body-stalk projects' from beneath 

 the extreme posterior end of the body of the embryo, but as growth in this part of 

 the body advances and the tail projects more and more, the body-stalk is brought to 

 the ventral surface of the abdominal region in close proximity to the yolk-stalk. 

 The allantoic blood-vessels grow from the embryo through the body-stalk to the 

 chorion, where they ramify in the chorionic villi. At first there is an extension of 

 the coelom about the yolk-stalk in the umbilical cord, but the mesoblast tissues of the 

 structures of the cord soon fuse together, obliterating this cavity. The area of attach- 

 rnent to the abdomen of the umbilical cord becomes relatively very much reduced in 

 size and is known in the adult, after the separation of the cord, as the umbiliczis 

 or navel. 



