THE BELLY STALK. 



113 



Ann iotic fold^ 



The Belly Stalk. During the formation of the amnion the anterior end of 

 the embryo sinks down into the blastodermic vesicle much more rapidly than the 

 posterior end, the latter, in 

 fact, remaining attached to 

 the surface of the blastoderm 

 (Fig. 87. A). As the forma- 

 tion of the amnion proceeds 

 the embryo becomes separated 

 more and more from the sur- 

 face of the vesicle, eventually 

 being united with it only by 

 a short stalk arising from its 

 ventral surface (Fig. 87, B). 

 This is the "belly stalk," in 

 the interior of which is to be 

 found the umbilical vesicle, 

 which has been carried back- 

 ward by the constriction 

 which produced the stalk, and 

 has been reduced to a small 

 pyriform vesicle supported 

 upon a long pedicle. This 

 pedicle is connected with the 

 digestive tract of the embryo, 

 and behind its attachment a 

 small outgrowth develops from 

 the ventral wall of the intes- 

 tine, and. pushing in front of 

 it the splanchnopleure which 

 forms the outer lining of the 

 intestine, extends out into the 

 belly stalk and forms what is 

 known as the aUantois. In 

 some animals the allantois is 

 a hollow projection and is 

 usuallv styled the aUant<jii.t 



* * 



h : but in most mam- 



Amniotic fold 



AUantois 



3 



FIG. 87. Diagrams showing the formation of the belly stalk. The 

 heavy black line represents the embryonic portion of the epiblast, 

 the dotted portions and broken lines the mesoblast, and the inner 



mals. and especially in man, continuous line the hypobiast 

 the external or mesoblastic 



element undergoes great development, while the internal or hypoblastic element 

 undergoes little increase beyond the body of the embryo, so that it is very 

 doubtful whether any cavity exists in the allantois beyond the limits of the 

 umbilicus. A portion of the allantoic vesicle icithin the body cavity is eventu- 

 ally destined to form the bladder, while the remainder forms an impervious cord, 

 the uraeJtits. stretching from the summit of the bladder to the umbilicus. The 

 belly stalk is at first hollow, its cavity being continuous with the pleuro-peritoneal 

 cavity of the embryo (Fig. 87), but it soon becomes solid by the extensive growth 

 of the mesoblastic tissue which it contains. Over that portion of the wall of the 

 blastodermic vesicle with which the outer end of the belly stalk is connected the 

 chorionic villi, already referred to, reach their greatest development, this being 

 the region of the placenta. 1 In the walls of the allantois vessels are formed 

 which extend their branches out into the surrounding mesoblast and into the 

 chorionic villi. The allantois, accordingly, though much reduced in man in 

 comparison with the lower mammals, is still the tract along which the vessels 

 extend which convey the blood of the embryo to the foetal chorion, where it is 



1 In some animals some of the vessels of the villi of the ohorion are derived from the yolk-sac 

 that is from the omphalo-mesenteric vessels. 



