54 EMBRYOLOGY 



and the hind-gut there exists for a time a wide opening into the yolk-sac, but the 

 latter is gradually reduced to a small pear-shaped sac (sometimes termed the 

 umbilical vesicle), and the channel of communication is at the same time narrowed 

 and elongated to form a tube called the vitelline duct. 



THE YOLK-SAC. 



The yolk-sac (Figs. 22 and 23) is situated on the ventral aspect of the embryo; 

 it is lined by entoderm, outside of which is a layer of mesoderm. It is filled with 

 fluid, the vitelline fluid, which possibly may be utilized for the nourishment of the 

 embryo during the earlier stages of its existence. Blood is conveyed to the wall of 

 the sac by the primitive aortse, and after circulating through a wide-meshed capil- 

 lary plexus, is returned by the vitelline veins to the tubular heart of the embryo. 

 This constitutes the vitelline circulation, and by means of it nutritive material is 

 absorbed from the yolk-sac and conveyed to the embryo. At the end of the fourth 

 week the yolk-sac presents the appearance of a small pear-shaped vesicle (umbilical 

 vesicle) opening into the digestive tube by a long narrow tube, the vitelline duct. 

 The vesicle can be seen in the after-birth as a small, somewhat oval-shaped body 



Heart 





Mandibular arch 

 Yolk-sac Maxillary process 



Ey 



Body-stalk Hind-limb 



FIG. 22. Human embryo of 2.6 mm. (His.) FIG. 23. Human embryo from thirty-one to thirty-four 



days. (His.) 



whose diameter varies from 1 mm. to 5 mm.; it is situated between the amnion 

 and the chorion and may lie on or at a varying distance from the placenta. As 

 a rule the duct undergoes complete obliteration during the seventh week, but 

 in about three per cent, of cases its proximal part persists as a diverticulum 

 from the small intestine, Meckel's diverticulum, which is situated about three or 

 four feet above the ileocolic junction, and may be attached by a fibrous cord to 

 the abdominal wall at the umbilicus. Sometimes a narrowing of the lumen of the 

 ileum is seen opposite the site of attachment of the duct. 



DEVELOPMENT OF THE FETAL MEMBRANES AND THE PLACENTA. 



The Allantois (Figs. 25 to 28). The allantois arises as a tubular diverticulum 

 of the posterior part of the yolk-sac; when the hind-gut is developed the allantois 

 is carried backward with it and then opens into the cloaca or terminal part of the 

 hind-gut: it grows out into the body-stalk, a mass of mesoderm which lies below 

 and around the tail end of the embryo. The diverticulum is lined by entoderm 

 and covered by mesoderm, and in the latter are carried the allantoic or umbilical 

 vessels. 



