552 EMBRYOLOGY. 



Mammals were originally provided with abundant yolk-material, like 

 those of Reptiles (compare p. 222), and, secondly, that the blasto- 

 clermic vesicle, which arises after the process of cleavage, becomes 

 greatly distended by the accumulation within it of a fluid very rich 

 in albumen, furnished by the walls of the uterus. Out of this vesicle 

 likewise the vitelline blood-vessels undoubtedly take up nutritive 

 material and convey it to the embryo, until a more ample nutrition 

 is provided by means of the placenta. 



In addition to the vitelline blood-vessels there arises in the higher 

 Vertebrates a second system of vessels, which is distributed in the 

 foetal membranes outside the embryo and for a time is more developed 

 than the remaining vessels of the embryo. It serves for the 

 allantoic circulation of Birds and Reptiles and the placental circu- 

 lation of Mammals. 



When in the Chick the allantois (PI. I., fig. 5 al) is evaginated 

 from the front [ventral] wall of the hind-gut, and as an ever 

 increasing sac soon grows out of the body-cavity through the dermal 

 umbilicus into the ccelom of the blastodermic vesicle between the 

 serosa and the yolk-sac, there appear in its walls two blood-vessels, 

 which grow forth from the ends of the two primitive aortas the 

 umbilical vessels, or arteries umbilicales. The blood is again collected 

 from the fine capillary network, into which these vessels have been 

 resolved, into the two umbilical veins (vense umbilicales), which, 

 after having arrived at the navel, pass on to the two Cuvierian 

 ducts (see p. 577) and pour their blood into these near the entrance 

 of the latter into the sinus venosus. The terminal part of the 

 right vein soon atrophies, whereas the left receives the lateral 

 branches of the right side and is correspondingly developed into a 

 larger trunk. This now also loses its original connection with the 

 ductus Cuvieri, since it effects with the left hepatic vein (vena 

 hepatica revehens) an anastomosis, which continually becomes larger 

 and finally carries the whole stream of blood. Together with the left 

 hepatic vein the left umbilical vein then empties directly into the 

 sinus venosus at the posterior margin of the liver (HOCHSTETTER). 



The umbilical and vitelline veins undergo opposite changes in 

 calibre during development : while the vitelline circulation is well 

 developed, the umbilical veins are inconspicuous stems ; afterwards, 

 however, with the increase in the size of the allantois they enlarge, 

 whereas the venae omphalomesentericze undergo degeneration and in 

 the same proportion as the yolk-sac by the absorption of the yolk 

 becomes smaller and loses in significance. 



