o84 A Manual of Veterinary Physiology. 



mesoblast with the hypoblast is formed the splanchnopleure. 

 From the somatopleure is formed the skin, muscles, and 

 bloodvessels ; from the splanchnopleure is formed the 

 intestinal canal. 



The embryo now undergoes a change in shape, for at 

 each end an inflection or tucking in of the layers surround- 

 ing it occurs. These inflections are spoken of as the head 

 and tail fold. As this folding under the body occurs, the 

 embryo (ventral surface downwards) is raised higher and 

 higher, so that at last it rests at the future umbilicus on a 

 narrow stalk or pedicle, termed the vitelline duct, which 

 communicates with a sac outside the body of the embryo 

 termed the umbilical vesicle ; in the body of the embryo 

 the vitelline duct communicates with the primitive intes- 

 tinal canal. 



From the splanchnopleure is now developed the heart, 

 then the aorta, from which is given off several vessels 

 termed omphalo-mesenteric arteries, which form, by passing 

 through the vitelline duct, a vascular network within the 

 umbilical vesicle, and from here arises the omphalo- 

 mesenteric veins, which return to the body of the embryo 

 by the vitelline duct, and terminate in the heart. 



This is the primary embryonic circulation. The um- 

 bilical vesicle is furnishing the embryo with nutriment, but 

 this does not last for long, for when the placental circula- 

 tion is established, the umbilical vesicle has no longer any 

 function to perform, and it gradually disappears. 



We must now describe the formation of two sacs which 

 are found outside the body of the embryo, which at this 

 time are destined to perform important functions, and in 

 their arrangement are different from those occurring in the 

 human subject. 



The Amnion is formed from the somatopleure, growing 

 over the embryo from all sides, meeting, and then forming 

 a cavity termed the amniotic, which eventually contains 

 a quantity of fluid in which the embryo resides. The 

 amnion surrounds the duct of the umbilical vesicle, and 

 we may describe it as terminating at the umbilicus, where 



