104 



OBSTETRICAL PHYSIOLOG Y 



larger herbivorous quadrupeds, the Hmbs of the new-born animal are 

 long to enable it to reach the teat, as well as to enable it to escape by 

 flight should danger be apprehended. 



The Circulatory System. 

 The development of the circulatory system is not apparent until some 

 days after the appearance of the embryo in the blastodermic layer, there 

 being no trace of vessels in the germinative space. It is not long, how- 



m 71 



Fig. 57. 



FcETAL Circulation in a Transition State. 



a, b, c', Circle or Envelope resulting from the Fusion of the Vitelline Membrane, 

 External Layer of the Blastodermic Vesicle, and the Transformation of the 

 Allantois ; c, The Diminishing Umbilical Vesicle ; d. Cephalic Portion of the 

 Embryo ; d', Caudal Portion ; e, Ventricle of the Heart ; /, Auricle of the 

 Heart ; i, Aorta, forming the Aortic Arches ; h, Trunk representing the 

 Thoracic Aorta ; g, Vessel which becomes the Anterior Vena Cava ; k, Vena 

 Azygos ; I, Confluents of the two Veins g and k ; m, Confluent of all the 

 Veins at their Entrance into the Auricle of the Heart ; n, Vessel resulting 

 from the Union of the Allantoid Veins p, p, and the Omphalo-Meseraic 

 Vein q ; o, Posterior Vena Cava ; p, p, Allantoid Veins ; q, Omphalo-Meseraic 

 or Umbilical Vesicle Vein ; r. Posterior or Abdominal Aorta ; s, s, Allantoid 

 Arteries ; t, Omphalo-Meseraic Artery. 



ever, before the central organ of circulation and some vessels appear in 

 the middle layer, and canals also spread to the surface of the umbilical 

 vesicle. While the contents of the vitelline vesicle are being imbibed 

 by the embryo, the heart is in course of formation ; bloodvessels are 

 increasing and extending, the allantois is completed, and, finally, the 

 placental circulation is established and continues until birth. 



The pleuro-peritoneal cavity in the embryo shows a diverticulum in 



