of Human Monstrosity. 247 



panded outer part of the allantois. As it expands, the terminating 

 branches of the aorta, follow the vascular external layer, and ran 

 upon its sides, and so pass out of the abdomen. In birds, and 

 the higher amphibia, the umbilical arteries ramify most minutely 

 on the external vascular layer, particularly on that portion of the 

 doubled bladder which is next the exochorion. In man the 

 allantois external to the abdomen soon dwindles to a capillary 

 tube, its original vascular layer still continuing as a conductor 

 of the umbilical arteries. In the other mammalia the allantois 

 persists throughout fetal life, and receives numerous branches 

 from the umbilical arteries. The veins which return the blood 

 are collected into one stem which accompanies the arteries as 

 far as the navel. There the two orders of vessels separate. The 

 veins pass as one large trunk to the general venous center, the 

 ascending cava, below the heart. This is the principal stem : 

 subordinate branches are given to the liver as it passes between 

 its great lobes, and a branch of communication to the Vena Porta?. 

 In man the umbilical arteries are immediately surrounded by 

 the vascular membrane which originally belonged to the allantois. 

 When the cord reaches the placenta, now beginning to be formed, 

 that membrane spreads itself over the whole inner surface of the 

 exochorion, and forms its vascular portion. Thus is the chorion 

 formed ; it consists of an external villous exochorion, and an inter- 

 nal vascular endochorion. In the case of twins the two embryos 

 have usually a common exochorion, with the other membranes 

 proper to each. This is an instance of the law which has been 

 well elucidated by M. Serres, viz. that in the primitive state of 

 embryos when two homogeneous organs are brought in contact, 

 they coalesce and form one. If the amnion be also common to the 

 two, the case, though not uncommon, is pathological, or monstrous. 



