246 Professor Clark on a Case 



attracted by the larger ones, and when they come into their 

 neighbourhood coalesce with them. Hence the veins which 

 accompany the subdivisions of the anterior aorta, and the in- 

 tercostal arteries are guided by these vessels directly to the 

 venous sac of the heart, whilst the same sac of the heart is 

 the centre of the veins of the abdomen and lower parts of the 

 body. The difference is that these two series of veins attain the 

 heart most directly, the one by following the course of the ar- 

 teries, the other by diverging in some degree from that course. 



But to return to the membranes of the ovum, which hitherto 

 we have described as an external exochorion, and an internal 

 serous amnion. 



The allanto'is is a production from the posterior part of the 

 bowel at its lower extremity. It proceeds from the cloaca, (for 

 as yet there is no urinary bladder,) as a thin transparent whitish 

 vesicle, through the unclosed abdomen at the part nearest to 

 the tail, whilst the umbilical vesicle passes out nearer to the 

 head. It consists of two layers of the blastoderma, the mucous 

 and vascular layers. Its existence is later than that of the liver, 

 but earlier than the production of the arteries which form the 

 umbilical cord. It is observed in the human embryo in the 

 third or fourth week, but here it does not attain any great size, 

 and disappears in the sixth : whilst in many other mammalia, 

 as also in birds, it is reflected over the whole amnion, which with 

 the embryo it includes as a sac doubled on itself, the sac being 

 somewhat distended with fluid. When the urinary organs are 

 developed, the bladder is formed out of that portion of the allan- 

 to'is within the abdomen which is connected with the intestine. 

 This at first does not increase in size with the rest of the sac, 

 but appears rather as a duct connecting the bowel and the ex- 



