338 Dr Duckworth, A critical description of three cases 
Dr Ballantyne’s paper deals with the anatomy of a symelian 
monster. The arrangement of the abdominal arteries found 
therein very closely resembles that described above in specimen F. 
Dr Ballantyne rightly finding a difficulty in regarding the single 
umbilical artery of his specimen as a true hypogastric or allantoic 
derivative, put forward the view that in such cases the artery is 
of “ vitelline ” (or what I have termed segmental splanchnic) 
origin. I believe that I have shewn good reason for regarding 
such a view as only partly correct, and that the artery in question 
owes its origin to the dilation of an anastomotic channel originally 
connecting a segmental splanchnic vessel with a branch from the 
hypogastric offshoot of the primitive posterior ventral aorta (one 
of the piers of the secondary caudal arch). 
A third category of the anomaly (viz. the single umbilical 
artery) is known. Herein the hypogastric arteries arise as usual, 
but converge as they pass over the bladder so as to meet and fuse 
just before they reach the umbilicus. This class is not as- 
sociated with the grave malformations described in connexion 
with the preceding examples. Bardeleben (Anatomischer An- 
zeiger, Band x, s. 725) has described several instances in dis- 
secting-room subjects. 
In drawing up the foregoing account, I have not overlooked 
the writings of Hochstetter ( Morphologisches Jahrbuch, Band xvi, 
1890, and Hertwig’s Handbucli, Band vi, s. 108), and of Tandler 
( Anatomische Hefte, xxm, 1903), but I have found no reason 
for enlarging on the special points dealt with by those authors. 
A few remarks may be added concerning the placenta. This 
has been described in comparatively few cases of symelian mon- 
strosities, in which the single umbilical artery (presumably of 
the second and more complicated type) is known to be of 
frequent occurrence. Dr Ballantyne seems to have expected 
that the placenta would present abnormal features, although he 
recognised the possibility of the functional replacement of the 
normal umbilical arteries, by vessels of different morphological 
value. If my suggestion as to the nature of the single um- 
bilical artery of symelian monsters is satisfactory, there seems 
no special reason for expecting the placenta to be structurally 
abnormal. 
Dr Ballant}me noted a very remarkable and primitive condition 
of the abdominal veins in the symelian specimen described in 
his paper. The ductus venosus was not developed, for the 
umbilical vein terminated by breaking up into capillary offshoots 
in the liver. In my three specimens in which the umbilical 
artery is single, no such anomalous condition of the umbilical 
vein was discovered, for in each case the ductus venosus was 
formed. 
