362 MALL. [Vou. XIV. 
the whole membrana reuniens filled with a minute plexus of 
veins which radiate from the myotomes towards the umbilical 
cord, while His describes this same region in the human embryo 
as filled with branches of the umbilical vein which empty into 
the sinus reuniens above, and into the umbilical vein below. 
According to His’s description they arise when the communica- 
tion between the umbilical veins and the sinus reuniens is 
severed. Although the picture given by Kolliker does not 
correspond with His’s description it does not contradict it, nor 
is it peculiar to the cow’s embryo. I have in my collection a 
well-preserved human embryo (No. LXXVI), in which the 
membrana reuniens is filled with a plexus of veins much like 
that in the cow’s embryo. The specimen was taken from the 
uterus seven hours after the death of the woman, and without 
opening the ovum was hardened in absolute alcohol. All the 
vessels down to the capillaries are filled with blood, thus making 
it an excellent specimen for the study of the blood vessels. It 
seems to represent a stage somewhat more advanced than the 
one pictured by Kolliker, as the plexus of veins does not cover 
the whole membrana reunieus. The ventral wall of the heart 
near the liver contains no vessels, while the membrana reuniens 
covering the upper end of the heart is filled with a plexus of 
vessels which communicate with the capillaries of the mandibu- 
lar arch. There is an extensive plexus through the arm and 
lateral body walls which extends through the membrana 
reuniens covering the liver, and finally encircles the cord and 
communicates with the umbilical veins. 
The specimen just described must be about 22 days old and, 
although I have six other good embryos between 14 and 28 
days old, I find no such plexus in the membrana reuniens, 
although in all but one of them (14 days) the arm shows a rich 
plexus of capillaries filled with blood. In stages older than 
four weeks I find no blood vessels in the membrana reuniens 
with the exception of that portion encircling the cord where 
there is a rich network of veins. Although I have a number 
of excellent specimens of five and six weeks, the membrana 
reuniens over the heart and liver contains no blood vessels 
until it is invaded by the ventral plate, which is accompanied 
