74 MALL. [Vou. XIX. 
Amnion formed.—One of the earliest specimens of vesic- 
ular forms in which the amnion is present but the embryo is 
pretty well destroyed is No. 13. There is much magma 
reticulé in the ccelom. It is quite clear that the double vesicle 
represents the amnion and umbilical vesicle, which are bound 
together by a mass of mesoderm. This contains two blood- 
vessels, which unite at the point where the mesoderm passes 
over into the chorion. There are no other structures of the 
embryo present. The tissues have become dissociated, the 
mesoderm cells are round, and other round cells are in the 
cavity of the yolk sac. In general, we have the remnants of 
an embryo a little younger than Eternod’s with the blood- 
vessels reaching to the chorion. 
An excellent specimen, No. 304, which was cut into serial 
sections with the entire chorion and decidua, is most instruc- 
tive. The villi of the chorion are normal in shape and are 
covered with a very active syncytium. They contain remnants 
of blood-vessels within them, showing that at one time there 
was vascular connection between the yolk sac and the villi. 
The decidua is encircled and infiltrated with leucocytes and 
between the villi there is a mucoid mass rich in leucocytes, 
showing that the inflammatory process has reached the ovum. 
The ccelom is well filled with magma reticulé, in which there 
is imbedded the umbilical vesicle attached to the remnants of 
the embryo. This is partly covered with the amnion, which 
runs out into a stem, containing an allantois, the latter not 
connecting with the chorion. There are remnants of a 
nervous system and numerous blood islands in the yolk sac, 
but no heart. 
This specimen corresponds well with No. 13 and gives, in 
them, by about ten days for embryos less than 22 mm. long. For 
embryos from 22 to 33 mm. long I believe his estimations fairly accu- 
rate. To make the proper correction it would be necessary to recast 
all of my tables and much of the text. The reader may make them 
by adding ten days to the age of embryos less than 43 days old. My 
new data, about 1,000 in number, relating to the age of human embryos 
will be published in Keibel-Mall’s Handbuch der Entwicklungsgeschichte 
early in 1909. 
