DEVELOPMENT OF THE VENTRAL ABDOMINAL 
WALLS, IN: MAN! 
FRANKLIN P. MALL. 
AFTER the neural plate is formed within the amniotic cavity, 
the tail end of the embryo remains attached to the chorion by 
means of the allantoic stalk (Bauchstiel). The amnion surrounds 
the embryo only on its dorsal side, as has been shown recently 
by the study of a number of young human embryos. The ven- 
tral side of the embryo, however, hangs free in the coelomic 
cavity, as the body walls have not been formed by the exten- 
sion of the amnion. The extension of the amnion proceeds 
hand in hand with the flexion of the embryo. It first extends 
over the face of the embryo, then tucks under the tail end, and 
at the same time it encroaches from both sides of the body. In 
covering the head it first crosses the mouth and then the heart, 
this movement being accelerated by the growth of the amnion 
over the head end of the body from left to right.2 While this 
is taking place, the stem of the umbilical vesicle elongates to 
produce the general condition, as represented in embryos four 
weeks old. Soon, however, the amnion covers the whole 
allantoic stalk, including within its folds the stem of the 
umbilical vesicle, thus forming the true umbilical cord. At 
this stage the intestine begins to enter the cord. 
After we have reached the stage in which the umbilical cord 
is formed we find that in transverse sections of human embryos, 
as well as in embryos of other mammals of the same stage, the 
. ventral abdominal walls are composed wholly of a membrane of 
connective tissue, without any ribs, muscles, or blood vessels as 
they are found in the adult. 
1 The present paper is to be considered a continuation of two papers published 
recently; one upon the development of the human coelom (/ourn. of Morph., vol. 
xii), and the other upon the development of the human intestine (His’s Archiv, 
Supplement Band, 1897). 
2 Mall, Journ. of Morph., vol. xii, p. 430, Figs. 27, 28. 
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