No. 3. ] UTERUS AND EMBRYO. 381 
gibility. The foetal skin is characterized by the ectoderm be- 
coming many layered, while the cutis remains for a long time 
undifferentiated from the mesoderm below, and the matrix is of 
low refrangibility. In comparing the ectoderm of the umbilical 
cord with the skin, therefore, we do not expect to find any differ- 
entiated cutis. The epithelium of the cord is at first, of course, 
single layered, the condition which is permanent over the am- 
nion. In the cord of a three-months embryo, Cut 3, I find the 
two-layer stage. The outer layer is granular, and in some parts 
each cell protrudes like a dome. Dome cells also appear on the 
young epidermis, and as I learn from Dr. J. T. Bowen, who has 
Cut 3. — Epithelial covering of the umbilical cord of an embryo of three months. 
X 545 diams. 
been investigating the subject in my laboratory, are probably 
the precursors of the epitrichium. The cells of the inner layer 
are larger and clearer than those outside. By the fifth month 
the epithelium is distinctly stratified, and the superficial layers 
consist of flattened cells similar to those of the horny layer of 
the skin at an early stage. The ectoderm of the cord agrees 
therefore entirely with that of the embryo proper in its general 
development, but the differentiation proceeds more slowly, so 
that at any given age the ectoderm of the cord is at a less 
advanced stage than that of the embryo. 
The appearance of the cord in cross-sections is instructive. 
Cut 4, A, is a section through a cord of sixty days; the right 
umbilical vein is already aborted; the caelom, coe, is a large 
cavity, and contains the yolk stalk, Y.S, with its two vessels, 
