usually seen at this level between it and the parietal
layer. Some of the viscera, such as the kidneys and pancreas, are
retro-peritoneal; others, such as the small intestines and transverse
colon, are surrounded, except at one point where they are attached to
the dorsal wall by a _mesentery_ or _mesocolon_ as the reflections are
called; others again are completely surrounded, and of these the caecum
is an example; while some, like the liver and bladder, have large
uncovered areas, and the reflections of the membrane form ligaments
which allow considerable freedom of movement.

The _tunica vaginalis_ is the remains of a process of the peritoneum
(_processus vaginalis_) which descends into the scrotum during foetal
life some little time before the testis itself descends. After the
descent of the testis the upper part usually becomes obliterated, while
the lower part forms a serous sac which nearly surrounds the testis, but
does not quite do so. Posteriorly the epididymis is in close contact
with the testis, and here the visceral layer is not in contact; there
is, however, a pocket called the _digital fossa_ which squeezes in from
the outer side between the testis and epididymis. The parietal layer
lines the inner wall of its own side of the scrotum.

For a full description of the topography of the serous membranes see
any of the standard text-books of anatomy, by Gray, Quain, Cunningham
or Macalister. Special details will be found in Sir F. Treves'
_Anatomy of the Intestinal Canal and Peritoneum_ (London, 1885); C. B.
Lockwood, _Hunterian Lectures on Hernia_ (London, 1889); C. Addison,
"Topographical Anatomy of the Abdominal Viscera in Man," _Jour.
Anat._, vols. 34, 35; F. Dixon and A. Birmingham, "Peritoneum of the
Pelvic Cavity," _Jour. Anat._ vol. 34, p. 127; W. Waldeyer, "Das
Becken" (1899), and "Topographical Sketch of the Lateral Wall of the
Pelvic Cavity," _Jour. Anat._ vol. 32; B. Moynihan, _Retroperitoneal
Hernia_ (London, 1899). A complete bibliography of the subject up to
1895 will be found in _Quain's Anatomy_, vol. 3, part 4, p. 69.

[Illustration: After Young and Robinson, Cunningham's _Text-Book of
Anatomy_.

FIG. 3.--Diagram of Longitudinal Section, showing the different areas of
the Blastodermic Vesicle.

_a_, Pericardium. _e_, Placental area.
_b_, Bucco-pharyngeal area. _d_, Entoderm.
_c_, Ectoderm.]

_Embryology._--As the mesoderm is gradually s