. Biology of the laboratory mouse. Mice as laboratory animals; Mice; Animals, Laboratory; Mice. i6 BIOLOGY OF THE LABORATORY MOUSE mesometrium (Fig. 4A). In terms of an older embryo, the ectoplaceiital cone is up and the embryonic portion of the egg cylinder is down (Fig. 4B). The dorso-ventral axis of the embryo is thus parallel to the mesometrium and perpendicular to the long axis of the uterus (Fig. 11). The anterior- posterior axis of the embryo likewise has a definite orientation with respect to the uterus, being, as a rule, perpendicular to the mesometrium. Depar- Mesometriuiti Uterus. E
. Biology of the laboratory mouse. Mice as laboratory animals; Mice; Animals, Laboratory; Mice. i6 BIOLOGY OF THE LABORATORY MOUSE mesometrium (Fig. 4A). In terms of an older embryo, the ectoplaceiital cone is up and the embryonic portion of the egg cylinder is down (Fig. 4B). The dorso-ventral axis of the embryo is thus parallel to the mesometrium and perpendicular to the long axis of the uterus (Fig. 11). The anterior- posterior axis of the embryo likewise has a definite orientation with respect to the uterus, being, as a rule, perpendicular to the mesometrium. Depar- Mesometriuiti Uterus. Embryo Fig. II.—Diagram showing the onentation of an 8 day embryo in the uterus, and of the planes in which sections are cut. Plane A: transverse to uterus, sagittal to embryo. In the early egg cylinder stages this may be referred to also as a longi- tudinal section of the egg cylinder. The orientation of the embryo is not always consistent and may sometimes depart by as much as 45° from this plane. Plane B: transverse section of embryo. Note, however, that in embryos past the egg cylinder stage this plane though transverse to head and tail regions is frontal with respect to the mid-trunk region. Plane C: frontal section of embryo. Note, however, that in embryos past the egg cylinder stage this plane though frontal to head and tail regions is transverse with respect to the mid-trunk region. In early egg cylinder stages this may be referred to also as a longitudinal section of the egg cylinder. tures from this orientation by as much as 45° may, however, occur. This orientation persists until about 8 or 8^ 2 days when the embryo begins to shift its position in the uterus. Amnion, chorion and exocoelom.—When mesoderm cells first appear between the ectoderm and entoderm at the posterior margin of the egg cylinder, they cause the ectoderm at the line of junction between its embry- onic and extra-embryonic portions to bulge into the proamniotic Please note that these im
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