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 cavity. 



