THE EARLY EMBRYOLOGY OF THE MOUSE 15 



an embryo of the thirtecn-striped ground squirrel. This reduction in total 

 volume involves little if any reduction in the volume of the embrvonic area 

 proper. 



The primitive streak and mesoderm formation. — At 6} 2 days the middle 

 germ layer or mesoderm makes its appearance (Fig. 10). The first meso- 

 derm cells are budded off from a narrow strip of embryonic ectoderm which 



r . . . ' 



extends dorso-ventrally from the line of junction of the embryonic and extra- 

 embryonic ectoderm about half way to the tip of the egg cylinder. This 

 J stfip of ectoderm is known as the primitive streak. Since the primitive 

 streak lies at the posterior end of the embryo, an anterior-posterior axis is at 

 once established with its appearance.* 



The mesoderm cells form a loose tissue of very characteristic appearance. 

 They multiply rapidly, wedging their way laterally between ectoderm and 

 entoderm toward the anterior margin of the egg cylinder (Fig. 14A). The 

 forward growth is particularly rapid along the line which marks the junction 

 between embryonic and extra-embryonic ectoderm, and in this line meso- 

 derm may be found at the anterior margin of the egg cylinder about 12 hours 

 after the first mesoderm cells appeared (Fig. 12) . Elsewhere the two lateral 

 wings of mesoderm do not penetrate to the mid-sagittal region until much 

 later. Some mesoderm cells also push dorsally between the extra-embryonic 

 ectoderm and the adjacent entoderm, thus leaving the region of the embryo 

 proper. These mesoderm cells, for the most part, are destined to take part 

 in the formation of the yolk-sac, an extra-embryonic membrane, which later 

 envelops the embryo and which is discarded at birth. 



The orientation of the embryo in the uterus. — Since the primitive streak 

 is at the posterior margin of the egg cylinder, its formation, heralded by the 

 appearance of the mesoderm, establishes an anterior-posterior axis in the 

 embryo. It is appropriate at this point to consider how this axis and 

 the other axes of the embryo are oriented in relation to the uterus. 



At the time of implantation the embryo settles to the ventral or anti- 

 mesometrial side of the uterus. When it first implants, the inner cell mass 

 is up or towards the mesometrium, the blastocoele is down or away from the 



* In our material we have noted that from 5 to $14 days, the egg cylinder and more 

 particularly the proamniotic cavity instead of being round in cross section, are slightly 

 flattened along an axis perpendicular to the mesometrium. This is the same as the 

 future anterior-posterior axis. However, it cannot be determined until the appearance 

 of the mesoderm which end of the axis is anterior and which end posterior. With the 

 appearance of the mesoderm the flattening of the egg cylinder, if any, is along the 

 opposite axis. 



