lO 



BIOLOGY OF THE LABORATORY MOUSE 



two regions is still quite distinct (Fig. 8). Strictly speaking the trophecto- 

 derm is also extra-embryonic ectoderm, but as a matter of convenience the 

 term will be used only for the extra-embryonic ectoderm of the egg cylinder. 



At about 5 days a cleft or cavity, the proamniotic cavity, appears in the 

 embryonic ectoderm (Fig. 7). This is followed very shortly by the appear- 

 ance of a similar cleft in the extra-embryonic ectoderm, and by the fusion of 

 these two, so that by 5}^ days the egg cylinder contains a narrow lumen 

 (Fig. 8). 



The ectoplacental cone. — Beginning at 5 or 5^^ days, active growth at 

 the dorsal end of the extra-embryonic ectoderm gives rise to a new structure. 



Secondary giant cell 

 -Extra-embryonic ectoderm 

 Distal entoderm 

 Proximal entoderm 

 Proamniotic cavity 

 Embryonic ectoderm 

 Primary giant cell 

 Yolk cavity 



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Fig. 7. — ^Longitudinal section of early egg cylinder. Age unknown, but probably 

 about 5 or 6 days. Projection drawing (X400). 



the ectoplacental cone, which joins the egg cylinder ventrally, and extends 

 dorsally towards the lumen of the uterus (Fig . 8) . This develops rapidly , its 

 cells showing numerous mitoses, and by 6I2 days it composes almost one- 

 half of the total length of the embryo. Its structure, particularly at the 

 upper extremity, is porous, and the interstices between the strands of cells 

 that compose it soon become infiltrated with maternal blood (Fig. 10). In 

 later stages it becomes part of the placenta. 



The inversion of the germ layers.— At sH days (Fig. 8) the egg cylinder 

 is a structure consisting of a double wall enclosing a narrow lumen. The 

 inner layer of the double wall is composed of ectoderm, the outer of ento- 

 derm. This relation of ectoderm and entoderm, found in the mouse, rat, 



