THE EARLY EMBRYOLOGY OF THE MOUSE g 



elongated nuclei, and a ventral, more lightly staining portion with round 

 nuclei* (Fig. 6). The former gives rise to various extra-embryonic struc- 



fa ^. 



t% r-3 uterine lumen 



Droplets of 

 secretion 



Extra-embryonic 

 ectoderm 



Embryonic 

 ectoderm 



Proximal 

 entoderm 



Distal entoderm 

 Trophectoderm 

 Yolk cavity 

 — Giant cell 



Degenerating 

 uterine 

 epithelium 



% 



(^W ^^' 



^. Soh*.n 



Fig. 6. — ^Longitudinal section of early egg cylinder stage at 4 days 15 hours after 

 mating. Projection drawing (X400). 



tures and is, therefore, called the extra-embryonic ectoderm; the latter 

 gives rise to the ectoderm of the embryo proper and is, therefore, called 

 the embryonic ectoderm. While the difference in staining reaction and in the 

 shape of the nuclei has disappeared by 5 3-^ days, the division between the 



* It is possible that the division between embryonic and extra-embryonic ectoderm 

 can be traced back to stages earlier than \V2 days. One author (41) contends that 

 the ectoderm of the inner cell mass at a stage corresponding to that shown in Fig. 5 is 

 divided into two regions, a lighter staining outer layer continuous with the trophecto- 

 derm and a darker staining area between this and the entoderm, but the existence of 

 such a division has also been denied {22, 61). In our preparations at the 4)^4 day 

 stage we find occasional llattened, dark-staining nuclei on the outer surface of the 

 inner cell mass and in some cases these appear to form a layer continuous with the 

 trophectoderm. It seems probable that these represent an early stage of the extra- 

 embryonic ectoderm. Phylogenetically the extra-embryonic ectoderm is probably 

 derived from the trophectoderm, so that a similarity of structure is not surprising. 



