456 MOUSE EMBRYO 



THE MOUSE EMBRYO: 3.5 TO 5.5 DAYS POST-CONCEPTION 



Fig. 20- Longitudinol section of gravid uterus at 3.5 days 



after conception, showing three blastulae in suspen- 

 sion within the uterine cavity. 



Fig. 21 - Enlarged view of portion of figure 20. 



Fig. 22 - Slightly later stage when the blastula is "trapped" 

 within a uterine crypt, prior to implantation. 



Fig. 23 - Embryo at 3. 5 days showing cell structure.- Pre- 

 implanted blastula. 



Fig. 24 - Inner cell mass, trophectoderm, and blastocoel 

 easily identified. 



Fig. 25 - Blastula at about 4 days showing embryonic and 



extra-embryonic ectoderm, proximal and distal en- 

 doderm, and blastocoel (yolk cavity). 



Fig, 26 - Blastula at the moment of adhesion to the uterine 

 epithelium, low power. 



Fig. 27- Same as fig. but at high magnification. 



Fig. 28- Active implantation processes have begun, at 4. 5 

 days. 



Fig. 29 - Embryo at 5. 5 days showing little change in the sur- 

 rounding uterus. 



Fig. 30 - Enlargement of the embryo to distinguish between 

 the inner solid embryonic ectoderm and the outer 

 thin layer of proximal endoderm. 



Fig. 31 - Separation of the first two primary germ layer. 



Fig. 32 - Separation of distal and proximal endodermal layers, 

 with a central core of solid ectoderm. Note early 

 dissolution of contiguous uterine epithelium. 



