BIOLOGY OF THE LABORATORY MOUSE 



and 8), but by 63^^ days they lie evenly spaced and quite close together over 

 the trophectoderm's entire inner surface (Fig. lo). The layer of cells thus 

 formed is known as the distal entoderm. Meantime the inner cell mass has 

 grown down into the yolk cavity to form the egg cylinder. This is composed 

 of an inner mass of ectoderm cells and an outer layer of entoderm cells 

 (Fig. 8). This layer of entoderm cells bounding the egg cylinder is known as 

 the proximal entoderm. The entoderm is thus divided into two distinct 

 parts, distal and proximal, lining the distal and proximal walls of the yolk 

 cavitv. 



uterine lumen 



Inner cell mass 



Entoderm 



Trophectoderm 



Blastocoele 



Uterine epithelium 







S. Bohtn 



Fig. 5.— Section of implanting blastula 4 days 5 hours after mating. Projection 



drawing (X400). 



Embryonic and extra -embryonic ectoderm. — At about 4}^ days, when 

 the egg cylinder first begins to form, it can be seen that the egg cylinder ecto- 

 derm is divided into two parts, a dorsal,* more darkly stainingf region with 



* Most authors have used the terms mesometrial and antimesometrial to dis- 

 tinguish the two poles of the egg, the former being toward, and the latter away from, 

 the mesometrium or supporting mesentery of the uterus. However, as the dorso- 

 ventral axis of the embryo coincides with the dorso-ventral axis of the mother for at 

 least the first 8 days of development, the usage dorsal and ventral would seem to be 

 perfectly clear in most cases besides having the advantage of simplicity. The dorsal 

 side is up in the drawings. 



I When counterstained with congo red. 



