MAMMALIAN EMBRYOLOGY 



(although formed in a different manner from either that of the chick or 

 the frog) has been established with two definite cell or germ-layers. 



ATTACHMENT OF THE BLASTODERMIC VESICLE TO THE 



UTERINE WALL 



There are two general ways in which the blastoderm may become 

 attached to the uterus. 



The trophoblast or sub-zonal layer may remain as an outer layer 

 around the entire blastoderm, or the developing embryo within the inner 

 cell mass may push through this outer layer and come to lie in close 

 relationship to the uterine wall. 



At about the same time that attachment of blastoderm and uterine 

 wall takes place, the amniotic cavity is formed (Fig. 357). The tropho- 



Fig. 357. 



Diagrams of the relations of the cavities and layers in the rat, showing 

 the "inversion" of the germ layers. Median sagittal sections. Embryo and 

 amnion, black ; ectodermal knob or "trager" in light tone ; endoderm and 

 mesoderm in darker tone. A. Early stage before the formation of the false 

 amnionic cavity. B. Late stage showing false and true amnionic cavities 

 and the interamnionic cavity. a, Amnion ; ac, true amnionic cavity ; c, 

 chorion ; E, embryo (anterior end; ea, endodermal rudiment of allantois) ; /, 

 false amnionic cavity ; i, interamnionic cavity ; m, mesoderm ; ma, mesoderm 

 of allantois; n, endoderm; o, trophoblast (ectoderm); p, anterior intestinal 

 portal ; ra, rudiment of true amnionic cavity ; rf, rudiment of false amnionic 

 cavity; s, marginal sinus; t, "trager" (ectoderm) ; y, yolk-sac; ye, yolk-sac 

 endoderm; x, amnionic folds. (After Salenka.) 



blast remains as an outer covering in man, in many primates, and in 

 such animals as the mouse, rats and guinea pig. When the trophoblast 

 remains as the complete outer covering such a condition is known as 

 entypy, and it is in animals in which this condition occurs that a definite 

 space is formed between the germ layers and the trophoblast. This 

 cavity is known as the amniotic cavity. 



Sometimes the trophoblast thickens in this particular region and 



