THE MAMMALIA 301 



cystic cavity, and the latter is nearly filled by the downward 

 projection due to the increasing size of the embryonic knob 

 and the cavity contained in it. 



During these changes the walls of the uterus have 

 been greatly thickened, the uterine epithelium has disin- 

 tegrated and has nearly completely disappeared, and the 

 blastocyst has come to lie in a deep pit in the uterine walls 



(fig. 75, F)' 



The embryo is now formed at the bottom of the downwardly 

 projecting embryonic knob. Differentiation sets in over an 

 area known as the embryonic area ; the cells become several 

 layers deep, and are divided into an epiblast above and a 

 mesoblast below ; the yolk epithelium of this region forms the 

 hypoblast (fig. 75, D). Both notochord and mesoblast are 

 formed largely from a linear proliferation of cells in the 

 middle of the embryonic area known as the primitive streak. 

 At first the mesoblast forms two lateral sheets a 1 ; the sides of 

 the notochord and these sheets bend upwards in conformity 

 with the folds of the yolk epithelium Presently the inner 

 edges of the mesoblast sheets abutting on the notochord are 

 divided into mesoblastic somites, and the lateral parts of the 

 sneets are split into somatopleuric and splanchnopleuric layers, 

 in the same manner as in the frog. Meanwhile two neural 

 folds enclosing a neural groove have been formed from the 

 epiblast, and the folds shortly grow up and unite to form the 

 neural tube. In these and all other details of organogeny the 

 development of the mammalian embryo so far resembles that 

 of the frog that it is unnecessary to describe it further. But 

 special attention should be paid to the fate of the somatopleur 

 and splanchnopleur. Both epiblast and mesoblast extend 

 right round the space lying above the embryo that is, round 

 the cavity formed at a previous stage in the embryonic knob. 

 When the mesoblast is split the somatopleur is adherent to the 

 epiblast, and outside the embryonic area the two layers 

 become widely separate from the splanchnopleur, which 

 latter, in the embryonic area and laterally, is closely applied to 

 the yolk epithelium, but above is in contact with the lower 

 layer of the trager (fig. 75, E). The somatopleur with its 

 lining of epiblast forms a hood, called the amnion, over the 

 embryonic area, and the space between the amnion and the 

 embryo is the amniotic cavity. This cavity is clearly the same 



