EMBRYOLOGY. 



of the primitive groove (pr) all three germ-layers are joined to 

 one another for a certain distance by means of a common cells 





Fig. 98. Cross section through the primitive groove (blastopore) of a Babbit's germ-disc, after 



ED. VAN BENEDEN. 

 ak, Outer, ik, inner, mk, middle germ-layer ; mV, parietal, mk', visceral lamella of the middle 



germ-layer ; ul, lateral lip of the blastopore ; pr, primitive groove. 



mass. At the same time one may observe, with tolerable dis- 

 tinctness, how the outer germ-layer (ak) bends around into the 

 parietal middle layer (mk 1 ) at the primitive fold (ul), while the 

 visceral lamella (mk 2 ) is continuous with the entoderm (ik), which 

 is only one cell thick. Indeed, in embryos of Eabbits and Bats, VAN 

 BENEDEN in some cases observed between the primitive folds, or 



fig 99. Cross section through a human germ-disc, with open medullary groove, in the 



vicinity of the neurenteric canal (pr), after GRAF SPEE. 

 at, Outer, ik, inner germ-layer; mk 1 , parietal, mk~, visceral lamella of the middle germ-layer; 



ul, lateral lip of the blastopore ; pr, primitive groove. 



blastoporic lips, a structure corresponding to the yolk-plug of 

 Amphibia. 



It is certainly of great general interest that the investigation of 

 an extraordinarily young human germ-disc at the hands of GRAF 

 SPEE has furnished a cross section (fig. 99) which is near enough 



