134 



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 



ink" mk 1 



pr uL 



ak 



mk 



ik 



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



ED. VAN BENEDEN. 

 ak, Outer, ik, inner, mk, middle germ-layer ; mk 1 , 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 Rabbits and Bats, VAN 

 BENEDEN in some cases observed between the primitive folds, or 



ink 1 id 



pr 



- --- *""^ 



., 

 ^k 



^^- 



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



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

 ak, 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 



