134 



EMBETOLOGT. 



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

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



niF mk' 



Fig, 98. — Cross section through the primitive groove (blastopore) of a Babbit's germ-disc, at't«r 



Ed. vajs Beneden. ' 

 ak, Outer, ik^ inner, mk, middle gertn-layei* ; mk^, parietal, mP, 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'-) at the primitive fold {ul), while the 

 visceral lamella {mk^) 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 



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



vicinity of the neurenteric canal (pr), after Geaf Spee. 

 ak. Outer, ik, inner germ-layer ; mk^, parietal, mP, 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 Gkaf 

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



