286 



EMBRYOLOGY. 



aside the middle germ-layers, which extend into this region, and 

 thus grow outward to the surface t where they unite with the epi- 

 dermis. The latter now become depressed into furrows along the 

 regions of contact (fig. 154), so that one can distinguish inner, deeper 

 throat-pockets, and outer, shallower throat- or gill-furrows. The two 



are separated from each other 

 for a time by a very thincZoa- 

 ing membrane, which consists 

 of two epithelial layers, the 

 epidermis and the epithelium 

 lining the fore gut. 



The bands of substance 

 which lie tetes^J^JHJfc 

 throat-pockets (figs. 



cessve 



154 and 157) are the mem- 

 branous branchial, throat-, or 

 visceral arches. They consist 

 of an axis r which is derived 

 from the middle germ-layer 

 and the mesenchyma, and of 

 an epithelial covering, which 

 on the side toward the pharynx 

 is furnished by the inner germ- 

 layer, on the outside by the 

 outer germ-layer. They are 

 designated according to their 

 sequence as the second, third, 



Tig. 151 Frontal (reconstruction) section of the 

 oro-pharyngeal cavity of a human embryo 

 (Bl of His) 4'5 mm. long, neck measurement, 

 from His " Menschliche Embrjonen." Mag- 

 nified 30 diameters. 



The figure shows four outer and four inner visceral 

 furrows, with the closing plates at the bottom 

 of them. In the visceral arches separated by 

 furrows one sees the cross sections of the 

 second to the fifth aortic arches. By reason 

 of the greater development of the anterior 

 visceral arches the posterior ones are already 

 somewhat pressed inwards. 



fourth, etc., visceral arches, 



inasmuch as the ridge which surrounds the mouth constitutes the 

 jfirst visceral arch. 



In all water-inhabiting Vertebrates which breathe by means of 

 gills the thin epithelial closing plates break through between the 

 visceral arches, and indeed in the same sequence as that in which they 

 arose. Currents of water therefore can now pass from the outside 

 through the open clefts into the cavity of the fore gut and be employed 

 for respiration, since they flow over the surface of the mucous mem- 

 brane. There is now developed in the mucous membrane, upon both 

 sides of the visceral clefts, a superficial, close network of blood- 

 capillaries, the contents of which effect an exchange of gases with 

 the passing water. Moreover the mucous membrane becomes folded, 

 for the increase of its respiratory surface, into numerous, close-set, 



