DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 



99 



yolk is designated as the yolk-wall (vitelline rampart). It serves 

 for the augmentation of the inner germ-layer, in that the free 

 nuclei increase in number by division, and keep up the process of 

 supplementary cleavage already mentioned. 



During incubation the liquefaction of the yolk makes further pro- 

 gress (fig. 56) and leads to the formation of a depression (ud), which 

 continually increases in depth and breadth, and over which the germ- 

 disc arches Hke a watch-glass. Upon examination from the surface 

 its middle, as far as the fluid reaches under it, appears clear and 

 translucent, whereas the marginal area, which lies upon the opaque 

 yolk, appears dark. Such a distinction is still more observable when 



one detaches the whole ^ 



germ-disc from the yolk, Je^K^^^™'®^^^^^^^^ 

 for in the region of the 

 fluid-filled space the thin 

 and transparent germ- 

 layers come off easily and 

 clean from their substra- 

 tum, whereas at the rim, 

 from the point where the 

 inner germ-layer merges 

 with the yolk-wall out- 

 ward, turbid yolk-substance remains clinging to the germ-disc. For 

 a long time the middle, clear, circular area has been designated 

 in embryology as the clear germinal area (area pellucida), and the 

 more cloudy, ring-like rim as the opaque germinal area (area opaca). 



In the next chapter I shall treat more in extenso of the important 

 changes which take place — up to the time when the egg is laid 

 and during the first hours of incubation — in the vicinity of the 

 crescentic groove and the anterior lip of the blastopore, because they 

 are connected with the development of the middle germ-layer. 



It is still more difficult than in the case of the Chick to interpret 

 in its details the development of the germ-layers in Mammals, and to 

 refer it back to the gastrulation of the other Vertebrates. Especial 

 service has been rendered through the painstaking investigation of 

 these conditions : in the earlier times by Bischoff, in later years by 

 Hensen, LiEBERKtHN, TAN Beneden, Kolliker, and Heape. The 

 object of investigation which has been made use of in this work, and 

 which we shall employ as the basis of our description, has usually 

 been the Rabbit ; besides this, the Bat and the Mole have also been 

 employed. 



Fi^. 57. — Section througli the margin of the germ-disc 

 of a Hen's egg that had been incubated for six 

 hours, after Du\'al. 



ak, Outer germ-layer ; dz^ yolk-cells ; dk, yolk-nuclei ; 

 cEw, yolk-wall. 



