LOPBIEN I OF Till: TWO HMMAllY .Kl:M LA 



yolk is d.-i_rnat'd as tii-- y Ik wall (vitellim rampart). It serves 

 for the augmentation of t lie inii'-r germ-layer, in that the free 

 nuclei incna>e in number by di\ i>i>n, and k--< p up tin- process of 

 Mippli meniary cleavage already mentioned. 



During incubation tlie liquefaction of the yolk makes further pro- 

 dig. 5G) and leads to tin- formation of a depression (ttt/), which 

 <-ontinually incn a-, s in depth and bn adth, and over which the g. rm- 

 arche> lik a watch-glass. Upon examination from the surface 

 i-- middle, as far MS the fluid r. -aches und -r it, appears clear and 

 transhuv nt, whereas the marginal area, which lies upon the opaque 

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



One (Itachts the whole ^ 



germ-disc from the yolk, fl*^**^^ 

 for in the region of the 

 thud-tilled >pace the thin 

 and transparent germ- 

 la vers come oil' easily and 

 clan from tluir substra- 

 tum, whereas at the rim, 

 from the point where the 

 inner g< rm-layc r merges 

 with the yolk-wall out- 

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

 a long time the middle, clear, circular ana 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 connect d 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-layw* in Mammals^ and to 



refer it back to the gastrulation of the other Vertehrat >. K.-p rial 



ice has been rendered through the painstaking inv stigation of 



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



IllNSEN, LlEBERKUHN, VAN BfiNEDEN, KoLLIKER, and HfiAPE. The 



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

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

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

 employed. 



Fig. 57. Section through the margin of the germ-diao 

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

 hours, after Duv \i . 



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

 dw, yolk-wall. 



