296 



EMBRYOLOGY. 



lamina being entitled the serous or animal layer, the inferior 

 the mucous or vegetative layer; the former is limited to the 

 extent of the area pellucida, the latter extends farther in the 

 periphery, stretching beyond the area vasculosa. The albu- 

 men disappears in a great measure over the germinal mem- 

 brane, and the vitellus approaches the lining tunic of the shell 

 more closely ; in this situation, the vitellus becomes more pro- 

 minent, forming a segment of a lesser sphere, like the cornea 

 of the eye ; a circumstance which may likewise be frequently 

 observed in the egg before incubation (fig. 287, over m). It 

 is not unimportant to observe that these, the earliest observ- 

 able changes, not unfrequently take place in eggs that are laid 

 in summer, and when the weather is very warm, though, of 

 course, much short of brood-heat. 



[ 483. About the middle 

 of the first day, after from 

 twelve to fifteen hours of in- 

 cubation, the blastoderma, 

 or germinal membrane, is 

 completely detached from 

 the vitellary membrane, and 

 may be cut out as a con- 

 nected lamina, and washed 

 away from the membrane of 

 the yolk (figs. 323 and 324.) 

 The germinal area (area pel- 

 lucida s. germinativa) has 

 now an elongated, often a 

 somewhat pyriform appear- 

 ance (figs. 323 and 325, b], 

 and is two lines in length. 

 The darker vascular area 

 (figs. 323 and 325, c) has 

 also lengthened out, and the 

 germinal membrane extends 

 as a foliaceous formation 

 indefinitely over it into the 

 halones, which now begin 

 to look less regular than they 

 were originally. This outer 



Fig. 323. Vitellus or yolk after 

 from twelve to fourteen hours' incuba- 

 tion, of the natural size (this and the 

 other figures of the vitellus look larger 

 than proper, from their having been 

 placed in flat saucers to be drawn, by 

 which they became somewhat flat- 

 tened) : a, the yolk ; b, area pellucida, 

 in the middle of which the notaprima- 

 tiv a, or prim ary streak, the first trace of 

 the embryo, is perceived ; c, outer area 

 pellucida, the future area vasculosa. 

 The halones are indicated by the three 

 concentric circles. 



