EARLIEST STAGES OF EMBRYONIC DEVELOPMENT. 581 



power of filling its lungs with air. The yolk-bag floats within 

 the albumen, and always tends to ta,ke the highest place, being 

 the lighter of the two ; but it is kept nearly to one place by 

 two cords (e, e) termed the chalazce, which seem formed of 

 peculiarly viscid albumen, and connect the yolk-bag with the 

 lining membrane at the two ends of the shell (d, d). In this 

 manner the yolk-bag is always kept at the part of the shell 

 where it can most favourably receive the warmth imparted to 

 it by the mother ; and the cicatricula or germ-spot (which is 

 the mass of cells first developed from the germ-yolk) is made, 

 by a similar contrivance, always to rise to the highest point. 

 In the eggs of Fishes there is no additional albumen ; and 

 in those of Frogs the albumen is common to the general mass 

 of ova, constituting the peculiar "gelatinous envelope," which 

 forms long necklace-like strings of " spawn," within which the 

 black yolk-bags are disposed at tolerably regular intervals. In 

 Mammals, each ovum receives a separate investment of a jelly- 

 like substance in its passage along the oviduct into the uterus ; 

 and around this there is formed a fibrous membrane termed 

 the Chorion, which is destined to take a very important share 

 in the subsequent nutrition of the embryo ( 761). 



756. In the eggs of Frogs, as in those of Mammals, the 

 whole of the yolk undergoes the process of segmentation 

 already described ( 736), and takes a share in the formation 

 of the " mulberry mass." But in the eggs of Fishes, Birds, 

 and the higher Reptiles, this process is limited to that small 

 portion of the yolk which is distinguished as the germ-yolk ; 

 and the formation of the germinal membrane takes place after 

 a different fashion. The mass of cells that immediately results 

 from segmentation, flattens itself out on the surface of the 

 yolk, forming the minute semi-opaque whitish spot, which is 

 known as the cicatricula, germ-spot, or "tread;" and by a 

 further extension it constitutes the "germinal membrane," 

 which gradually spreads itself over the food-yolk ; at the same 

 time dividing itself into two layers, between which a third 

 is afterwards interposed. Thus the " germinal membrane," 

 which may be compared to the seed-leaves or cotyledons of 

 Plants, forms a sort of temporary stomach round the mass of 

 nutriment prepared for the sustenance of the embryo ; the 

 whole of which nutriment, as will be presently seen, is absorbed 

 into the body of the embryo through its instrumentality. 



