REPRODUCTION BY GERM-CELLS 



285 



beyond itself, so to speak, and provides the means for a growth 

 which could obviously not be attained by means of the ordinary 

 nourishment supplied by the blood. 



We now understand why the eggs of many animals should be 

 of such enormous size 



and often of such com- ™ DM GD Bl WD FAY Ks 

 plex structure. The eggs 

 of birds are especially 

 remarkable in this re- 

 spect, and it has till 

 recently been disputed 

 whether they are really 

 morphologically equiva- 

 lent to a single cell. But 

 this is undoubtedly the 

 case, and though only 

 the small thin germinal 

 disk (Fig. J 3, Bl) with 

 its nucleus is the active 

 part of this cell — the 

 cell - body proper — yet 



all, the rest— the enormous sphere of yolk with its regular layers of 

 yellow (GD) and white (WD) yolk, the concentric layers of fluid 

 albumen (EW) round about this, the chalazse (Ch), and finally, the 

 delicate shell membrane (3) and the limy shell (KS) — belong to this 

 cell, and have arisen in connexion with it (Fig. 73). 



FiG. 73. Diagrammatic longitudinal section of a 

 hen's egg before incubation, after Allen Thomson. 

 Bl, germinal disk. GD, yellow yolk. WD, white yolk. 

 DM, vitelline membrane. EW, albumen. Ch, chalaza. 

 S, shell membrane. KS, shell. LR, air chamber. 



