REPKODUCTION BY GERM-CELLS 



285 



^.LR 



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 ^Y\\y the eggs of many animals should be 

 of such enormous size 



and often of such com- ^, D}1 GD Bl WD EW K>S 

 plex structure. The eggs " v . • . , . 



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. 73, 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 {WB) yolk, the concentric layers of fluid 

 albumen (EW) round about this, the chalaztB (Oh), and finally, the 

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

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



Fie. 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. LB, air chamber. 



