34 THE FIRST PRINCIPLES OF HEREDITY 



nucleus — is always small, generally visible only with the 

 aid of the microscope, the remainder of the egg being 

 made up by various extrinsic additions, but chiefly the 

 yolk. The latter is material stored up within the egg to 

 serve the forthcoming embryo as nourishment. We can 

 see how the large size of the bird's egg, e.g., is mostly made 

 up by the yolk, which lies around the very minute germinal 

 vesicle. 



DM GD Bl WD EW KS 



CH 



Fig. 23. — Hen's Egg : Diagrammatical Longitudinal Section. 



(After Allen Thomson.) 



[From Weismann, " The Evolution Theory.") 



CH, chalaza ; DM , vitelline membrane ; GD, yellow yolk ; Bl, ger- 

 minal disc with germinal vesicle ; WD, white yolk ; EW , albumen ; 

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



According to the disposition of the yolk within the egg, 

 we distinguish four types of eggs, each of which is charac- 

 terized, as w^e shall see later, by a typical method of seg- 

 mentation. We have eggs with (i) diffuse yolk, where 

 there is a small amount of yolk evenly distributed through- 

 out the egg-cell, as in Invertebrates (Sponges, Corals, Star- 

 fish, Worms, etc.), and Mammals ; (2) central yolk, the 

 yolk being in the centre of the egg, as in Arthropods 



