GENERAL ACCOUNT. 



221 



shell membrane are protective envelopes, which are not repre- 

 sented in the eggs of Amphioxus or of the frog. 



The yolk or ovum is, as in other animals, a single cell ; its 

 great size being due to the enormous quantity of food yolk ac- 

 cumulated within it, and distending it. As regards the quantity 

 of food-yolk contained within it, the hen's egg is at the opposite 

 extreme to that of Amphioxus ; the frog's egg being midway 

 between the two. 



It is in consequence of the abundance of food material 



SH 



sv 



FIG. 97. The Hen's Egg at the time of laying, x g. 



BA. blastoderm. SH, epr? shell. SM, shell membrane. .SV, air chamber. 

 "WA, white or albumen. "WC, chalaza, or twisted cord of denser albumen. Y, yolk. 

 Z, vitelline membrane. 



present in the egg itself, that the chick embryo is enabled to 

 complete its development within twenty-one days, while the frog 

 requires three months or more, and Amphioxus an even longer 

 time. The Amphioxus larva hatches in about eight hours, but in 

 an extremely immature condition (Figs. 25, 26, p. 59) ; the frog 

 hatches in about a fortnight, in a form utterly unlike the parent, 

 and devoid of mouth and limbs (Figs. 72, 73, p. 157); the chick 

 does not leave the egg until the twenty-first day, but is already a 

 fully developed bird. 



A large amount of food-yolk is undoubtedly an advantage, 



