Egg- 



119 



The akcithal owiva {e.g. Earthworm, AnipMoxus, Rabbit) is 

 a minute egg, with only a little yolk in the form of a few 

 spherules scattered uniformly through the protoplasm. 



The telolecithal ovum (e.g. Fowl, Dogfish) has a quantity of 

 yolk massed at one pole, and occupying the greater part of the 

 ovum. 



The centrolecithal ovum, finally (e.g. Astacus), has a 

 quantity of yolk, which lies centrally, and is surrounded by 

 a peripheral layer of protoplasm. 



FiG. 5g.— Egg of Fowl in longitudinal section. (From Marshall.) 



BA, germinal disc : Y, yolk ; z, vitelline membrane; \va, albumen; wc, chalaza; sv, air 

 space ; sm, shell membrane ; SH, shell. 



The frog's egg is intermediate between the alecithal and 

 the telolecithal; there is a great deal, but not an enormous 

 quantity, of yolk, which is more dense at one pole than at the 

 other, though the protoplasmic pole of the ovum is not entirely 

 free from yolk, as in the fowl. 



Alecithal ova either produce embryos which are hatched 

 and lead a free larval hfe at a very early period of development, 



