REPRODUCTION AND LIFE CYCLES 



42.3 



dysentery, passes from the alimentary canal of man safely protected 

 by a thick, hyaline coat, until such time as ingestion by a suitable 

 host brings about its dissolution in the host's stomach. The eggs of 

 some of the tapeworms and roundworms are surrounded by dense 

 impervious shells, rendering them viable, in the ca.se of Ascaris, for 

 five or six years. Some of the parasitic roundworms are ovovivipa- 

 rous, retaining the eggs 

 within the body of the 

 parent until thoy are 

 nearly ready to hatch. 



A few fishes, like some 

 of the skates, produce 

 an egg surrounded by a 

 hard, leatherlike ca.se, 

 which is drawn out into 

 entangling tendrils 

 that readily become 

 enmeshed in seaweeds, 

 thus affording protec- 

 tion to the egg. Most 

 of the fresh-water fishes 

 and amphibians, how- 

 ever, lay eggs which are 

 protected by nothing 

 more than a gelatinous 

 mass which .swells after 

 the eggs are laid in the 

 water and are fertilized 



by the sperm. Among the reptiles and birds a hard shell is usually 

 produced which gives protection to the enclosed ovum with its 

 stored food. Only one small group of mammals, the monotremes, 

 lay eggs, all others being viviparous. 



The Yolk Sac 



Among the fishes which lay telolecithal eggs containing enough 

 yolk to render the cleavage pattern irregular, a mass of undivided 

 yolk accumulates beneath the developing embryo. Soon, however, 

 the blastoderm upon which the embryo lies grows down over the 

 yolk, eventually enclosing it. This mass of tissue is composed of an 

 inner layer of endoderm and an outer lining of mesoderm and is called 

 H. w. H. — 28 



Embryo and egg case of skate. Such cases afford 

 protection against wave action. What other types 

 of adaptations are there for the protection of eggs 

 and embryos.^ (After Walker.) 



