OF THE EGG. 



135 



281. Laying. After leaving the ovary, the eggs are 

 either discharged from the animal, that is, laid ; or they 

 continue their development within the parent animal, as is 

 the case in some fishes and reptiles, as sharks and vipers, 

 which, for that reason, have been named ovo -viviparous 

 animals. The eggs of the mammalia are not only developed 

 within the mother, but become intimately united to her ; this 

 peculiar mode of development has received the name of 

 gestation. 



282. Eggs are sometimes laid one by one, as in birds ; 

 sometimes collectively and in great numbers, as in 



the frogs, the fishes, and most of the invertebrates. 

 The queen ant of the African termites lays 80,000 

 eggs in twenty-four hours ; and the common hair- 

 worm, (Gordius,) as many as 8,000,000 in less than 

 one day. In some instances they are united in 

 clusters by a gelatinous envelop ; in others they are -pig. 99. 

 enclosed in cases or between membranous disks, 

 forming long strings, as in the eggs of the Pyrula shell, (Fig. 

 99.) The conditions under which the eggs 

 of different animals are placed, on being laid, 

 are very different. The eggs of birds, and of 

 some insects, are deposited in nests constructed 

 for that purpose by the parent. Other animals 

 carry their eggs attached to their bodies ; 

 sometimes under the tail, as in the lobsters 

 and crabs, sometimes hanging in large bun- 

 dles on both sides of the tail, as in the Mo- 

 noculus, (Fig. 100, .) 



Fig. 100. 



283. Some toads carry them on the back, and, what is 

 most extraordinary, it is the male which undertakes this 

 office. Many mollusks, the Unio for example, have them 

 enclosed between the folds of the gills during incubation. 

 In the jelly fishes and polyps, they hang in clusters, either 



