106 EMBRYOLOGY. 



become intimately united to her ; this peculiar mode of de- 

 velopment 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. In 

 some instances they are united in clusters by a 

 gelatinous envelop ; or are enclosed in cases or be- 

 tween membranous discs, forming long strings, as in 

 the eggs of the Pyrula (Fig. 99). The conditions 



FigToi. 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 at- 

 tached to their bodies ; sometimes under the 

 tail, as in the lobsters and crabs, sometimes 

 hanging in large bundles on both sides of the 

 tail, as in the Monoculus (Fig. 100, a). 



283. Some toads carry them on the back, Fig. 100. 

 and, what is most extraordinary, it is the male which under- 

 takes this office. Many mollusks, the Unio for example, have 

 them attached to the gills during incubation. In the polyps 

 they hang in clusters (Fig. 77, o), either inside or outside, at 

 the bottom of the cavity of the body. Some insects, such as 

 the gad-flies, deposit their eggs on other animals. Finally, 

 many abandon their eggs to the elements, taking no fur- 

 ther care of them after they have been laid ; such is the 

 case with most fishes, some insects, and many mol- 

 lusks. As a general rule, it may be said that animals take 

 the more care of their eggs and brood, as they occupy a 

 higher rank in their proper class. 



284. The development of the embryo does not always take 

 place immediately after the egg is laid. A considerable 

 time even may elapse before it commences. Thus, the first 



