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PRINCIPLES OF ANIMAL BIOLOGY 



male grasps the female, with his forelegs around her body (Fig. 151), 

 and pours out the fluid containing the spermatozoa as she lays her eggs 

 in the water. One of the salamanders, Notophthalmus viridescens, shows 

 a curious combination of methods, the value of which is obscure ; the male 

 first clasps the female, but instead of pouring out the spermatozoa into 

 the water, he then deposits them in spermatophores, from which the 

 female takes them into her cloaca as just described. 



In many other animals the spermatozoa are introduced into the 

 body of the female by direct act of the male, a process known as copula- 

 tion. Fertilization then occurs internally. This method is employed 



Fici. 151. — Clasping in a species of toad, Bnfo typhonius (Linnaeus). The small individual 

 is the male, the larger the female. {Photograph by A. G. Ruthven.) 



by some parasitic worms, snails, fishes, and amphibia, and by all insects, 

 reptiles, birds, and mammals. 



Place of Development. — From the methods of ensuring fertilization, 

 it will be seen that the eggs may be fertilized either before or after they 

 are laid. That is, fertilization is either internal or external. When 

 fertilization is internal the eggs may be retained for a long time after 

 fertilization, or they may be laid very soon thereafter. Whatever period 

 of time the eggs remain in the organs of the female after fertilization is 

 utilized in development, so that the embryo may be far advanced before 

 it is separated from the mother, or it may have attained only an early 

 stage of development, or development may scarcely have started. Thus, 

 in most of the insects and in all the birds the eggs are laid soon after 

 fertilization. In these cases only a few divisions of the egg, or of its 

 nucleus, have taken place at the time of oviposition, or it may not have 

 divided even once. On the contrary, development may proceed until a 



