Development 



Investigation has yet to prove tiiat any given North American 

 species has more than one true brood each year. The fertiliza- 

 tion of the egg is external and the grasp of the male is back of 

 the arms, except in the DiscogIossida2 and the Pelobatidae, where 

 it is inguinal, as among the Salamanders. When several species 

 are breeding in a given pond, they may divide the space and form 

 themselves into colonies. The same exclusiveness is true later 

 of the tadpoles also. 



For details of the development of the frog's egg, see pp. 176 to 

 181, and for those of the toad's egg, see pp.67 to 71. The Salientia 

 show their close relationship by the similarity in the development 

 of the eggs. The time element varies with the species and with 

 the temperature in the case of a given species. Some embryos 

 hatch in less time than others — in two or three days even (Bufo 

 americanus) instead of in as many weeks (Rana piptens) — and 

 thus may be at an earlier stage of development at the time of 

 hatching. They may therefore either pass through a quiescent, 

 clinging stage after hatching (Figs. 35 and 248), or omit this 

 and becom.e active at once. There may be differences in colour, 

 in the extent of the development of the external gills, and in other 

 details, but, on the whole, if we know the story of development 

 in one species, we know what to look for in other species also. 



This applies equally to the metamorphosis of the tadpole. 

 (See American Toad, pp.69 and 70, and Bullfrog, pp. 235 to 238.) 

 The length of life of the .tadpole before its change into the frog 

 may vary from two or three weeks to as many years. Some of the 

 interesting points possible to observe during the metamorphosis 

 are the following: The ^rms and legs develop simultaneously, 

 but the arms are concealed under the opercular membrane. (Fig. 

 280.) The left arm appears first, because of the presence of the 

 breathing-pore orspiraculumontheleftside. (Figs. 284 and 285.) 

 The eyes are elevated, become free, and have movable lids per- 

 fected. The lachrymal canal shifts upward and backward tow- 

 ard the eye and enters the lower eyelid. (Fig. 287.) The horny 

 parts of the mouth are dropped and the mouth cavity increases 

 in size. The tail becomes smaller by absorption from within. 

 The tadpole takes on habits of rushing to the surface or of resting 

 wholly out of the water (Figs. 287 and 288), showing that the 

 lungs, which for some time before had been functional in company 

 with the gills, now take on all of the respiratory work (except 



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