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earliest, i. e. , the most remote ancestors. Temporary organs 

 may be of actual use, but there is no good reason why a 

 frog should pass through an aquatic stage of existence when 

 a straight development towards the adult condition could be 

 explained as more seemingly normal, were it not for the per- 

 sistence of hereditary traits. The Hy lodes (West Indian frog) 

 omits the tadpole stage entirely. 



The tendency to repeat the history of lower ancestral condi- 

 tion is believed to explain the fact that the red blood corpuscles 

 of mammals are at first nucleated, as are those of adult frogs, 

 and the still more striking one that during the period when the 

 unborn human infant gets aerated blood from the lungs of the 

 parent, the septum dividing the right and left auricles has an 

 aperture ( K forai?ie?i ovale) which is represented by a constant 

 opening in the adult frog. 



In the development of the frog, the embryo leaves the egg 

 in a condition so far removed from that of the adult as to de- 

 serve a somewhat detailed account of its growth. The eggs 

 are laid in masses or strings, and are impregnated as they 

 leave the body of the female. 1 



They are about one-sixteenth of an inch in diameter, and 

 contain enough food yolk to carry the young to the tadpole 

 stage. 2 



The eggs are laid in gelatinous envelopes, which swell after 

 leaving the adult. At the time of hatching there are three 

 pairs of external gills, but no mouth or anal opening. To pre- 

 vent the young tadpole from dropping to the bottom of the 

 pond, where it would soon be smothered in the mud, it is pro- 

 vided with two small " suckers " just back of where the mouth 

 is to appear, and by these it clings to aquatic plants. Soon 



1 The male uses his anterior limbs to seize the female near the fore- 

 legs and presses the eggs from the body. The salamanders seize the 

 female with the hinder feet. 



2 The chick is developed fully before leaving the egg at the expense 

 of the food yolk. The egg of the frog is sixteen times as large as that of 

 a rabbit, but the embryo of the latter is developed from the time of using 

 food yolk by the placenta, thus receiving nourishment directly from the 

 blood of the mother. As the frog has no membranes for foetal nourish- 

 ment, the large quantity of food yolk and consequent size of egg are 

 explained. 



