12 DUBLIN UNIVEESITT 



mity'with the atmosphere seems a necessity; neither is there duckweed 

 or vegetable matter to mount upon, when, at a later period, the lungs 

 actually exist. In glass vessels a foot deep, I have often watched a 

 tadpole resting for a time at the bottom, slowly and with the least 

 movement on its own part, it begins to ascend, gradually reach the sur- 

 face, there expel a minute bubble of gas, and its specific gravity being 

 altered by the expulsion, it as gently sinks to the bottom again. After 

 a time another bubble is secreted within ; it again ascends like a little 

 balloon, and so on. It is probable that the first development of the 

 lung commences by a secretion of gas, and, as the homology of the 

 organs would lead us to suppose, it is at first a bilobed swim bladder, 

 and afterwards a lung. 



Many die in the artificial form, from a disease accompanied with 

 unequal development and over-distention of those little air-sacs; they 

 are buoyed up so as to be with difficulty able to swim down, and are 

 turned on one side, or even belly upwards. 



"With reference to the influence of light on the development of the 

 tadpole, Higginbottom speaks with authority. "I am enabled," he 

 says, " from the most minute observation to state that their develop- 

 ment advances equally well in the dark and in the light, and that 

 absence of light has no influence in retarding their development." Of 

 this I am also convinced: increase of temperature unquestionably accele- 

 rates, within certain limits, their development ; if light has any effect, it 

 is only when the creature, being unfed, the light, by producing vegetable 

 growth, produces food for them. In fact, as is natural to suppose, in 

 creatures the most remarkable part of whose metamorphosis is the 

 change which the respiratory system undergoes, any influence which 

 interferes with the natural performance of the respiratory function in- 

 terferes with the progress of development more readily than any other. 

 Hence, no doubt, the explanation of why so few in the foregoing ex- 

 periments passed into the state of frogs, where no facilities were afforded 

 by floating vegetable matter, of coming in contact with the air, and 

 where the depth of the water and the vertical sides of the vessels 

 offered absolute obstacles to the same. . The same, I fancy, is the cause 

 of the mortality being at first, in the three experiments of the table, 

 greater in the light than in darkness. In the first, a complete exclusion 

 from the air was offered by the diaphragm ; in the others (sixth and seventh 

 experiments) the form of the vessel offers an obstacle to reaching the 



