THE STUDY OF AMPHIBIANS 



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scale. The little animals often associated with the frog's spawn 

 may help to loosen the envelopes and thus assist the emergence of 

 the embryo frogs, but some appear to attack the latter. Further- 

 more, the slippery nature of the jelly may protect the eggs from 

 water-birds, especially from those that are not broad-billed like 

 the ducks. It is also noteworthy that only a few of the voracious 

 smaller water animals attack frog-spawn, perhaps because of 

 some unpalatability in the jelly. There is interest in an inquiry 





FlG. 40. The division of the frog's eggs. (After Ecker.) The numbers (2, 4, 8, 16, 32, 

 64) indicate the number of cells into which the egg has divided. It should be noted 

 that although there are 64 cells in the last figure on the second row, it is no larger 

 than the unsegmented egg (i). Growth has not begun. The first cleavage is vertical, 

 dividing the egg into a right and left half ; the second is also vertical, at right angles 

 to the first, dividing an anterior from a posterior half ; the third cleavage is equa- 

 torial, at right angles to the two preceding. With the help of a lens these early 

 cleavages can be seen occurring. As division goes on there come to be numerous 

 smaller cells in the upper part of the egg, and fewer, larger, yolk-laden cells towards 

 the lower pole. 



of this kind, for although there are some uncertainties, the general 

 fact is clear, that the spheres of jelly are useful in many ways. 

 They are instances of the fitness of minute details in organic 

 nature. In many amphibians the gelatinous envelope serves to 

 attach the eggs to water-weeds, but those of Rana temporaria 

 float freely. 



Early Development. For about three weeks the time re- 

 quired varying with the temperature the eggs go on developing. 

 Out of apparent simplicity comes obvious complexity, an embryo 

 is formed. With a lens the egg may be seen dividing, as if it was 



VOL. I. Q 



