Life of Plants and Animals 



early spring, being found in shallow pools, often 

 floating on the surface of the water, or lying near 

 the banks of quiet streams in large gelatinous masses, 

 or attached to submarine plants in small clusters. 

 Frog-spawn may be put into jars of fresh water and 

 kept in the school-room, where the development of 

 the eggs into tadpoles can be observed. 



At first the egg of the frog has a white and a black 

 side, the former the vegetative, the latter, the animal 

 pole. Development begins when the egg has been 

 fertilized. 



Fertilization takes place in the water as soon as 

 the egg is oviposited by the female frog. At first two 

 small bodies, the polar globules, are pinched off from 

 the egg. This can be seen by the aid of a hand-lens 

 if closely watched. 



Segmentation. Maturation being completed, the 

 egg as a whole begins to divide by the formation of 

 furrows, visible on the surface, and passing at first 

 from the animal to the vegetative pole. Furrows then 

 form at right angles to those first formed. The 

 result of this cleavage, or segmentation, as it is called, 

 is the gradual obliteration of the white vegetative 

 pole by the growth over it of the animal pole, the 

 last stage of which is called the closing of the blasto- 

 pore. 



Formation of the Embryo. Ridges and furrows 

 then make their appearance, which, growing together 

 above, constitute the neural canal or spinal cord and 

 brain. From this stage the egg passes rapidly into 

 the tadpole form, being gradually elongated, a tail 

 forming opposite the head end of the animal. Already 

 the spherical egg has been converted into an elongated 

 animal, having bilateral symmetry and fore and aft 

 polarity. How has it all been accomplished? It is 

 characteristic of the thoughtless to take such things 

 for granted, as if they needed no explanation. 



