42 8 



HouD Animals and Plants Reproduce unit viii 



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Fig. 376 Eggs laid by a frog. Each is coated 

 with a jellylike substance which swells in water. 



(AMERICAN MUSEUM OF NATURAL HISTORY) 



eggs come out, the sperms are deposited 

 on them. The chances of fertihzation are 

 good. As each t^^ passes through the 

 oviduct it is coated with a thin jelly Hke 

 material. The sperms pierce this layer, 

 which then swells as water is absorbed. 

 The jelly like material protects the eggs 

 by holding them together. The large 

 slippery mass is difficult to grasp and is 

 not easily eaten by other animals. 



The early stages of the development 

 of the fertilized &^^ are much like those 

 of the fish. But when the organs form 

 and the body takes shape, the embryo 

 does not resemble the parent frog; it is 

 fishlike. We call it a tadpole or a polly- 

 wog. It gets dissolved oxygen from the 

 water by means of two feathery gills 

 which project just back of the head. 

 Young tadpoles wriggle occasionally or 

 hang quietly attached to water plants by 



Fig. 377 A tadpole changes into a frog. What 

 is the first sig7i of its becoming a frog? (new 



YORK ZOOLOGICAL PARK) 



means of a sucking organ at the front of 

 the head. Soon the tadpole swims by 

 means of its tail; a mouth forms with 

 horny jaws well fitted for eating plants; 

 and internal gills replace the external 

 gills with which it began. You can see 

 these chan^'es if you are fortunate 

 enough to have live material. If not, use 

 museum preparations as described in 

 Exercise 6. 



After living this way for several 

 months, or longer in some species, the 

 tadpole goes through other changes. 

 Two lumps appear near the hind end of 

 the tadpole's body, one on either side of 

 the tail. Slowly legs push out here. If 

 you cut the tadpole open at this time, 



