AN INTRODUCTION TO ANIMAL BIOLOGY 



61 



mass of many hundred cells ; and it is then ready to arrange 

 the cells so as to form the body of a tadpole. 



60. Later Development. The changes in external form 

 of developing frogs' eggs should be observed, either in eggs 

 preserved in formalin-solution, or in living eggs (only in 

 early spring) kept in aquaria (e.g., fruit-jars) and examined 

 daily during development. In brief, the changes are as 

 follows : The spherical mass of cells formed by the numerous 







FIG. 23. Stages in division or cleavage of frog's egg. The figures indicate 

 the number of cells. (From Thomson, after Ecker.) 



divisions of the egg-cell becomes elongated, the various 

 organs are formed by complicated changes which cannot be 

 described in the limited space of this book, and gradually 

 the embryo becomes a tadpole. Finally, the tadpole 

 hatches, that is, it breaks through the jelly that has sur- 

 rounded the egg throughout the development. 



The tadpole at hatching appears larger than a fertilized 

 frog egg; but if dried would not weigh more, for no food 

 can be taken until after hatching. The larger size of the 

 tadpole as compared with the egg is due to water absorbed 

 during development. 5 



The time necessary for a fertilized egg-cell to develop into 

 a free-swimming tadpole varies with the temperature of the 

 water. Also, the eggs of some species develop faster than 



