8o 



AN INTRODUCTION TO ZOOLOGY. 



covered by Mr. Caldwell to be animals that produce 

 eggs provided with a yolk. In these animals, which 

 in other respects present a number of characters link- 

 ing them with the lower vertebrates, we have there- 

 fore a direct link between egg-laying or oviparous 

 animals and viviparous animals. 



A. 



Fig. 21.— The process of conjugation between similar cells. A, in 

 Spirogi/ra, a plant belonging to the Algoe ; B, in an Infusorian. A, slightly, B, 

 considerably magnified. See page 63. 



