190 



The Study of Animal Life PART in 



occur in some worm types (some flukes, threadworms, etc.), 

 and as high up in the series as Tunicates ; while among 

 plants analogous alternations are very common, e.g. in the 

 life-cycles of fern and moss. 



FIG. 35. Diagram of a hydroid colony, some of the individuals of which have 

 been modified as swimming - bells or medusoids ; one of these has been 

 liberated. 



3. Historical, In the seventeenth and eighteenth cen- 

 turies, naturalists had a short and easy method of dealing with 

 embryology. They maintained that within the seed of a 

 plant, within the egg of a bird, the future organism was 

 already present in miniature. Every germ contained a 

 miniature model of the adult, which in development was 



