190 



Tlic Study of Anhnal Life part in 



occur in some Avorm-types (some flukes, threadworms, etc.), 

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

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

 life-cycles of fern and moss. 



Fig. 35. — Diagr.im of a liydioij crilony, sonic of the iddhidii.ils of wliich 

 been inodificd as swimming - bi;lls or nicdusoids ; oiiu of Lhesu !ias 

 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 futui-e organism was 

 already present in miniature. Every germ contained a 

 miniature model of the adult, which in development was 



