PREFACE. 



THE following chapters comprise the substance of a 

 course of public lectures which were delivered by the 

 author in the hall of the Lowell Institute, in Boston, 

 Mass., during the months of February and March, 1864. 

 Since that time a considerable amount of matter has 

 been added, chiefly in the form of notes ; and some alter- 

 ations have been made in the sequence of the subjects, 

 during which it has been found necessary to arrange 

 the chapters in such a way that they do not correspond 

 with the original succession of the lectures. To obviate 

 any difficulty, however, which might arise from the fre- 

 quent references to previous lectures, care has been taken 

 to mention the page upon which the subject alluded to 

 may be found. 



Although these lectures were originally given to the 

 public in a popular form, it must not be supposed that 

 they were altogether based upon what was already 

 known to the scientific world ; on the contrary, no small 

 proportion of the facts and ideas promulgated herein are 

 claimed by the author to be original with himself. In 

 this respect the attention of naturalists is invited to the 

 following subjects. The structure of Bacterium termo. 

 The organization of Vibrio baccillus. The theory of the 

 egg. The polarity and bilaterality of the egg. The eel- 



