vi PREFACE. 



In order to facilitate the use of the work by students 

 I have employed two types. The more general parts of 

 the work are printed in large type; while a smaller type 

 is used for much of the theoretical matter, for the details 

 of various special modes of development, for the histories 

 of the less important forms, and for controversial matter 

 generally. The student, especially when commencing his 

 studies in Embryology, may advantageously confine his 

 attention to the matter in the larger type; it is of course 

 assumed that he already possesses a competent knowledge 

 of Comparative Anatomy. 



Since the theory of evolution became accepted as an 

 established doctrine, the important bearings of Embryo- 

 logy on all morphological views have been universally 

 recognised ; but the very vigour with which this depart- 

 ment of science has been pursued during the last few 

 years has led to the appearance of a large number of 

 incomplete and contradictory observations and theories; 

 and to arrange these into anything like an orderly and 

 systematic exposition has been no easy task. Many 

 Embryologists will indeed probably hold that any attempt 

 to do so at the present time is premature, and therefore 

 doomed to failure. I must leave it to others to decide 

 how far my effort has been justified. That what I have 

 written contains errors and shortcomings is I fear only 

 too certain, but I trust that those who are most capable 

 of detecting them will also be most charitable in excusing 

 them. 



The work is fully illustrated, and most of the figures 

 have been especially engraved from original memoirs or 

 from my own papers or drawings by Mr Collings, who 

 has spared no pains to render the woodcuts as clear and 



