vi PREFACE. 



that they do not profess to form a complete biblio- 

 graphy of the subject. 



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 Embryology 

 on all morphological views have been universally recognised ; 

 but the very vigour with which this department of science 

 has been pursued during the last few years has led to the 

 appearance of a large number of incomplete and contradic- 

 tory 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 



