viii EMBRYOLOGY OF THE LOWER VERTEBRATES 



accessibility or ease of investigation. It follows that at the present 

 time when we have knowledge of the more archaic subdivisions 'of 

 the vertebrata not accessible to the early builders, of the science, it 

 is necessary to regard the historical foundations of vertebrate 

 embryology rather critically in the light of the fuller knowledge 

 of to-day. In essaying the writing of this volume I have been 

 fortunate in having at my disposal — for the first time in the history 

 of embryology — developmental material of all three genera of Dipnoi 

 as well as of Polypterus — in addition to the more accessible material 

 of the other relatively archaic groups constituted by the Elasmo- 

 branchs, Actinopterygian Ganoids, and Urodele Amphibians. This 

 has rendered possible an all-round survey of the chief problems of 

 vertebrate embryology which would otherwise have been quite 

 impossible. 



As already indicated I do not intend this volume as a work of 

 reference on the details of vertebrate embryology : that role is 

 fulfilled by the wonderful and indispensable Handbuch edited by 

 0. Hertwig — of which incidentally I have made constant use and 

 to which I must express my sincere acknowledgments. Nor do I 

 attempt to give full historical accounts of the development of various 

 parts of the subject. The literature lists are merely guides to point 

 the way to the student who desires to extend his reading to original 

 papers. The dates given in these lists are as a rule the dates given 

 on the title-page of the complete volume, and are merely to facilitate 

 finding the particular paper : they must not be taken as giving the 

 actual date of publication of the individual memoir. 



I have to express my grateful thanks to various friends. As 

 regards the first three chapters I had the benefit of the wise counsel 

 of Mr. Walter Heape, who unfortunately however found himself com- 

 pelled by the exigencies of war work to withdraw from the Editorship. 



Various chapters have benefited by the help and advice of my 

 friend and colleague, Dr. W. E. Agar. The entire volume has been 

 read in proof by Mr. James Ohumley and Dr. Monica Taylor, to 

 both of whom I am deeply indebted. To Dr; John Love and to Dr. 

 Jane Robertson I am indebted for helpful criticism in regard to 

 special sections of the book. 



The illustrations which form a marked feature of the volume I 



