Manchester Memoirs, Vol. I. (1906), No. 3. 



III. Remarks on the Germinal Layers of Vertebrates 

 and on the Significance of Germinal Layers in 

 general. 



By J. W. JENKINSON, M.A., D.Sc. 

 Exeter College, Oxford. 



(Communicated by Dr. F. W. Gamble.) 

 Received January 12th, igo6. Read January i^th, igo6. 



INTRODUCTION. 



That theory plays a part of predominant importance 

 in the acquisition of new facts is a matter of common 

 knowledge, and embryology has afforded no exception 

 to this rule. The cell theory, the recapitulation theory, 

 and theories of germinal layers have here not only 

 supplied a stimulus which has produced but also pro- 

 vided conceptions which have dominated and guided a 

 long series of developmental investigations. In the light 

 of these theories the embryologist has set himself to 

 complete the task already taken in hand by the com- 

 parative anatomist, the search for homologies, believing 

 that he has had in the homology of the germinal layers 

 an absolute and infallible criterion of the homogeny, or 

 community of descent, of the organs of the adult. 



Now while it is true that the morphologists of the 

 older school and their pupils, to say nothing of the more 



^ I have to thank Dr. Haldane, Dr. Ritchie, Dr. Bourne, and Mr. 

 Assheton for their kindness in reading through the manuscript of this 

 paper ; to Mr. Assheton I am particularly indebted for much friendly and 

 valuable criticism. 

 Alarch 26i/i, igo6. 



