XXIV. A RENEWED STUDY OF THE GERMINAL LAYERS OF 

 THE CHICK. BY F. M. BALFOUR AND F. DEiGHTON 1 . 



(With Plates 43, 44, 45-) 



THE formation of the germinal layers in the chick has been 

 so often and so fully dealt with in recent years, that we consider 

 some explanation to be required of the reasons which have in- 

 duced us to add to the long list of memoirs on this subject. 

 Our reasons are twofold. In the first place the principal results 

 we have to record have already been briefly put forward in a 

 Treatise on Comparative Embryology by one of us ; and it seemed 

 desirable that the data on which the conclusions there stated 

 rest should be recorded with greater detail than was possible in 

 such a treatise. In the second place, our observations differ 

 from those of most other investigators, in that they were pri- 

 marily made with the object of testing a theory as to the nature 

 of the primitive streak. As such they form a contribution to 

 comparative embryology ; since our object has been to in- 

 vestigate how far the phenomena of the formation of the germinal 

 layers in the chick admit of being compared with those of lower 

 and less modified vertebrate types. 



We do not propose to weary the reader by giving a new 

 version of the often told history of the views of various writers 

 on the germinal layers in the chick, but our references to other 

 investigators will be in the main confined to a comparison of 

 our results with those of two embryologists, who have published 

 their memoirs since our observations were made. One of them 

 is L. Gerlach, who published a short memoir'"' in April last, and 



1 From the Quarterly Journal of Microscopical Science, Vol. XXII. N. S. 1882. 



2 " Ueb. d. entodermale Entstehungsweise d. Chorda dorsalis," Biol. Centralblatt, 

 Vol. I. Nos. i and 2. 



