A Renewed Study of the Geeminal Layers of the 

 Chick. By F. M. Balfour, LL.D., F.B.S., 

 Trinity College, Cambridge, and F. Deighton, 

 B.A., St. Peter's College. (With Plates YII, 

 YIII and IX.) 



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 induced us to add to the long list of memoirs on this 

 subject. Our reasons are twofold. In the first place the prin- 

 cipal 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 con- 

 clusions 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 primarily made with the object of testing a theory as 

 to the nature of the primitive streak. As such they form a con- 

 tribution to comparative embryology ; since our object has been 

 to investigate 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 

 the other is C. Koller, who has published his memoir~ still more 

 recently. Both of them cover part of the ground of our investi- 



^ "Ueb. d. entodermale Entstebungswiese d. Chorda dorsalis," 'Biol. 

 Centralblatt,' vol. i, Nos. 1 and 2. 



^ " Untersuch. nb. d. Blatterbildung im Hiihnerkeim," * Archiv. f. 

 mikr. Anat.,' vol. xx, 1881. 



11 



