112 THE DEVELOPMENT OF THE CHICK 



first place the position of the first somite was marked with a 

 deUcate electrolytic needle which left a permanent scar. The 

 eggs thus operated on were closed up and permitted to develop 

 to a stage of 10-12 somites or more; and then the mark was found 



F.B. 





jY.A^ 



^u. ep. 



S.2 



•S.J2. 



2> 



G- 



pr.sir. 



^t^■ 



Fig. 65. — Embryo of 12 s, from above, drawn in alcohol with re- 

 flected light. 

 au. ep., Auditory epithelium. Other abbreviations as before. 



to coincide with the first somite of the series. In the next place 

 it was possible by similar means to mark out the topography of 

 the embryonic head in the stage of one or two somites. Thus 

 it was determined that a mark made immediately in front of the 

 first somite formed appeared later in the region of the otocyst; 



