112 THE DEVELOPMENT OF THE CHICK 



tion of the first somite was marked immediately after the 

 appearance of the first cleft with a delicate electrolytic needle 

 which left a permanent scar. The eggs thus operated on were 

 closed up and permitted to develop to a stage of from 10 



MB. 



//.£. 



F.B. 



3u. ep. 



3.^ 



S.I2. 



pr.str. 



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

 flected Hght. 

 au. ep.. Auditory epitheUum. Other abbreviations as before. 



to 25 somites; and then the mark was found 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; 



