THE ORDER OF APPEARANCE OF THE ANTERIOR 

 SOMITES IN THE CHICK. 1 



J. THOS. PATTERSON. 



A. Introduction. 



The statement that in the chick, somites arise in front of the 

 first somite formed in the series has been widely accepted by em- 

 bryologists. This view, nevertheless, is not in accord with our 

 knowledge concerning the early development of birds, for it is 

 well known that differentiation usually begins at the anterior end 

 and progresses posteriorly. 



Although workers in this field agree that somites arise anterior 

 to the one first formed, yet they differ as to the exact number. 

 Thus Balfour ('85) states that there is one, while His ('68) and 

 von Baer ('28) have estimated it at two. Kupffer and Benecke 

 ('79) would lead one to believe that there were at least three or four. 

 So far as I am aware the latest work done to determine this num- 

 ber is by Miss Piatt ('89), who concludes from a study of sections 

 that there are two, or, to be more exact, one and a half. 



From the results of certain experiments, made in connection 

 with an experimental study of the early development of the 

 pigeon, the writer was led to believe that no somites were formed 

 in front of the first mesodermic cleft, except, of course, the so- 

 called rudimentary or incomplete anterior cephalic somite. At 

 the suggestion of Professor Lillie I have performed a number of 

 experiments to test the validity of this view. These experi- 

 ments, in connection with others, were conducted on a farm 

 in Ohio, where I had at my disposal the eggs from fifty laying 

 hens. It was possible, therefore, to collect and incubate the 

 eggs hourly. 



It gives me pleasure here to express my thanks to Professor 

 Lillie for his kindness in sending all the necessary equipment for 

 this work from these laboratories, and for his valuable criticisms. 



'Unless otherwise indicated, the word somite will be used throughout this work 

 to mean protovertebra. 



