APPEARANCE OF THE SOMITES IN THE CHICK. 



127 



periments performed to throw light on the order of development 

 of the somites. In all cases the results support the view that 

 only the incomplete somite arises anterior to the first mesodermic 

 cleft. In the cases cited, the oldest embryo was carried to the 

 sixteen-somite stage, but other embryos were allowed to de- 



■ ■ '• ' ■■•■•■;.'?:??'. " ' ■ 



Fig. 4. Twenty-two hours old when operated on, and then incubated twenty and 

 one half hours. There are sixteen pairs of somites. The glass pin is located in the 

 incomplete somite on the left side, at O. X 2 °- 



Fig. 5. Twenty-two hours old when operated on, and then incubated ten hours. 

 The injury is shown at O, and ten pairs of somites are present. X 2 °- 



velop until twenty or twenty-five somites appeared, with the in- 

 jury still found in the rudimentary somite. In another series the 

 injury was made just posterior to the first cleft. In such cases 

 the first complete somite was either greatly disturbed or entirely 

 destroyed. 



