36 FRANK R. LILLIE! 



mined after the operation. It agrees, however, fairly well with 

 the original operation diagrams which were simply rough esti- 

 mates of the amount destroyed by the operation. It should be 

 distinctly understood that the figure is a result of study of the 

 anatomy of the defective embryos. In further explanation it 

 should be added, that the numbers to the right are the numbers 

 of the experiments ; those to the left of the somites. The lines 

 leading to the numbers indicating the experiments are drawn 

 across the body to indicate, that, in the experiment in question, all 

 back of the line was destroyed ; the cross on each line marks the 

 junction of reference and operation lines. 



A word concerning the enumeration of the somites ; the somite 

 numbered I possesses a short anterior process, that is probably 

 an independent somite. It is, however, so inconspicuous in many 

 embryos that it seemed better for the present purpose not to 

 enumerate it. Somites 17, 18 and 19 are especially marked 

 because they are the wing-somites, i. e., the somites that will 

 form the major portion of the bone and muscle of the wing. 

 The twenty-sixth is the first leg somite. By this reckoning, there 

 are three cephalic somites, or, reckoning in the incomplete one, 

 four. 



Referring to Fig. I again, it will be seen that the organs 

 destroyed in such operations are : (i) The hind end of the neural 

 tube ; (2) the hind end of the notochord ; (3) the mesoblastic 

 segmental plate and often certain of the posterior mesoblastic 

 somites ; (4) the hind gut including the rudiment of the allantois ; 

 (5) the hind end of the Wolffian body and ducts. 



I may say at once that no true regeneration of these structures 

 takes place. (For discussion of this, see p. 50.) Therefore the 

 problems of interest became narrowed down to the differentiation 

 of the uninjured parts, and my attention has been particularly 

 drawn to the behavior of the mesoblastic somites. In the somites 

 we have an originally homonymous series of structures, the seg- 

 mentation of which becomes strikingly heteronymous as develop- 

 ment proceeds, some entering into the head, others the neck, 

 others the wing, the thorax, abdomen, leg and tail. Definite 

 somites, i, e., somites in a definite numerical position in the series, 

 form the skeleton and muscles of each of these regions. It would 



