470 THE AMERICAN NATURALIST [Vol. XLII 



pearance of the somites to this case, it would seem that 

 this embryo must have had the first of the two anterior 

 somites, protovertebra a, already partly formed, at the 

 time of the operation, and that there should, therefore, 

 have been one more, protovertebra b, to arise in front of 

 this. But no such somite appears in Fig. 2, and its ab- 

 sence led to the conclusion that there is no such somite 

 as protovertebra b, in other words, that but one somite is 

 formed in front of the first cleft which appears. The 

 simplest explanation of Miss Piatt's error is that she 

 mistook protovertebra a in sections of older embryos for 

 protovertebra b. This is much more probable than that 

 she could have mistaken, as Mr. Patterson suggests, 5 the 

 most posterior transitory shallow depression in the head 

 mesoderm for the first cleft. 



If it be objected that the experiment does not prove 

 that one or two somites may not arise in front of the one 

 first formed, it may be said that if they do arise, the rate 

 of their formation, compared with the rate of formation 

 of those that appear behind, is contrary to the descrip- 

 tion of this process by Miss Piatt, according to whom the 

 rate of formation is much greater behind the first formed 

 somite than it is in front. Either then somites are not 

 formed in front, or, if they do arise, the description of 

 the rate of their formation is not correct. 



In conclusion, then, this experiment, in proving that 

 not more than two somites could arise in front, showed 

 the inaccuracy of Kupffer and Benecke's estimate of the 

 number formed. 



It showed further, in regard to Miss Piatt's work, 

 either that her description of the time of formation of the 

 somites was incorrect, or, if development proceeds ac- 

 cording to her account, that no somites, except the rudi- 

 mentary one, arise in front of the first cleft. 



Thus the result of the experiments, with reference to 

 the condition of the problem up to the time when Mr. 



* Loc. ext., pp. 129, 132. 



• Loc. cit., p. 177. 



