Xo. 499] SUCCESSION OF SOMITES IN CHICK 467 



forming somites. The somite behind the cleft is called 

 the first one in the series. The one anterior to it, proto- 

 vertebra a, forms slowly, while four or five are making 

 their appearance behind. After five or six somites are 

 visible in all, another, protovertebra h, arises slowly in 

 front of a. Protovertebra h is said to be rudimentary, 

 never becoming completely cut off from the mesoderm in 

 front. 



It will be noted in this account that although two 

 somites are described as arising in front of the one first 

 formed, in reality there is but one to be considered— 

 protovertebra b — for protovertebra a makes its appear- 

 ance at the same time with the one behind it. An ex- 

 amination of Miss Piatt's sections 2 would lead one to 

 agree in the main with her account of the order of forma- 

 tion of the somites, except in regard to the appearance 

 of protovertebra b, whose growth has to be followed in 

 a series of sections from different embryos at succes- 

 sively older stages. The difficulty of identifying a grow- 

 ing somite in this way casts much doubt upon even its 

 existence, and it was to test the question therefore that 

 these experiments were devised. 



The aim of the experiments was to mark or destroy, 

 in embryos with a small number of somites (not more 

 than five or six) the most anterior somite on one side, 

 and so to determine whether any more were later formed 

 in front of this. The ideal stage to have secured would 

 have been that of an embryo with only a single pair of 

 somites, but repeated failures to obtain this condition 

 verified the statement made by Miss Piatt, 3 founded upon 

 a study of sections, that the first cleft occurs between 

 two forming somites. An operation, then, even as early 

 as at the time of the first cleft would have had to take 

 into account the first two pairs of protovertebra?. 



The methods employed in the experiments were in gen- 



8 Loc cit.,' p. 177. 



