APPEARANCE OF THE SOMITES IN THE CHICK. 1 29 



nterior to the first mesodermic cleft, in the time occupied by the 

 formation of six or seven proto vertebrae posterior to that cleft." 1 



The first indication of somites is a depression on the dorsal 

 surface of the mesoderm lying at the sides of the notochord, just 

 anterior to the fore-end of the primitive streak. This is soon fol- 

 lowed by a corresponding- indentation on the under side (Figs. 

 6 and 7, a). Between the cleft and the anterior end of the prim- 

 itive streak, indications of the two succeeding clefts are already 

 present (Fig. 6, b and c). In front of the cleft, the strip of meso- 

 derm which extends forward into the head gradually thins out 

 anteriorly. At some points this thinning out has progressed more 

 rapidly than at others, giving rise to shallow, transitory depres- 

 sions (Figs. 6 and 8, s), which have been wrongly interpreted as 

 clefts by Miss Piatt. 



The mesoderm immediately anterior to the first cleft is destined 

 to form the rudimentary somite, and the manner in which this 

 structure arises can be followed with no small degree of certainty. 

 At first the mesoderm is quite uniformly thick (Fig. 9, t), but by 

 the time three or four somites are formed its posterior edge has 

 become much enlarged. Apparently this thickening takes place 

 at the expense of the mesoderm just anterior to it (cf. Figs. 9—1 1, 

 t). It should be remembered, however, that this incomplete so- 

 mite is never so large as the others, really being, as Miss Piatt 

 states, only a half-somite. It must be considered as a part of the 

 head mesoderm, from which it never becomes separated (Figs. 

 9-12, r). 



Miss Piatt used the relative depths of the clefts as a means for 

 determining the priority of somites. After very justly criticising 

 Kupffer for judging either the fourth or fifth somite to be the 

 oldest on account of its size, she says : " I think enough has been 

 said to show that neither the size of the proto vertebrae, their rel- 

 ative distance from the primitive streak, nor yet their obliquity to 

 the main axis, is a sufficient ground to warrant a decisive answer 

 to the question in regard to the order of their development," 2 

 and I should add, that neither can the depth of the cleft be taken 

 as a criterion for ascertaining seniority. The error into which one 



1 Loc cit., p. 178. 



2 Loc. cit., p. 174 



