162 THE DE\ ELOPMENT OF THE CHICK 



source of much of the mesenchyme, and it is an interesting ques- 

 tion whether or not such mesenchyme has a different fate from 

 that of different origin. Nothing definite, however, is known in 

 regard to this, owing to the impossibiUty of separating the various 

 kinds after they have once merged. 



The Neural Crest in the Region of the Somites. The neural 

 crest is very sUghtly developed in the region of the first five so- 

 mites, which is correlated with the fact that these somites are 

 devoid of ganglia. But the mode of origin is the same through- 

 out the somitic region. Shortly after the closure of the neural 

 tube in any region the neural crest forms an aggregation of cells 

 in the roof, more or less sharply separated from the remainder 

 of the tube both by the arrangement of the cells and also by their 

 lighter stain (Figs. 107, 109, 112, 113). The early history may be 

 followed in a single embryo, by comparing the conditions opposite 

 the last somite with that of more anterior somites where develop- 

 ment is more advanced. Figs. 107, 108, 109, 110 represent 

 transverse sections through the twenty-ninth, twenty-sixth, 

 twentieth, and seventeenth somites of a 29 s embryo. In Fig. 

 107 the cells of the crest are extending towards the upper angle 

 of the somite, with which they are connected by protoplasmic 

 strands. The aggregation in the roof of the neural tube is thus 

 decidedly diminished; the lateral wings of the crest lie in the angle 

 between the neural tube and the ectoderm. In the twenty-sixth 

 somite (Fig. 108) the lateral wings extend farther from their point 

 of origin, and appear to have a more intimate connection with 

 the myotome. In the more anterior and older somites, twenty 

 and seventeen (Figs. 109 and 110), the process has progressed 

 much farther and the neural crest cells are completely expelled 

 from the neural tube, which closes after them (Fig. 110). A yet 

 later stage is shown in Fig. Ill, through the twenty-third somite 

 of a 35 s embryo. 



The dorsal commissure uniting the right and left sides of the 

 crest ruptures, and the cells of the crest aggregate so as to form 

 a pair of ganglia in each somite. Thus, although the neural crest 

 is primarily a median structure, it becomes divided into two 

 lateral halves, and although it is primarily a continuous structure 

 it becomes divided into a series of pairs of metameric ganglia. 

 The fate of the interganglionic commissures is conjectural. The 

 ganglia are ill-defined from the mesenchyme when they are first 



