276 The Nature of Biological Diversity 



PLATE I. Figures 6 to 11 illustrate the formation and migration of the pregangli- 

 onic thoracic column in the spinal cord of the chick embryo between 3 and 11 days. 

 All sections are from material impregnated with the Cajal-De Castro silver tech- 

 nique. The microphotos in this and in the following plates are not retouched. FIG. 

 6. 3-day embryo. FIG. 7. 4-day embryo. FIG. 8. 5 x /2-day embryo. FIG. 9. 6-day 

 embryo. FIG. 10. 7-day embryo. FIG. 11. 11-day embryo. See explanation in text. 



