in a series of normal 12- or 13-day-old chick embryos in which the neuroanatomical tracer wheat germ agglutinin conjugated to horseradish peroxidase ( wga-hrp ) was injected into one eye , about 25,000 neurons were found to be retrogradely labeled in the isthmic region of the brain .
C0019941
horseradish
C0078473
wga-hrp