At any time after or during {TAIL ~ staining}, the sample is illuminated with a wavelength of light selected to give a detectable optical response, and observed with a means for detecting {HEAD ~ the optical response} 
At any time after or during { TAIL ~ staining } , the sample is illuminated with a wavelength of light selected to give a detectable optical response , and observed with a means for detecting { HEAD ~ the optical response }
OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD