It has become possible to distinguish two essential subdivisions of the PRA complex, namely developmental and degenerative. According to whether the photo-receptor disease (i.e. elements in the retina which convert the light energy into electrical energy and give the so called vision in the brain) commences before or after maturation of the retina. As a generalisation it has been noted that in the developmental diseases eg Collie, onset of night blindness tends to occur before the occurrence of obvious changes in the retina when viewed by an ophthalmoscope, while in the degenerative condition eg Labrador, a degree of retinal change is usually present at the time of night blindness onset.