The effect of light and of darkness on blood cell number of the growing Albino rat.

Rats when 28 days old, at which time they were weaned, were placed in groups of five in the dark room, in the stock room, and given daily exposures of about two hours to direct sunlight and to diffuse daylight on the roof as described in the preceding paper. Counts were made of the blood cells at the ages given in the tables under the four conditions indicated. The red counts were made from the first drop of blood exuding after the tip of the tail was severed. Both a direct and an indirect method were used for counting platelets. Details will be given in a later paper. The same pipettes were used throughout for all counts.

Aside from reestablishing the “normal” count at these ages, the stimulating influence of direct sunlight and diffuse daylight, perhaps of fresh air, and the inhibitory influence of darkness on the reds and platelets is obvious from the table. The figures given for the reds represent the averages of counts on five rats, i. e., 30 rats under each of the four environmental conditions, at the ages indicated. The whites and platelets have been studied on a much larger scale.

The whites show a range of counts so wide that the average is of no real significance. The differential shows a decided drop in the lymphocytes of the rats kept in darkness for six months, and a corresponding increase in the polymorphonuclears. The significance of this change is questionable.