Stimulation of brain development in chick embryo by elevated temperature.

Experimental chick embryos were incubated at 37.5°C till day 7 and after day 10, and at 40.5°C on days 7-10; their optic lobes and cerebral hemispheres at day 10 and at hatching were compared with controls incubated at 37.5°C only. Cell numbers at day 10 were directly counted by a new method involving formalin fixation and cell disaggregation by gentle sonication. At hatching, body weights, organ weights and organ DNA (cell numbers) were the same in experimentals and in controls, for both optic lobes and cerebral hemispheres, though the protein contents were significantly higher in experimentals. However, at 10 days (end of neuron proliferation) the weights and the cell numbers in experimentals were significantly higher. Two possible explanations have been offered: 1. Elevated neuron population in experimental animals at day 10 is followed by their elevated death rate, or 2. The increment in neuron number is permanent but at hatching it is overshadowed by the population of other cells.