Abstract
SUGITA (1917) described in detail the postnatal changes of the albino rat brain. He pointed out its immaturity and the lack of differentiation at birth when only four layers of cortex are present (Fig. 1). SUGITA divided the subsequent development into three phases: Phase I, starting at birth and lasting 10 days, is a period of rapid growth during which brain weight increases from 250 mg to 960 mg and the thickness of the cortex from 0-74 mm to 1-7 mm. During this phase cells migrate from the ventricles, multiply, and become organized, with the result that all fundamental cortical layers are present at 10 days of age, and all the major areas of the adult cortex are recognizable. Phase II extends from the 10th to the 20th day after birth. It is characterized by cellular enlargement and the development of axons, but few new cells make their appearance. Myelinization begins in the latter half of this phase. Brain weight increases from 960 mg to 2-0 g and cortical thickness increases to within 4 per cent of the final adult average. By the 20th day the cortex is completely organized. Phase III extends from the 20th to the 90th day and is the period in which myelinization is completed. The rat brain is peculiar in that it continues to increase in weight throughout life; however, there is no appreciable increase in cortical thickness or cellular arrangement after the 20th day. Comparable physiological changes follow this anatomical develop- ment. Electrical activity recorded from the cerebral cortex is irregular and of low amplitude from 0 to 7 days of age, but by 10 days it is essentially similar to that in the adult (CRAIN, 1952). From a blind, poorly integrated, dependent animal at birth, the rat becomes, by 20 days, an active and autonomous unit. Recently FLEXNER and his co-workers (1948, 1950, 1951, 1953) have investigated the biochemical changes during the "critical period" as it occurs (in utero) in the guinea-pig. They demonstrated rapid rises in ATPase, succinic dehydrogenase and cytochrome oxidase activity in whole brain homogenates. The levels of phosphocrea- tine and ATP were also studied throughout gestation. SUGITA (1917) in his investi- gation of the pre- and post-natal cellular changes of the rat cerebral cortex, found a rapid increase in the average cell and nuclear diameter of the pyramidal cells of layers III and V during the first 10 days after birth. These findings were similar to those of PETERS and FLEXNER (1950) for the guinea-pig. POTTER (1945) measured
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