e during the first three years.2)Maternal deprivation for a period varying for days within the first and second years, and weeks or months during the third and fourth years.3)Changes from one mother-figure to another during the first four or five years.With respect to the reality of the critical developmental periods, it is, of course, well known that such critical developmental periods exist in the development of the body, with the principle well established that the earlier the interference with their development the more widespread will the disturbance in growth be. Tissues must reach a certain stage of differentiation before they can respond; later such tissues achieve a certain relative fixity, so that they can yield. Prenatal environmental influencesFor many years , in fact, it was believed that the baby in the uterus was completely insulated form all outside influences, but now we know that is not entirely true. Environmental influences ranging from radioactivity and stress in the outside world to drugs, chemicals, hormones, and viruses in the mother’s bloodstream can affect prenatal development. Even though the placenta acts as a filter, keeping the blood of the mother and the fetus from mixing, a number of potentially dangerous substances can pass through it. And, of course, if the mother’s blood lacks the nutrients required by the growing fetus, it may not develop fully.Lack of proper nourishment or the introduction of harmful substances will affect the fetus in different ways at different stages of prenatal development. This is because the body organs and parts develop at different speeds but through definite phases. First they go through a phase of rapid multiplication in the number of cells. Then there is an increase in both the number of cells and in cell size. In the third and final phase of development, cell size continues to increase rapidly but cell division slows down. The second phase, when a bo...
