Conditions Of Development
( Originally Published 1923 )
3. Conditions of Development.—The single cell which is the beginning of a new life, whether it is an independent individual, or a spore, or a fertilized egg, can develop into a complete plant or animal only in the presence of certain conditions. Like a living being in any later stage, this cell must have water, food, and oxygen; it must get rid of its wastes; and it can live only within certain limits of temperature. The one-celled animals and plants, which usually live independently in water, or as parasites upon larger plants or animals, or upon plant and animal remains containing food materials (as for example, milk, meat, fruit juices, etc.), get their supplies directly from their surroundings.
The one-celled stage of the one-celled animal or plant is about as well able to meet the conditions of life when it is first formed as it is likely to be later. The spore of a plant or an animal must also shift for itself from the first, although its has not the organs for making or obtaining food immediately. When we come to species that reproduce sexually we find that the newly formed individual, that is, the fertilized egg, is quite incapable of obtaining for itself the essentials for healthy development. The higher in the scale of life, the more dependent is the new individual upon others for its survival during the early period. In the evolution of life there has been not only a differentiation of reproductive cells from vegetative or growing cells, and of male gametes from female gametes; there has also been a progressive differentiation of infancy, with its dependence, from maturity, with its relative independence.
In the lowest plants and animals, every cell may be a food-getting and growing cell, and also a reproducing cell; among the highest species, some cells are related only to the growth of the body, while others are specialized as reproducing cells. In the lowest forms, the two gametes that unite in reproduction are quite alike in every way; among the higher forms, there is a great deal of difference between the male gamete and the female gamete. In the lowest species there is little or no difference between a newly formed individual and a mature, adult individual; among the highest species the difference is so great that the new individual can hardly be recognized, from its appearance, as belonging to the same species as its parent.