Embryonic Development

the development of the body of an animal that goes on within the egg membranes, either outside or inside the mother’s body.

Embryonic development is preceded by a period of proembryonic development, when the ovum grows, takes shape, and matures. A period of postembryonic development follows it. In the course of embryonic development a multicellular organism, consisting of various organs and tissues and capable of independent existence, is formed from a single, relatively simply organized egg cell. In some animals, such as echinoderms, the embryo leaves the membranes at very early stages and the principal processes of development occur during the post-embryonic period. In all animals, embryonic development includes fertilization or (in parthenogenesis) activation of the ovum, the divisions of cleavage, gastrulation, organogenesis, and the exit from the membranes, or birth. The structure of the ovum and the character of embryonic development vary according to the biology of reproduction (the number of ova, type of fertilization, duration of embryonic development, sources of nourishment of the embryo, and the degree of care for the young).

The start of embryonic development, fertilization, takes place either in the mother’s body or in an aqueous medium. The male germ cell, a motile spermatozoon, reaches the ovum and penetrates it, often through special openings in the membranes called micropyles. The ovum and the spermatozoon contain single (haploid) sets of chromosomes; in fertilization, the paternal and maternal chromosomes unite in a single nucleus, reconstituting their normal double (diploid) number. The biological meaning of fertilization consists in the exchange of genetic information between the animals of a single population, since each new organism combines within itself the inherited characters of both parents. After fertilization, during the period of cleavage, the ovum successively and repeatedly divides, first into large and then into smaller and smaller cells, or blastomeres; subsequently a multicellular embryo, or blastula, is formed, usually with an interior cavity. As a result of the cleavage divisions, conditions are created for the emergence of differences between parts of the embryo. This is called differentiation. Cells formed from different sections of the ovum receive different cytoplasm (which determines primary differentiation) and become capable of movement, which ensures the formation of the organs of the future organism.

During gastrulation, there is separation of the germ layers, which are distributed by means of various shifts in such a way that the endoderm is inside, the ectoderm outside, and the mesoderm is between the two. Gastrulation proceeds differently in different animals, but it results in the creation of a general design of the organism’s structure that is similar even in taxonomically distant groups of animals.

During the period of organogenesis, the germ layers divide into the rudiments of organs and systems; large rudiments differentiate into smaller ones, thus creating a more and more complex body structure. Organogenesis is accomplished basically through various cell migrations and the differentiation of the cells themselves. In order to effect the embryo’s exit from the membranes or birth, an enzyme that dissolves the membranes is synthesized at the end of embryonic development and adaptations appear that aid in breaking the shell, where present.

The early embryos of different animals resemble one another more than do the adult animals, since evolutionary changes have to a greater degree affected the later stages of individual development. Thus, the course of embryonic development to some extent resembles the course of evolution. However, this similarity of embryos is only relative, chiefly because at each stage of development embryos are adapted to their environments. In the embryos of fish a large yolk sac is formed; in birds, a yolk sac and special embryonal organs, the allantois.and the amnion; in mammals, in addition, a trophoblast and a placenta are formed.

Present-day embryology sets itself the task of investigating the mechanisms of embryonic development and the causal relationships that determine differentiation. In the early stages of embryonic development, embryo cells are capable of developing in many directions. After the effect of a series of factors the cells gradually become determinate—that is, they acquire the ability to develop in a single, definite direction. As they develop, the cells become increasingly well differentiated and specialized in structure and function. Thus, for example, in the part of the ectoderm that forms the rudiments of the nervous system, the brain becomes isolated, part of it develops into the rudiments of the eyes, part of which are isolated as retinas, in which the rods and cones, which have characteristic, narrowly specialized structure and function, in turn become differentiated.

Embryonic development is determined by the hereditary apparatus of the cell, which is enclosed in the nucleus. The chromosomes, which are contained in the nucleus, consist of many genes, each of which carries information on the structure of one of the proteins. The characters of the parent organism, which are coded in the genes, are realized in the course of embryonic development. Upon dividing, cells receive a complete set of genes, but in each tissue only a portion of the genes function—those that provide for synthesis of the proteins characteristic of that tissue. Hence, on the genetic level, the process of embryonic development consists in the “switching on” of certain genes, as a result of which the appropriate ribonucleic acid (RNA) is synthesized, which transfers hereditary information from the nucleus to the cytoplasm, where the molecule of the specific protein is synthesized. The genes begin to function as early as the proembryonic period of development, when there occurs in the growing egg cell an active accumulation of yolk and of all types of RNA necessary to insure synthesis of proteins during early development. In the course of embryonic development, the various genes that determine the synthesis of the proteins necessary for each type of differentiation are switched on in the various rudiments at some stage of development. Thus, the realization of heredity in the course of embryonic development consists in the fact that the factors of differentiation determine the switching on of specific genes, which cause the synthesis of the appropriate proteins, which in turn ensure the differentiation of the cells. The role of many proteins in this process is already known: hemoglobin is synthesized during the differentiation of erythrocytes, myosin during the formation of muscles, and enzymes and hormones during the development of the glands. Not yet studied, however, are the proteins that determine changes in the forms of cells, their movement, and their behavior in the course of embryonic development. Also unknown are the mechanisms by which differentiation factors lead to the switching on of specific genes.