The study of the development or morphogenesis and growth of the invertebrates. The same general principles of development apply to the invertebrates as do to the vertebrates. Much of the basic knowledge of embryology has been the result of studies on the invertebrates. A common phenomenon in the invertebrates is the release of a free and independent form, the larva, before development is completed. The larvae vary considerably and are characteristic of the different animal groups.
Embryonic development begins with the formation of the gametes in a specialized cell bearing the haploid or N number of chromosomes. The process of spermatogenesis consists of a stage of cell proliferation, followed by a period of progressive concentration and streamlining. The essential, heredity-determining material of the chromosomes is packed tightly into a tiny nucleus. The cytoplasm forms the locomotor apparatus, usually a single long flagellum with a centriole at its base and a mitochondrion nearby, as well as an organelle (acrosome) for penetrating the egg coverings. Millions upon millions of such cells are produced in the testis, where they remain quiescent until they are spawned. See also Sperm cell; Spermatogenesis.
The egg is specialized for large size and protection of its contents, with less concern for numbers and none at all for motility. In addition, its cytoplasm possesses intrinsic capacities for differentiation and building in exact accordance with the specifications contained in its chromosomes, so that a spider egg, for example, always produces a spider and never a fly. The reserve building and energy-yielding materials are stored in the egg cytoplasm as minute spheres or platelets of yolk, a stable lipoprotein substance. Eggs are large cells even without this inert material. At the end of their growth period, when they have accumulated the full amount of yolk, they are huge in comparison to the body cells of the parent animal. The largest are found among the arthropods, while some marine animals have very small eggs.
During the growth period, while the egg cell is actively synthesizing yolk and increasing the amount of cytoplasm, it has a very large nucleus, the germinal vesicle. When it reaches full size, however, and this synthetic activity subsides, the nuclear membrane breaks down, releasing its contents into the cytoplasm. The two successive nuclear divisions of meiosis follow, but the cytoplasm, instead of dividing equally, pushes out one of the daughter nuclei each time as a polar body. These two minute bodies have no further function in development. The chromosome material left in the egg forms the egg pronucleus, which is ready to unite with the sperm pronucleus. The zygote nucleus, formed by their union, is comparable in size to those of the body cells.
The eggs of invertebrates are always surrounded by a protective covering. In some forms the eggs are laid in batches which may be enclosed in a leathery sac or embedded in a mass of jelly. In other cases each egg has its own separate membranous case, a layer of jelly, or a more complex system of protective structures. Sperm and egg of each individual species have been shown by light and electron microscopy to be characteristic of its particular species. Mechanisms have evolved which normally prevent the egg of one species from being fertilized by the sperm of another. Reproduction among the invertebrates takes place in a variety of ways which differ widely from phylum to phylum.
Fertilization has been studied in several invertebrates, but especially in the sea urchin. Egg and sperm of the sea urchin are released into the seawater. The eggs are covered with a jelly coat to which a receptor on the plasma membrane of the fertilizing sperm binds. The plasma and outer acrosomal membranes of the sperm break down and fuse with each other as a Ca2+ influx occurs; the hydrolytic enzymes within the acrosome are released to lyse the egg coat. Next the inner acrosomal membrane everts by the polymerization beneath it of actin, and forms the acrosomal process which makes contact and fuses with the egg plasma membrane. The egg responds to the sperm by forming a fertilization cone. The sperm nucleus enters the egg, and its DNA swells to form the male pronucleus. As the sperm binds to the receptors on the egg plasma membrane, the electrical potential of the egg membrane changes and establishes a rapid block to prevent further sperm from making contact and fusing with the egg. With sperm-egg membrane fusion, Ca2+ is released to activate a series of changes in the egg. As changes occur at the egg surface, the egg pronucleus and the sperm pronucleus with associated astral rays move toward the center of the egg, where they fuse.
The union of the two pronuclei (syngamy) marks the completion of the fertilization process. The fusion forms the zygote nucleus, with the full complement of chromosomes, and the dormant egg cell has been aroused to start the series of changes which will produce a new sea urchin. With different time schedules and allowance for the individual characteristics of each species, these basic processes of sperm entry, aster formation, and syngamy make up the complex phenomenon of the fertilization reaction as it occurs in all animals.
The fertilized egg, or zygote, sets about at once to divide the huge mass of the egg into many small cells in order to restore the usual ratio between the amounts of nuclear and cytoplasmic substances. The energy for these repeated mitoses comes from the yolk, which also furnishes at least part of the materials required for synthesis of new nuclear structures. During this cleavage period, which commonly occurs during the first 12 h after fertilization, the blastomeres, as the cleavage stage cells are called, divide more or less synchronously. Generally, cleavage follows one of several patterns characteristic for large groups of animals and often correlated with the amount and mode of distribution of the yolk. Small eggs, which contain little yolk, divide completely and usually very regularly, forming a mass of cells that shows spiral, bilateral, or radial symmetry.
Insect eggs contain a large store of yolk. Following fertilization, the nuclei alone divide and move apart in the layer of cytoplasm after each division so that they distribute themselves all around the egg. After nine such nuclear divisions have taken place (producing 512 nuclei), the cytoplasm also cleaves at the next division, forming a single layer composed of about 1000 cells surrounding the central yolk mass. See also Cleavage (embryology).
Among all the invertebrate forms except the insects, the result of 6–10 successive cleavage cycles is the formation of a sphere (blastula) composed of small cells which lie in a single compact layer around a central cavity (blastocoele).
The end of the brief blastula stage occurs when the process of gastrulation begins. In its simplest form, this consists in an indenting (invagination) of the blastula wall in the vegetal region. Meanwhile cell division is going on steadily, and since the larva has as yet no way of taking in solid food from the outside, all the form changes which occur during this period are accomplished with the material originally present in the fertilized egg. The only addition is water (blastocoele fluid) and such dissolved substances, mostly salts, from the environment as can enter through the cell membranes. As the blastomeres become smaller and the blastular wall becomes correspondingly thinner, cells are provided to extend the vegetal indentation into a pocket. With the appearance of this structure (primitive digestive tract) the larva becomes two-layered, possessing an outer layer, the ectoderm, which will later produce the nervous system as well as the outermost body covering, and an inner layer, the endoderm, from which will be formed the lining of the functional digestive tract and its associated organs and glands. As the primitive digestive tract extends into the blastocoele, its opening to the outside becomes smaller and is known as the blastopore. See also Blastulation; Gastrulation.
At this time the first few cells belonging to a third body layer, the mesoderm, make their appearance. This mesodermal tissue spreads out between the ectoderm and endoderm, and in all phyla more advanced than the flatworms it splits through its center into an inner and an outer layer. The cavity thus formed within the mesoderm is the true body cavity in which the various internal organs lie. The outer layer of mesoderm becomes closely applied to the inner side of the ectoderm, forming body-wall muscles and other supporting layers, while the inner layer of mesoderm surrounds the endoderm with layers of muscle. The organs of circulation, excretion, and reproduction, as well as all muscles and connective tissue, are eventually formed from this mesodermal layer which surrounds the endoderm.
So far it is possible to summarize the development of invertebrate animals as a group but beyond this point each subgroup follows its own course, and these are so widely divergent that every one must be considered separately. Meaningful generalizations are not even possible within a single class in some cases, as attested to by the various modes of development occurring among the Insecta, some of which proceed directly from egg to adult form, while others go through an elaborate series of changes. In very many species there is a sharp break in the life history when the larva, after passing through a number of morphological phases which lead from one to the next with a steady increase in size and complexity, abruptly forms a whole new set of rudimentary adult organs which take over the vital functions. This metamorphosis represents the end of the larval period. The tiny animal which it produces is for the first time recognizable as the offspring of its parents. See also Insecta.
