Brendan Massey

GENES AND DEVELOPMENT

This chapter describes genes and their development. Development of nearly all animals is determined up to the phyletic stage, by the spatial organization of the egg cytoplasm and its mRNA, proteins, and other important molecules that were stockpiled during oogenesis. These early embryological processes are usually fuelled by food and energy reserves that have also been passed from the mothers surplus. Liver, kidney, or skin cells all have nuclei with the same genes; yet they all exhibit different forms and functions, in terms not only of the major proteins secreted or contained but also in their shapes, sizes, division rates, susceptibility to radiation, or cell poisons. Theories of differentiation used to start by assuming that cells were initially similar and attempt to account for later differences. The acquisition of differentiation may be sudden, but is usually progressive. The other case in which de-differentiation has been postulated is that of regeneration. Because all individuals of a species have different cell antigens the virus cannot mutate to mimic the natural antigens, because these vary from individual to individual as do the antibodies which recognize them.

EMBRYOS AND EVOLUTION

This chapter describes embryos and their evolution. Animal evolution has produced and is producing a great variety of animal forms. The ways in which this variation appears in the life history of the animals concerned is part of the province of embryology. Caenogenetic modifications are those variations that benefit the young form, embryo or larva, and have no particular significance for the adult. Very often they are lost at hatching, birth, or metamorphosis. The converse situation, paedomorphosis, is the early acquisition of the adult state while the organism still has the larval modifications and general body form. The insects are probably derived by paedomorphosis from a form resembling modern millipedes, which have a six-legged stage in their embryology. The vertebrates may have been derived from echinoderm-like ancestors by retention of their larval characters. In many ways the primates, and especially the anthropoid apes and man, may be said to be neotenous mammals. Deviation, on the other hand, describes those changes that are adaptations, primarily by adults, to meet adult requirements. Because of the conservative nature of developmental changes, the early stages of two related animals usually resemble one another more than later stages.

THE NERVOUS SYSTEM

This chapter discusses the nervous system. The induction and early development of the primitive neural tube have already been described for the frog and are similar in most vertebrates except the teleost fishes. The changes that occur in the gross morphology of this primitive tube to become the definite central nervous system are also remarkably uniform among the vertebrates. The cells within the wall of the spinal cord have a special way of developing the histology and circuitry of the adult nervous system. When the neural tube closes most of these cells undergo mitosis; the nucleus of each cell drops toward the inner surface of the tube and undergoes a transverse division. Along the whole length of the vertebrate head and trunk and usually tail, the mesoderm on either side of the spinal cord and brain becomes divided up into a series of somites. This results in the appearance of segmental ventral roots, corresponding in number and position with the somites.

THE URINOGENITAL SYSTEM

This chapter describes the urinogenital system. The rudiment of the excretory system in the tadpole arises well before hatching as a longitudinal thickening of the somatic mesoderm on each side of the coelom, below the myotomes. Near the mouths of the funnels, a sacculated outgrowth of splanchnic layer appears. It is known as the glomus and becomes filled with blood from the dorsal aorta. The frog kidney, mesonephros, consists of a series of paired masses of cells in the nephrotomes in these middle segments, each of which develops into one of the kidney tubules, having at one end an opening into the segmental duct and at the other a Malpighian capsule with a glomerulus. In frog, and in amphibians generally, the segmental duct tends to lose its organizing capacity after it has traversed about two-thirds of its journey to the cloaca; no more mesonephros results behind this as the tube passes under or through the remaining posterior nephrotomes. The functional kidney of mammals, birds, and reptiles is called metanephric and its development is more complex.

THE NASAL ORGAN

The chapter describes the nasal organ. The development of this organ is deceptively simple to describe. In the jawed vertebrates, the appearance of the anterior end of the neural plate occurs in response to influences from the prechordal plate and the front end of the gut; these cause the anterior epidermis to thicken and form symmetrical nasal placodes. Sensory nerve cells appear in the epithelium of the placodes and send processes back into the brain, where in response to this invasion the olfactory lobes appear and grow. These nerves do not seem to make many cross-connections on their way into the brain and it seems, therefore, that integration of the incoming signals must take place in the circuitry of the brain tissue itself, rather than peripherally as in the ear and eye. Between patches of sensory cells the epithelium becomes ciliated and mucus-secreting and is usually thrown into complex folds.

THE MUSCULAR SYSTEM

This chapter provides an overview on the muscular system. This system of vertebrates arises partly from the segmental somatic mesoderm, partly from the more ventral unsegmented mesoderm and partly from ectomesenchyme derived from neural crest cells. Along each side of the spinal cord, and usually of the brain, is a row of mesodermal somites. As these develop each becomes divided, probably under the inductive influence of neural tissue medially, skin laterally, and perhaps gut ventrally, into four regions. These are myotome, sclerotome, dermatome, an intermediate cell mass soon becoming nephrotome and gonotome. All of the segmental muscles of vertebrates arise in the embryo as paired concentrations of somatopleure on either side of the nerve cord and notochord, myotomes. Myotomes of the three most anterior segments provide the extrinsic eye musculature, myotomes four and five degenerate more or less completely; six, seven, and eight contribute to the gill musculature and may perhaps grow ventrally and contribute to the muscles of the lower jaw and even the tongue.

DEVELOPMENT OF THE CHICK

This chapter provides an overview on the development of the chick. The egg is telolecithal, but the situation is complicated to some extent by the addition of large quantities of albumen, the shell, and other tertiary membranes. The blastoderm is visible as a circular white area on the surface of yellow yolk, underneath the vitelline membrane, really the fertilization membrane. After a sequence of vertical divisions, occasional horizontal divisions occur that separate the lower layer or hypoblast from the upper layer or epiblast. The posterior end of the hypoblast probably has a more complicated origin from the epiblast. This blastula should be regarded as having nominal blastocoele between epiblast and hypoblast. When the eggs are incubated, however, there may soon be noticed an odd appearance in the center of the epiblast. The appearance is very yellow yolk, underneath the vitelline membrane, really the fertilization membrane.

FURTHER CONTRIBUTIONS BY THE MOTHER

This chapter provides an overview on the contributions provided by the mother to her offspring. It is usual for the mother to provide all of the instructions for early development, as well as the wherewithal to achieve it. Father only contributes a genotype that is not involved in development until the phyletic stage. The mother provides one set of chromosomes to the zygote nuclear chromosomal complement. She also provides the egg with an architecture taht will take it through early development to the point where these nuclear genes can begin to determine the course of its further development. There are many informational DNA molecules other than those associated with the mothers nucleus and these may be passed to offspring. The mothers phenotype will also affect the start in life that she can give the embryos. Although this depends to some extent on her genotype, so many accidents are involved during her maturation that it is very difficult to tie any individual property of a good mother solely to her genetics. Some mothers may accidentally have properties not at all related to their genetics, which will contribute good or ill to their embryos. Such effects range from mothers who expose embryos to nicotine or thalidomide, to bottle or breast feeding or even the choice of a language.

THE ALIMENTARY SYSTEM

This chapter focuses on the alimentary system. In the frog, as in other mesolecithal forms, the gut is a complete tube right from its inception. During development, these two endodermal pockets lengthen; that anterior to the AIP forms the foregut, that behind the PIP the hind gut. The anterior end of the gut tube approaches the ectoderm of the anterior end ventrally, and this ectoderm thickens and forms a pocket, the stomodeum. The liver arises, usually from two outpushings of the gut tube, about midway between the AIP and the transverse septum. The pancreas arises from several outpushings near the origin of the bile duct and it spreads anteriorly over the forming stomach. The anterior end of the stomach is anchored by the esophagus in the transverse septum; the posterior end is also anchored via the bile duct and hepatoenteric ligament. Therefore, as the stomach extends in length it has no choice but to curve, always into the left half of the anterior peritoneal cavity.

THE SEQUENCE OF DEVELOPMENTAL EVENTS

This chapter discusses the sequence of developmental events. There are also evolutionary consequences of the association and dissociation of developmental events. Not only is the sequence of developmental events in closely related organisms often different, but also one organism may produce a structure not comparable to anything in the other, or one may omit characteristic structures that the other possesses. Such interpolations into the developmental sequence are called caenogenetic and are very common. Perhaps, the largest single category of them is the embryonic membranes. The embryonic membranes of mammals also show extreme heterochrony in that they are usually formed from the egg before the embryo itself appears.