Chapter 11 Reproduction in lower vertebrates is characterised by females of the species laying large numbers of small, yolk‐laden oocytes in an aquatic environment with subsequent discharge of sperm by males in the same location. Fertilisation in these circumstances is described as external. The yolk provides the nutritional requirements for the developing embryo while oxygen is obtained from the aquatic environment and metabolic waste is discharged into the same watery habitat. In species such as amphibians, in which the embryo is provided with a limited supply of nutrients, an intermediate free‐feeding larval stage develops which subsequently undergoes metamorphosis and grows to the adult form. The large number of oocytes produced in these circumstances compensates for the high mortality associated with this type of reproduction. Species which produce a small number of large oocytes, with high yolk content and in which the young are at a more advanced stage of development when emerging from the egg, have an enhanced probability of survival. In this type of development, which is commonly encountered in cartilaginous fish, some bony fish, reptiles, birds and some mammals, a larval stage does not occur. Species at a more advanced stage of evolutionary development produce oocytes which are retained within the body of the female. The male deposits sperm into the female tract and fertilisation is internal. In these species, the yolk content of the egg is relatively low and the embryo receives its nutritional and oxygen requirements from the maternal vascular system. This pattern of reproduction is found in most vertebrate species. Species in which the developing embryo is retained within the body of the female and the young are born alive are termed viviparous. The term oviparous is used to describe those species in which embryos hatch from eggs incubated outside the body. A number of species exhibit an intermediate developmental pattern between oviparity and viviparity. In these species, termed ovo‐viviparous, yolk‐laden oocytes are retained within the mother’s body and the embryo receives its nutritional requirements from the oocyte itself, its respiratory needs supplied by the maternal vascular system. Embryos which develop in an aquatic environment rely on their own egg‐derived food supply as they acquire oxygen from the water and their metabolic waste diffuses into their aquatic surroundings. The yolk is stored either within the endodermal cells of the ventral gut wall, as in amphibians with medialecithal oocytes, or as an extracellular mass ventral to the developing embryo, as in fish, reptiles and birds with megalecithal oocytes (Fig 11.1). In embryos which develop from megalecithal oocytes, the body wall of the embryo grows around the yolk mass forming a trilaminar sac, the yolk sac. The mesoderm of the trilaminar yolk sac becomes vascularised and the enclosed nutrients are absorbed via the endodermal layer and then transported to the embryo by the vitelline vessels. The development of the head, tail and lateral body folds raises the embryo above the yolk, so that a demarcation becomes evident between the embryo itself and the yolk sac. The initial broad connection between the embryo and the yolk sac becomes constricted until they are connected only by a stalk. As the yolk is consumed, the sac diminishes in size until it is eventually withdrawn into the abdominal cavity through the umbilicus (Fig 11.2). Figure 11.1 A. Amphibian embryo with yolk stored within endodermal cells of ventral gut wall. B. Avian embryo with yolk stored as extra‐cellular ventral mass. Figure 11.2 Formation of the trilaminar yolk sac in a chick embryo. Coelom formation resulting in the separation of the extra‐embryonic mesoderm into splanchnic and somatic layers. The evolution of terrestrial vertebrates necessitated changes in the developing embryos to enable them to survive in non‐aquatic environments. Although some species, such as amphibians, have evolved to a terrestrial existence, they return to an aquatic environment for egg laying. Other species, such as turtles, lay in damp surroundings from which the eggs can derive water. Birds and reptiles have adapted to their non‐aquatic environment by laying eggs with protective membranes or shells secreted by their reproductive tracts. In addition, albumen, also secreted by the female reproductive tract as an additional source of nutrients, is enclosed in the shell. Terrestrial species also evolved additional extra‐embryonic membranes, the amnion, the chorion and the allantois, to provide further protection, conserve water and store waste products. In mammals, there is a tendency towards a reduction in protective membranes and in yolk content. The oocytes of most eutherian mammals are 80 to 140 μm in diameter and miolecithal. Structures or tissues which develop from the zygote and which do not form part of the embryo itself and are of functional importance only in embryonic life are called extra‐embryonic or foetal membranes. Their function is concerned with the supply or storage of nutrients, respiratory exchange, excretion and mechanical protection of the embryo. In some species, they are also associated with the transfer of immunoglobulins from the mother to the embryo which confer passive immunity. In mammals, foetal membranes are involved in hormone production and formation of the placenta. As these membranes are solely required for embryological development, they are either shed or absorbed at hatching or birth. In the avian oocyte, the developing embryo is positioned on the large yolk mass. Towards the end of gastrulation, the ectoderm spreads peripherally from the area opaca over the yolk mass. Endoderm forms beneath the ectoderm, and the bilaminar layer advances beyond the area from which the embryo develops. These two layers form the wall of the bilaminar yolk sac. Mesoderm then extends between the ectoderm and endoderm forming a vascular trilaminar layer around the yolk, the trilaminar yolk sac (Fig 11.2). Formation of the coelom splits the trilaminar layer into an outer avascular somatopleure layer and an inner vascular splanchnopleure layer which is in contact with the yolk and forms the definitive yolk sac. With the formation of the body folds, the embryo becomes distinguishable from the more peripheral extra‐embryonic somatopleure and splanchnopleure. The extra‐embryonic tissue continues to extend peripherally until it almost surrounds the yolk. Blood vessels develop in the mesoderm of the trilaminar layer and establish communications with vessels which form in the splanchnic mesoderm of the definitive yolk sac. Two distinct areas, a proximal vascular area and a distal non‐vascular area, can now be recognised in the yolk sac. Vessels on the periphery of the vascular area anastomose forming a vessel known as the terminal sinus, which demarcates the boundary between the vascular and non‐vascular areas (Fig 11.2). The vascular system of the yolk sac establishes connection with the embryonic vascular system following the formation of left and right vitelline veins which join the embryonic venous system, and left and right vitelline arteries which join the embryonic arterial system. With the development of a pulsating cardiac tube and of anastomoses between the extra‐embryonic and intra‐embryonic blood vessels, a functional vascular system becomes established approaching 48 hours of incubation. Although the yolk sac is connected to the embryonic midgut at the yolk stalk, yolk is not transmitted to the embryo by this route. The endodermal cells produce digestive enzymes which convert the yolk to a form suitable for absorption by the vitelline vessels. As the embryo develops, the splanchnopleure undergoes folding, which extends into the yolk mass thus increasing the absorptive area (Fig 11.3). Oxygen, which diffuses through the shell, is taken up by the vitelline blood vessels and carbon dioxide is eliminated by the same route. At approximately the 19th day of development, the yolk sac is withdrawn into the abdominal cavity. Figure 11.3 Chick embryo showing folding of splanchnopleure and convergence of amniotic folds. As they are so closely related in their origins, the amnion and chorion are usually considered together. Both membranes are formed by a dorsal folding of the extra‐embryonic somatopleure (Fig 11.4
Foetal membranes
Development of the foetal membranes
Avian species
Yolk sac
Amnion and chorion
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