Development of Sense Organs of Animals Lamellated corpuscles are differentiated from a mass of mesenchymal cells clustering around a nerve termination. These multiply, flatten and give rise to concentric fibrous lamellae, the Pacinian corpuscles. A tactile corpuscle originates with a looping plexus of terminal nerve fibres located just beneath the epidermis and this plexus becomes […]
Bone formed in this process has a spongy character and consists of irregular plates of bone, which branch and unite with one another and in the meshes is enclosed embryonic connective tissue rich in blood vessels forming the red marrow.
Development of Muscular System of Animals The muscular system consists of specialised cells – the muscle fibres which are of three varieties, skeletal, plain and cardiac. These are differentiated from the myoblasts which are derived from the mesoderm. The smooth muscle fibres of the iris and sweat glands are ectodermal. The muscles originate in the myotomes. The
Development of Foetal circulation in animals Oxygenated blood returning from the placenta, enters the embryo by the large umbilical vein and is conveyed to the liver. From here it is conveyed to the posterior vena cava for the most part directly through the ductus venosus and to a certain extent through the hepatic- veins. The
Development of Arteries and Veins of animals DEVELOPMENT OF ARTERIES The aorta is dilated at its origin forming the aortic sac from which the ventral aortae ascend and continue as right and left dorsal aortae. The dorsal aortae unite behind to form a single descending aorta. From the aortic sac, five pairs of aortic arches
Development of Heart of Animals The Heart is a specialized blood vessel with a lumen and thick muscular walls. In mammals, it arises from a cardiogenic plate located in front of the head in the splanchnic mesoderm and beneath the pericardial coelom. Due to a reversal in position, the heart primordium comes to lie above the pericardial
Development of Haemopoiesis in animals The first site of blood formation is in the area vasculosa of the wall of the yolk sac. Here between the extraembryonic splanchnic-mesoderm and endoderm the blood islands of Pander develop. These are solid masses of mesenchymal cells united by cellular stands to form a network. As they become differentiated the outer
Development of Cardiovascular system in animals Both the blood vessels and blood cells arise from mesenchyme. The earliest formative tissue of this kind is the angioblast which differentiates in the mesodermal wall of the yolk sac where groups of angioblasts form the blood islands of Pander. Originally solid, these soon hollow out and in this process the
Development of Mammary gland of animals An ectodermal thickening called milk line or mammary ridge extends on either side as longitudinal bands between the bases of limb buds. The primordial band is developed in the pectoral region (Primates, elephants) inguinal region (undulates) or throughout (axilla to groin region) in carnivores and pigs. The primordium of
Development of Genital System in animals The genital organs are undifferentiated when they first appear (indifferent period), either by gross or microscopic appearance and hence during this period the term gonad is applied to them. The gonad appears within a thickening called the genital ridge on the medial aspect the urino-genital ridge. The ventral surface
