Kaspar Wolff observed in a chick embryo that the individual species arose from gradual enfolding & differentiation of an unorganized tissue into organs.
Intercellular substance is the product of the cells and surrounds the cells, providing nutrition, taking up waste products, and providing shape to the body.
Important structures of a cell include the cell nucleus, ribosomes, Golgi complex, lysosomes, nuclear membrane, cell cytoplasm, endoplasmic reticulum, mitochondria, peroxisomes, centrioles or diplosomes, and cytoskeleton.
The cell nucleus is responsible for the metabolism, growth & reproduction of the cell and is present in all the cells of the body except for mature red blood cells & blood platelets.
The Golgi complex shows various forms but the most common is in the form of plates or lamellae or plates may dilate to bowl-shaped structure (cisternae).
Endoplasmic reticulum consists of minute fluid containing membrane bound tubules known as canaliculi cisternae or vesicies which may form a network (reticulum) or may be in the form of granules (when free are known as ribosomes or polysomes).
In epithelial cells, the threadlike structures are called tonofilaments; of fibroblast, cytoplasmic fibril; of nerve cells, neurofilaments; and of muscles, myofilaments.
Types of Epithelium include Simple (squamous, cuboidal, columnar), Pseudostratified columnar (composed of single layer of columnar cells arranged in a way that the epithelium appears to have several layers of cells, caused by difference in location of nuclei), and Stratified (consist of 2 or 3 more layers of cells).
Connective tissue types include Fibrous tissue in lamina propria (less dense in lamina propria of skin & mucosa), Areolar tissue (has small amount of cells, loose & thin network of fibrous intercellular substance held together by large amount of amorphous substance), Fat tissue, Hemopoietic tissue, Cartilage, and Bone.