May defined species as “interbreeding individuals of natural population which are reproductively isolated by other such groups.” 

A Sertoli cell is a "nurse" cell of the testicles that is part of a seminiferous tubule and helps in the process of spermatogenesis; that is, the production of sperm.

Lysozyme is an antibacterial enzyme present in phagocytic cells, tears, saliva, sweat and other body secretions. It digest peptidoglycan in bacterial cell wall. 

Retina is the innermost thinnest and softest but most complicated tunic of an eyeball. It has two main layers an outer, simple epithelium of cuboidal or low columber and heavily pigmented (melanin contaning) cells and an inner layer. The inner layer in the optic region is thick and highly complex containing light sensitive elements nerve cells and fibres. This is the actual neuro-sensory layer of eye. It consists of outer or peripheral layer of rods and cones, middle layer of bipolar nerve cells and inner or medial layer of large ganglionic nerve cells. The layer of rods and cones conisists of the highly specialized neurosensory cells which are actual light sensitive cells, shaped like rods and cones and placed perpendicularly upon the pigmented epithelium.

Chloragogen cells of Pheretima posthuma is considered to be analogous to liver of vertebrates. Chloragogen cells are said to be excretory but their exact nature is not definitely known. Some zoologist are of the opinion that the chloragogen cells store the reserve food material and also manufacture glycogen form fatty acids. 

Turner’s syndrome is a human adnormality in which the affected individuals are pheotypically females but have rudimentary sex organs and mammary glands is caused by the absence of one X-chromosome. Turner’s syndrome is characterised by monosomy of XO type. 

Oparin-Haldane theory is also called chemical theory or naturalistic theory.A soup-like broth of chemicals formed in oceans of the early earth from which living cells are believed to have appeared, was termed by J.B. Haldane (1920) as ‘prebiotic soup’ (also called ‘hot dilute soup’).

Kaziranga national park is situated at Sibsagar district in Assam. This national park is famous for one horned Rhinoceros. 

Myelin sheath is a layer convering vertebrate nerve fibre. It is produced by flattened cells called Schwann's cells. Myelin serves as insulating material and causes slatatory conduction of impulses.  

The glomerular filtrate contains a large amount of wate rand other dissolved substances such as urea, uric acid, creatinine, amino acids, glucose, sodium, potassium, vitamins etc. Glomerular filtrate and blood plasma are similar except that glomerular filtrate does not have proteins. 

Animals have two systems of co-ordination i.e., nervous system and endocrine system. Neurones are the basic structural and functional units of the nervous system which spread through out the organism forming a complex communication network. Endocrine system release hormones for intercellular communication.

Archenteron cavity is found in gastrula. It is a primitive or embryonic digestive tube and communicating with outside by blastopore. 

Germ cells produce gametes and are the only cells that can undergo meiosis as well as mitosis. These cells are sometimes said to be immortal because they are the link between generations.

In Klinefelter's syndrome individual has 47 chromosomes (44+ XXY). Such persons are sterile males with undeveloped testes, sparse body hair, mental retardation, long limbs and some female characters.

There are three auditory ossicles in the middle ear. Malleus (hammer shaped), incus (anvil shaped) and stapes (stirrup shaped). Stapes is the smallest bone in the body. Malleus, incus and stapes are modification of articular, quadrates and hyomandibular bones respectively.

The mobility of a motile sperm is controlled by the mitochondria located in the middle piece. The mitochondria, being carrier of oxidative enzymes, are the “power plants” furnishing the work energy for the fibre system of the spermatozoon. 

The term enzyme was coined by W. Kuhne in 1678. J.B. Summer (1926) purified and cystallised unrease enzyme and suggested that enzymes are proteins J.H. Northrop and M. Kunitz (1930) prepared pure cystalline pepsin and trypsin and showed that these enzymes were also proteins. 

Cardiac muscles from the walls of heart. These are intermediate in structure being striated and involuntary. It is meant for automatic and rhythmic contraction. On the other hand skeletal muscles are striated and voluntary, they are most abundant and found attached to skeleton. They are supplied by somatic nerve and hence are under voluntary control.

Gene ecology is the branch of science which deals with the study of genetic composition and changes in relation to the environment. 

Binomial nomenclature is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin grammatical forms, although they can be based on words from other languages. It is described in Systema Naturae.

Malaria is caused by a protozoan Plasmodium vivax. Infective stage of Plasmodium is sporozoite. Schizogony is an asexual reproduction in which schizont is formed. It undergoes Pre-erythrocytic schizogony, exoerythrocytic schizogony and erythrocytic schizogony. During erythrocytic schizogony RBCs gets ruptured to release merozoits. At this stage malaria fever is felt. 

At cellular level, GH affects growth by controlling the production of m-RNA, within cells. This induces growth by increasing protein synthesis, increases cell division adn cell differentiation. 

Adipose tissue is a fat storing connective tissue which consists of several sphereical or oval adipose cells (= adipocytes or fat cells). There are two types of tissues white or yellow fat and brown fat. White fat cells are monolocular or unilocular whereas brown fat cells are multilocular. 

On the basis of morphology, 23 chromosomes in human beings are arranged in 7 groups.Tijo and Levan in 1956 demonstrated that human karyotype consists of 23 pairs (total 46) chromosomes.