12th Class

The male reproductive system consists of a scrotum, a pair of testes, vasa efferentia, a pair of epididymis, a pair of vasa deferentia, a pair of ejaculatory ducts, a urethra, a penis and certain accessory sex glands. Reproductive organs (1) Scrotum : The scrotum is a pouch of pigmented skin arising from the lower abdominal wall and hanging between the legs. The testes originate in the abdominal but latter, during the seventh month of development, descend permanently into the respective scrotal sac through passages termed inguinal canal.     A spermatic cord connects testis with abdominal cavity. It consists of connective tissue that encloses an artery, a vein, a lymph vessel, a nerve, cremaster muscle and a vas deferens. A testis rests in it chamber over pad called gubernaculum. The scrotal sac of male homologous to more...

Post gastrulation involves two main process. Neurulation is process of laying the neural plate to form the nervous system. The establishment of the germ layers initiates the final phase of embryonic development, i.e., organogenesis. The latter involves differentiation and specialization of groups of cells in the individual germ layers. The cells of such groups change their form and give rise to morphologically recognizable tissues and organs of the new individual. The groups of differentiated cells separate from their germ layers in an orderly manner and with unique precision. Separation of the differentiated cell groups may occur by folding off from the germ layer or by migration of cells individually and reaggregation at a new place. In this manner, the primordial cells of the germ layers gradually and accurately give rise to the tissues and organs of the offspring. By four weeks after fertilization, the embryo has a simple heart, limb more...

Embryonic development involves following dynamic changes and identifiable process. (1) Gametogenesis : It involve the formation of haploid sex cells or gametes called sperms and ova from diploid primary germ cells called gametogonia present in the reproductive organs called gonads (testes and ovary). It is of two types (i) Spermatogenesis : Formation of sperm. (ii) Oogenesis : Formation of ova (2) Fertilization : It involve the fusion of haploid male and female gametes to form diploid zygote. The fusion of gametic pronuclei is called Karyogamy while the mixing of two sets of chromosomes of two gametes is called amphimixis. (3) Cleavage : It includes the rapid mitotic division of the zygote to form a single layered hollow spherical larva called blastula and its formation is called blastulation. (4) Implantation : The process of attachment of the more...

(i) Definition: Placenta is defined as a temporary intimate mechanical and physiological connection between foetal and maternal tissues for the nutrition, respiration and excretion of the foetus.   (ii) Structure: Human placenta consist of chorion only. Hence, it is called a chorionic placenta. Allantois remains small. The allantoic blood vessels, however, extend to vascularize it. A large number of branching villi from the vascular chorion penetrate the corresponding pits, the crypts, formed in the uterine wall. The latter becomes very thick and highly vascular to receive the villi. The intimate connection established between the foetal membrane and the uterine wall is known as the placenta. The placenta has two parts: the part contributed by the foetus, i.e., chorionic villi, is called the foetal placenta; and the part shared by the mother, i.e., part of uterine wall, is termed the maternal placenta. The chorionic villi more...

Reproduction : Reproduction is the ability of living organism to produce a new generation of living individuals similar to themselves. Basic features of reproduction : All organisms reproduce. Modes of reproduction vary in different organisms. However, all modes have certain common basic features. These are (1) Replication of DNA. This is the molecular basis of reproduction. (2) Cell division, only mitotic, or both mitotic and meiotic. This is cytological basis of reproduction. (3) Formation of reproductive bodies or units. (4) Development of reproductive bodies into offspring. Types of reproduction : These are of two main types (1) Asexual (Non-gametic)                           (2) Sexual (gametic) (1) Asexual reproduction Definition : Production of offspring by a single parent without the formation and fusion of gametes is called asexual reproduction. The young one receives all its genes from one parent. Asexual reproduction is also known as agamogenesis or agamogeny. It involves only mitotic cell more...

Definition : Ageing is the slow deterioration in the structure and function of body cells tissues and organs of an animal and starts after the adulthood. Gerontology : The field of developmental biology that deal with the process and problems of ageing is known as gerontology – (Gr. geron = old man; logos = discourse). The scientists involved in the science of ageing are called gerontologist. Life cycle and life span : In all metazoan animals, the life cycle includes two developmental period; embryonic period (pre-natal developmental period) which extends from zygote to offspring till hatching or birth, and post embryonic period (post-natal developmental period)- which includes growth, adulthood, reproduction, ageing. Thus, the life cycle comprises five main events : birth, growth, maturity, old age and death, that follow in the sequence named. Maximum life span is the maximum number of years survived by any member of a species, while average more...

Definition : Death may be defined as the permanent cessation of all the vital function in an organism. Characteristics (1) It is the last event in the degenerative processes of ageing. (2) Death of an organism involves the death of the body cells. But all the cells of the body do not die at the same rate e.g., ciliated cells lining the respiratory tract of mammals continue to beat their cilia for a long time even after animal's death. Brain cells of body are last to die. (3) There is no natural death in the protozoans e.g., Amoeba. (4) Death involves widespread cell breakdown and cell death. (5) It usually occurs due to lack of oxygen supply to body tissues. Causes of death : Causes of death are many. These can be separated into following main categories : (1) The weakening of the body tissues and of vital organs like heart, more...

Meaning and definition of growth : Growth is an important properties of all living organisms. All organisms grow from a young stage to an adult stage. Growth is a permanent increase in dimensions of the body and its parts. It results from the addition to the body tissues. Growth at different levels (1) Molecular level : At molecular level, the growth involves synthesis of new molecules and their aggregation into organelles and storage products in the cells. (2) Cellular level : At the cellular level, the growth involves. (i) Cell expansion (hypertrophy) : Increase in the size of the cells due to addition of new cell material, called protoplasm. (ii) Cell division (hyperplasia) : Increase in the number of cells by cell division. (iii) Cell differentiation : Specialisation of cells for specific roles, in its broad sense, growth includes. (iv) Matrix formation : Addition of intercellular materials, termed apoplasmatic substances, more...

Definition : It is that post-embryonic morphogenetic phenomenon which when temporarily stimulated brings about repair of the damaged cells/Tissues, or replacement or redevelopment of severed body parts or reconstruction of whole body from a small body fragment. Capacity for regeneration : Among animals, power of regeneration was first discovered in Hydra by Trambley, in 1740. The capacity of repeated regeneration, though, present throughout the animal kingdom, but to varying degree. It is more marked in the lower animal than in the higher animals. Among invertebrates, protozoans, sponges and coelenterates, the regeneration capacity is very high. In higher animals, regenerative ability is much greater in the embryonic and larval stages than in the adult. In man, it is restricted to healing of injured tissues such as skin, muscles, bones, blood vessels and nerves; the lost organs cannot be regenerated. Types of regeneration : Regeneration is of two main type - Reparative more...

Community are never stable but keep on changing. This relatively definite sequence of communities over a period of time in the same area is called ecological succession. Types : Succession is of two types : (1) Primary succession : It includes changes which occur when living things become established on a previously uninhabited area such as a newly exposed sea floor, lake sediments or sand dunes. (2) Secondary succession : It occurs where early communities have been damaged, leaving a few organisms and considerable organic matter. These remnant species, along with some new ones, regenerate a new community.

• According to another classification, the succession is of following types :

(1) Autogenic succession : When the succession has begun, the vegetation itself is responsible for replacing itself by changing existing environmental conditions. (2) Allogenic succession : When in succession other conditions (not vegetation itself) are responsible for more...