Easyanswers.blog.com

Placenta: The organ which is formed by the combination of foetal tissue and maternal tissue for the exchange of nutrients and gases are called as placenta.     The foetal tissue is the part of foetus and maternal tissue is from uterine wall of mother. Placentation: The intimate relation between uterine wall of mother and foetal tissue which is  mainly from chorionic membrane of foetus for the exchange of nutrients, minerals, respiration and excretion is called Placentation.

Neoteny: The occurance of both adult and juveline characters together in an organism is called as Neoteny.    This is seen in Amphibians. Example is Axolotol larva of Ambylostoma.   Neoteny can also be defined as the loss of sexual maturity in the organisms and retain of larval characters along with adult characters. Why Neoteny is observed in an organisms? Neoteny occurs due to lack of thyroid stimulating harmone which stimulates thyroid to produce thyroxine. The hypothalamus of the brain fails to stimulate the thyroid gland to produce growth harmone which stops the metamorphosis. Axolotol of Ambylostoma can again get metamorphose by giving injections of Iodine or by giving shocks of Thyroxine harmone.

Prokaryotes: Prokaryotes includes bacteria, blue-green algae, mycoplasmas and PPLO. They are smaller and multiply more rapidly. They vary greatly in shape and size. All prokaryotes have a cell wall and cell membrane. No well defined nucleus. Matrix is filled with cytoplasm. In addition to genomic DNA, it has smaller DNA's called Plasmids which is resistant to antibiotics. No organelles are present except ribosomes. It has unique Inclusions. Inclusions is the reserve material stored in the cytoplasm. Differentiated form of cell membranes called mesosomes is the characteristic of prokaryotes. Eukaryotes:  Eukaryotes includes protists, plants, animals and fungi. Compare to prokaryotes, they multiply slowly. The cells are not identical. All eukaryotes do not have cell walls. Well defined nucleus with a nuclear envelope. Cytoplasm have many compartments due to presence of cell organelles. Genetic material is organised into chromosome. Many membrane bound cell