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The entire earth shows animal fauna. As per the distribution of the fauna the earth is divided into Zoogeographical regions. Australian region contains Australia, Newzealand, New Guinea and nearby Islands in the Pacific ocean Walice” includes Celebas islands in Australian region. But now a days it was kept in…
This region includes S. America, Central America, Mexico and West Indies. This is divided into 4 sub-regions. Chillian sub region Brezelian sub region Mexicon sub-region West Indies sub-region. This region shows tropical conditions. The southern part of South America shows temperate zones, because of these varied environmental…
NEARCTIC REGION The region above the tropics of North America is the Nerctic region. Towards north it includes New found land, the Arctic Archipelago and Greenland. This region is known as ‘Head quarters of urodels'. Nearctic region shows great variations in climatic conditions and temperatures. In the North…
PALAEARCTIC REGION This is the largest of the Six regions is covering an area of 14,000,000 square miles. In this region Europe, Asia, North of Himalayas and Northern parts of Africa. This region shows wide range of temperature fluctuations. It also shows wide range of fluctuation in the…
Zoogeography: The study of geographical distribution of animals is Zoogeography. The vertebrates have characteristic patterns of distribution on the land masses. Zoogeography is helpful in understanding the evolution. The increase in number of animals by reproduction makes them distribute in all directions. Dispersal continues until barrier is reached.…
FOETAL MEMBRANES OF CHICK In amniotes the developing embryo in order to grow properly foetal membranes are formed. When few membranes are produced by mother, it should take more care for their survival. If the number are more, care will be less. In amniotes when the developing embryo is…
The mesoderm is proliferated by primitive streak. It is formed as two layers. In front of the primitive streak an area without mesoderm is present. It is called proamnion. After 48 hours of incubation the proamnion is also occupied by mesoderm. The mesoderm is divided into dorsal and intermediate and…
The second step in gastrulation is the formation of primitive streak. At the posterior region of area pellucida in the mid dorsal line primitive streak will appear as a thickened area. It starts eight hours after incubation. The thickening is because convergence of cells of blastoderm towards the centre. Usually in…
Formation of endoderm: Endoderm of hypoblast develops as a single layer of cells in side of blastocoel. After the formation of endoderm, upper layer is called epiblast. There are different theories to explain the formation of endoderm. Infiltration theory: This was proposed by Peter in 1923. According to this theory…
In the upper region of oviduct fertilization will takes place. One sperm will penetrate into hens egg and fertilizes with the egg. The fertilized egg will travel through oviduct. It takes nearly 22 hours. Hence the early development of egg will take place in oviduct. Cleavage: Cleavage is restricted to…
Shape: The 48 hours chick embryo will show this shape? (In mirror view). In this stage of development the head region shows prolific growth. Because of which the embryo shows this peculiar shape. Flexure: In the head region bendings are seen. These bandings are called flexure. This bending is…
At 24 hrs. incubation period the chick embryo is oval in shape. The primitive streak is seen. From the hensen's node a notochord is developed. In the 24 hrs. chick embryo four pairs of somities are present. NEURALFOLDS: On either side of mid dorsal line the ectoderm becomes…
The fully formed and freshly laid hen's egg is large. It is 3cm. in diameter and 5cm. in length. It contains enarmous amount of yolk. Such egg is called macrolecithal egg. The egg is oval in shape. The ovum contains a nucleus. It is covered by yolk free cytoplasm. It…
EMBRYOLOGY OF CHICK The following list clearly describes about embryological changes in chick egg during incubation. FRESHLY LAID HEN'S EGG CHICK EMBRYO AT 24 HOURS CHICK EMBRYO AT 48 HOURS GASTRULATION IN CHICK-I GASTRULATION IN CHICK-II - FORMATION OF ENDODERM GASTRULATION IN CHICK-III - FORMATION OF PRIMITIVE STREAK…
The following list shows most commonly used symbols in biology and their meaning arranged in a alphabetical manner. A is a biology symbol for Androecium A Adenine ABA Abscisic acid ATP Adenosine triphosphate Aa Heterozygous dominant AAAA Autotetraploid AABB Allotetraploid A Angstrom ( = 0.0001 of a micron) ADP is a biological symbol for Adenosine diphosphate ADA Adenosine…
Adaptive Convergence in Evolution In this type of evolution, the organisms with completely different body organization possess superficial resemblance. It means that the organisms of different classes acquire similar characteristics independently and separately to avail similar environment. This is also known as ‘parallel evolution’. Examples: The most common…
