The trophectoderm is a layer of cells on the outer edge of a blastocyst. These cells provide nutrients for the developing embryo, facilitate implantation into the uterine lining and become part of the placenta. Answer : (d)

Metadiscoidal: Placenta is diffused first but later becomes discoidal. Example: human beings, apes and rabbit. Answer : (a)

In mammals, the blastula is referred to as a blastocyst. The blastocyst contains an embryoblast (or inner cell mass) that will eventually give rise to the definitive structures of the fetus, and a trophoblast which goes on to form the extra-embryonic tissues. Answer : (d)

Brain, spinal cord is derived from ectoderm. Heart and notochord is derived from mesoderm. Answer : (a)

Liver and pancreas develop from endoderm. Answer : (b)

Meoblastical cleavge is seen in insects. Insect eggs are centrolecithal in nature. Answer : (c)

Epidermis of skin, nails, enamel of teeth, skin glands, eye lens etc., originate from ectoderm. Answer : (c)

Discoblastula: In such blastula the floor of blastocoel is formed of yolk sac, while roof is formed of epiblast. Example: birds and reptiles. Answer : (a)

Non-cleidoic eggs: Egg is not covered with shell. Egg membranes are squashy in these eggs e.g. All viviparous animals and in oviparous animals that put down eggs in water. For example – fish. Answer : (b)

The extraembryonic membranes of the mammalian embryo are derived from trophoblast, after the cleavage of the zygote, the blastocyst is formed. Answer : (b)

The primary gut that forms during gastrulation in the developing zygote is known as the archenteron, the gastrocoel or the primitive digestive tube. Answer : (c)

Cleavage begins soon after karyogamy. The first cleavage occurs along primary axis. It takes about 30 hours. The first cleavage divides the egg into two blastomeres. One is slightly larger than the other. Therefore, first division is holoblastic but slightly unequal. The difference in size makes subsequent divisions asynchronous. Mammalian cleavage is slow, irregular, and is asynchronous. Answer : (b)

Cleavage starts in the zygote almost immediately. So the cleavage in the fertilized egg of humans starts in the fallopian tube. Answer : (c)

Cleavage is a series of rapid mitotic divisions of the zygote, charcterised by absence of growth of daughter cells, which convert the the single celled zygote into multicellular structure called blastula (blastocyst). Answer : (c)

Eye lens are derived from ectoderm. Muscles of eye ball originated from mesoderm. Answer : (d)

All the three options are derived from ectoderm. Answer : (d)

Transformation of the blastocyst into gastrula with primary germ layers by rearrangement of the cells is called gastrulation. In all the triploblastic animals three germ layers - ectoderm, mesoderm and endoderm are formed. These germ layers are formed by charcteristic movements of some of the cells. Thses cellular movements are known as morphogenetic movements. Answer : (a)

Blastopore is found in the gastrula. It is the pore which leads to the development of gastrula by forming an opening into the developing embryo or gastrula. Answer : (d)

Notochord originates from mesoderm. Answer : (a)

Holoblastic, unequal:- the cleavage in which segmentation line divides egg into two unequal halves. This type of cleavage is seen in amphibians. Rana (frog) and salamander are amphibians. The egg of amphioxus is isolecithal and the cleavage is of holoblastic type. Answer : (c)

Hypolast and epiblast form a disc called embryonic disc. Gastrulation involves the rearrangement and migration of cells from the epiblast. A primitive streak which is a faint on dorsal surface of the epiblast is formed. It elongates from the posterior to the entire part of the embryo. Posterior part of the embryo is caudal end. Answer : (c)

Meroblastic cleavage: The incomplete cleavage in telolecithal or megalecithal eggs of animals, such as birds and reptiles. Answer : (c)

Cleavage divisions produce a solid ball of cells called morula. Answer : (a)

At the onset of puberty, estrogen plays a role in the development of so-called female secondary sex characteristics, such as breasts, wider hips, pubic hair and armpit hair. Estrogen also helps regulate the menstrual cycle, controlling the growth of the uterine lining during the first part of the cycle. Answer : (c)

Centrolecithal: Yolk is concentrated in the centre in the centre and the cytoplasm is peripheral. Example insect eggs. Cockroach is an insect. Answer : (c)

Mammals which lay eggs have sufficient amount of yolk. While placental mammals has little to no yolk. Egg laying mammals are called prototherians. Answer : (c)

Interphase in cleavage division is short and does not involve growth so that the resulting blastomeres become smaller in size as their number increases. Answer : (a)

The one cell embryo undergoes a series of cleavage divisions, progressing through 2-cell, 4-cell, 8-cell and 16 cell stages. The cells in cleavage stage embryos are known as blastomeres. Answer : (a)

Yolk sac: It is formed below the embryo as a vascularized membrane. It contains fluid, not yolk. Its wall first consisits of trophoblast and endoderm (primary yolk sac), but later comes to consist of endoderm and mesoderm (secondary yolk sac). The yolk sac is nonfunctional in human beings except that it functions as site of blood cell formation unitl about the 6th week, when the liver takes over this role. Thereafter, the yolk sac strats to shrink. Answer : (b)

With the formation of blastocoel, morula is converted into blastula. Inner cell mass now looks like a small knob at one pole. The knob give rise to embryo and is called embryonal knob. Trophoblast cells in contact with the embryonal kob are called cells of Rauber. Answer : (d)