Question

List and describe the early stages of animal development, beginning with fertilization. Then describe two similarities and two differences between early plant and animal development.

Answer 1

Solution:

Early embryonic development include three stages starting from fertilization then cleavage and blastula stage and then gastrulation.

Fertilization is the process in which an egg and sperm fuse to form a zygote. Each of them contain one set of chromosomes. Only one sperm must fuse with an egg so that offspring will have a diploid set of chromosomes. In mammals a layer of extracellular matrix consisting of glycoprotein protect the egg is called zona pellucida. When sperm binds to this layer, a series of biochemical reactions occurs known as acrosomal reaction. Acrosome contains digestive enzymes that initiate degradation of the glycoprotein layerand allow the sperm to enter the egg plasma membrane. To ensure no more sperm will enter the egg, the egg releases proteins in the other locations to prevent other sperm from fusing with the egg. The fusion of egg membrane and sperm membrane creates opening through which sperm nucleus is transferred into the ovum.The nuclear membrane of egg and sperm break down and the two haploid genomes condense to form diploid genome.

Cleavage and Blastula Stage - The multiple round of cell division are going on rapidly in the cell causing cleavage. After cleavage is done over hundreds of cells the embryo is called blastula. The blastula is sperical shapesurrounding with fluid filled cavity called blastocoel. Mammals at this stage forms a structure called blastocyst which is characterized by inner cell mass called embryoblast which undergo to form embryo and outer layer called trophoblast helps the placenta and nourish the embryo.

Gastrulation - The next stage of embryonic development is the body plan in which the cells of blastula rearrange themselves spatially to form three layers of cells. This process is known as gastrulation. During this process the blastula folds itself to form these three layers namely the endoderm, the ectoderm and the mesoderm. Each layer is germ layer and each will differentiate into different organ system. Endoderm give rise to columnar cells of digestive system and many internal organs, ectoderm give rise to the nervous system and the epidermis and the mesoderm give rise to the muscle cells and connective tissue in the body.

The two similarities between the early plant development are:

1) The process of fertilization occurs in both plants and animals. For example, in moss plant fertilization occurs by sperm swimming to the egg and in vertebrate animals also reproduction is done in the same way of sperm and egg.

2) Both plant and animal cell undergo meiosis and mitosis in producing the new organism.

The two difference between the early plant development are:

1) The embryonic development of plant and animals are different. Plant show immediate growth, animals do not show this growth at first. The cells of plant differ immediately but animal cells do not show this.

2) Plants are solid and animals have two body cavities. Animals bend inwards and make a body cavity and digestive tract etc while plant do not form an inner space.