Table of Contents
Introduction Page 3
Cleavage and blastulation Page 5
Morula and blastocyst Page 6
Implantation and placentation Page 7
Placentation Page 8
Formation of the three primary germ layers Page 9
Embryonic acquisition of external form Page 11
Initiation of pregnancy: another new individual is made when the components of a powerful sperm combine with those of a fruitful ovum, or egg. Various motile sperms are saved in the vagina, pass through the uterus, and attack the uterine (fallopian) tube, where they encompass the ovum. After it enters the tube, the ovum loses its external layer of cells as a consequence of activity by substances in the spermatozoa and from the lining of the tubal wall. Loss of the external layer of the ovum permits various spermatozoa to enter the egg's surface. One and only sperm, however, regularly turns into the treating creature. When it has entered the substance of the ovum, the atomic leader of this sperm differentiates from its tail. The tail continuously vanishes, yet the head with its core survives. As it goes at the core of the ovum (at this stage called the female pronucleus), the head expands and turns into the male pronucleus.
"The two pronuclei next approach, meet midway in the egg cytoplasm, and lose their nuclear membranes. Each resolves its diffuse chromatin material into a complete, single set of 23 chromosomes. Centrioles apparently supplied by the sperm, appear, and a mitotic spindle organizes with the two sets of chromosomes arranged midway on it—ready to proceed with a typical mitosis. This climax in the events of fertilization creates a joint product named the zygote. It contains all the essential factors for the development of a new individual." [Railana, April 13,2014]
After the chromosomes join together and seperate in a method called mitosis, the prepared ovum, or zygote, as it is presently called, divides into two equivalent-sized little daughter cells. The mitotic division gives every cell 44 autosomes, 50% of which are of maternal and 50% of fatherly origin. Every little daughter cell additionally has either two X-chromosomes, making the new singular female, or a X- and a Y-chromosome, making it a male. The sex of the daughter cells is dead set, accordingly, by the sex chromosome from the male parent.Fertilization occurs in the uterine tube. The zygote reaches the uterine cavity about 72 hours after fertilization. It is nourished during its passage by the secretions from the mucous membrane lining the tube. By the time it reaches the uterus, it has become a mulberry-like solid mass called a morula composed of 60 or more cells. As the number of cells in a morula increases, the zygote forms a hollow bubblelike structure, the blastocyst, which is nurtured by the uterine secretions. It floats free in the uterine cavity for a short time and then is implanted in the uterine lining. Normally, the implantation of the blastocyst occurs in the...