Chapter 18 - Chapter 18: Pregnancy, Development, and...

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Chapter 18: Pregnancy, Development, and Lactation 1. Why is the timing of copulation so important? How is the precise timing accomplished? Timing is important because the spermatozoa must arrive at the oviducts before the ovum to have time to undergo capacitation, a process that enhances their fertility. Nature has an exquisite method for arranging the proper timing. Breeding is only allowed by the female during the estrus, or "heat," period. So spermatozoa enter the female reproductive tract when the oocyte in the follicle is fully developed but has not yet been released. Release of the ovum (ovulation) is delayed until near the end of the estrus period in most species. This nifty bit of traffic control helps ensure that the spermatozoa arrive at the oviducts first and have time to undergo capacitation before the ovum shows up ready to be fertilized. 2. Describe what happens to a zygote between fertilization and implantation. The fertilized ovum is called a zygote . Immediately after fertilization, the nucleus of the spermatozoon is called the male pronucleus , and the nucleus of the ovum is called the female pronucleus . Each carries the haploid chromosome number. The male and female pronuclei quickly join together to restore the diploid chromosome number and determine the unique genetic makeup of the offspring. As soon as the two pronuclei join to form a single nucleus, the zygote begins to divide rapidly by the normal process of mitosis. This rapid division is called cleavage. The single cell divides into two cells, which quickly divide into four, then eight, then sixteen, and so on. Cleavage occurs so rapidly that the cells of the zygote do not have time to grow between divisions. The number of cells making up the zygote is increasing dramatically, but its overall size is still about the same as the original ovum, even after several days. While cleavage is taking place, the zygote slowly moves down the oviduct toward the uterus. Delicate, muscular contractions and the movements of cilia are gently propelling it along. After a few days the zygote is a solid mass of cells that looks like a tiny raspberry; this is known as the morula stage. The cells of the morula continue to divide and gradually form a hollow cavity in the center. By the time it reaches the uterus a few days later, it is formed into a hollow ball of cells with a bump on one side that eventually forms into the embryo. It is now called a blastocyst , and it is ready to implant itself in the lining of the uterus.
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This note was uploaded on 04/03/2012 for the course NRM 2406 taught by Professor Dabbert during the Spring '12 term at Texas Tech.

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Chapter 18 - Chapter 18: Pregnancy, Development, and...

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