Answer to Question 1
For convenience, we think of the gestation period as consisting of three integrated trimesters, each lasting 3 months in duration: (1 ) The first trimester is the period of embryological and early fetal development. During this period, the rudiments of all the major organ systems appear. (2 ) The second trimester is dominated by the development of organs and organ systems, a process that nears completion by the end of the sixth month. During this period, body shape and proportions change; by the end of this trimester, the fetus looks distinctively human. (3 ) The third trimester is characterized by rapid fetal growth and deposition of adipose tissue. Early in the third trimester, most of the fetus's major organ systems become fully functional.
Answer to Question 2
Powerful acrosomal hydrolases released from the sperm head create openings between adherent granulosa cells (the corona radiata) and penetrate the zona pellucida. A solitary sperm penetrates to the oocyte membrane to which it fuses, sending its nucleus into the oocyte. This is the moment of fertilization. This triggers maturation of the haploid maternal pronucleus, followed by development of the male pronucleus. These then fuse in the act of amphimixis, the true combining of maternal and paternal haploid chromosome sets to form a unique diploid set of 46 chromosomes. Spindle fibers organize the chromosomes on the metaphase plate after the nuclear envelopes break down. After mitosis is complete, cytokinesis takes place, dividing the original zygote into two daughter cells or pair of blastomeres. This takes more than 30 hours to complete.
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Cradle the head briefly in both hands to end this section. Hold for a few seconds until you can feel ...
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