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..... .... CONCEPTUAL LIFE SCIENCE DevelopmeDt Fertilization Fertilization is the beginning ofllfe. The sperm and egg unite to form a fertilized egg, which is also known as a zygote. The zygote begins to develop by undergoing a rapid series ofcell divisions known IS cleavage divisions. Stages ofdevelopment • Fertnlzed Elg 2<ell sta.e (1 st cle.age) (ZJGote) • 8-ceUstale (ard cleavage) a • Morula • 4-4:ell sta... (2nd cleaya.e) Blastula Q Gastrula Figure 18·1. Early stages ofdevelopment. Cleavage After fertilization, the egg becomes a zygote. The zygote begins a series of rapid . cleavage divisioDS. During the cleavage divisions, the embryo Rmains about the same size IS the original egg. The result is called a morula, which is a solid ball ofcells. As the eell divisions continue, the ceD sizes continually become smaUer· in these stages. Blastula Cell division continues in the embryo. The blastida is a hollow ball ofcells that results from these divisions. Early differentiation is seen because there are larger cells at oneend of the embryo. This end with the larger cells is called the -vegetalpole. The opposite end with the smaller ceDs is called the animal pole. The hollow iDterior part of the blastula is called the blastocoel. 18-1 18-2 Gastrula Gastrulation is the infolding ofthe embryo at the vegetal pole. The result is the gastrula. The outside layer of the gastrula is called the ectoderm. The inside cell layer is the endoderm. As cell division and differentiation continue, the interior space fills up with cells ofa third type tailed mesoderm. The mesoderm forms between the othq two cell layers. Differentiation Differentiation is the process of a cell or cells becoming different from the other cells in the same embryo. The cells become different as the embryo develops. The primary germ layers; ectoderm, endoderm, and mesoderm; give rise to the structures of the body. Table XVm-1. Em Skin Lining ofthe mouth Nervous system d, 8D8l C8DBl r Lcnsof muscles e CODDeCtive tissue Others Endoderm Alimentary C8DBl Pharyngeal derivatives .