. A civic biology : presented in problems. Biology; Sanitation. DIVISION OF LABOR 177 partnership between algae and fungi), approximately 55,000 species of fungi, and about 16,000 species of algae. Development of a Simple Animal. - - Many-celled animals are formed in much the same way as are many-celled seed plants. A common bath sponge, an earthworm, a fish, or a dog, - - each and all of them begin life in the same manner. In a many-celled animal the life history begins with a single cell, the fertilized egg. As in the flowering plant, this cell has been formed by the union of two other cells
. A civic biology : presented in problems. Biology; Sanitation. DIVISION OF LABOR 177 partnership between algae and fungi), approximately 55,000 species of fungi, and about 16,000 species of algae. Development of a Simple Animal. - - Many-celled animals are formed in much the same way as are many-celled seed plants. A common bath sponge, an earthworm, a fish, or a dog, - - each and all of them begin life in the same manner. In a many-celled animal the life history begins with a single cell, the fertilized egg. As in the flowering plant, this cell has been formed by the union of two other cells, a tiny (usually motile) cell, the sperm, and a large cell, the egg. After the egg is fertilized by a sperm cell, it splits into two, four, eight, and sixteen cells; as the number of cells increases, a hollow ball of cells called the blastula is formed; later this ball sinks in on one side, and a double-walled cup of cells, now called a gastrula, results. Practically all animals pass through the above stages in their development from the egg, although these stages are often not plain to see because of the presence of food material (yolk) in the egg. In animals the body consists of three layers of cells: those of the outside, developed from the outer layer of the gastrula, are called ecto- derm, which later gives rise to the skin, nervous system, etc.; an inner layer, developed from the inner layer of the gastrula, the endoderm, which forms the lining of the digestive organs, etc.; a middle layer, called the mesoderm, lying between the ectoderm and the endoderm, is also found. In higher animals this layer gives rise to muscles, the skeleton, and parts of other internal structures. HUNTER, CIV. BI. 12. A moss plant. G, the* moss body; S, the spore-bearing stalk (fruiting body).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble th
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