. Beginners' Zoology . e a cell ivall,but many cells are naked, or without a wall. Hence theindispensable part of a cell is not the wall but the nucleus, 77/^5 PRINCIPLES OF BIOLOGY and a cell may be defined as a bit of protoplasm containinga nucleus. This definition includes naked cells as well ascells with walls. One-celled Animals. — There are countless millions ofanimals and plants the existence of which was not sus-pected until the invention of the micro-scope several centuries ago. They areone-celled, and hence microscopic in is believed that the large animals andplants are desce
. Beginners' Zoology . e a cell ivall,but many cells are naked, or without a wall. Hence theindispensable part of a cell is not the wall but the nucleus, 77/^5 PRINCIPLES OF BIOLOGY and a cell may be defined as a bit of protoplasm containinga nucleus. This definition includes naked cells as well ascells with walls. One-celled Animals. — There are countless millions ofanimals and plants the existence of which was not sus-pected until the invention of the micro-scope several centuries ago. They areone-celled, and hence microscopic in is believed that the large animals andplants are descended from one-celled ani-mals and plants. In fact, each individualplant or animal begins life as a singlecell, called an ^%g cell, and forms itsorgans by the subdivision of the &%g cell into many egg cell is shown in Fig. 6, and the first stages in thedevelopment of an egg cell are shown in Fig. 7. The animals to be studied in the first chapter are one-celled animals. To understand them we must learn how. Fig. 6. — Egg cell ofmammal with yolk.
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