What are portable, durable, dependable, convertible, and altogether incredible? Eggs! BY MARY HOFF Illustrations by Taina Litwak SLUG EGGS Some are smaller than grains of salt. Others are big as grapefruit. They come 5 DOUGWECHSLER, VIREO in beautiful colors and intriguing shapes. Each holds a wonder of the most wonderful sort: a new life, bundled up in this tiny container that provides everything the creature needs to grow. Most animals develop outside their mothers in special shelters we know as eggs. Some eggs are soft, protected from the world by thin membranes. Others are hard, surrounded by a tough shell or case. Turtles, toads, spiders, snails, SKINK EGGS5 fish, birds, insects, and crayfish all hatch from eggs. Eggs develop inside female animals. An egg starts with a single cell, called an ovum, in the mother’s ovary. The mother’s body makes food, called yolk, for the ovum—much as someone might pack you a lunch if you were going away. Then the ovum sets off on a journey. It travels down a tube called an oviduct. Along the way, WALLEYE EGGS CENTIPEDE EGGS it gathers other parts, such as membranes and 6 5 BILL BEATTY A.B.SHELDON, DEMBINSKY PHOTOASSOCIATES FROG EGGS6 a shell. At the end, the mother squeezes the finished egg out of her body. To make a new animal, most eggs must be HUMMINGBIRD EGGS fertilized—the ovum must combine with a 6 sperm cell from a male. In some animals, such as birds and insects, the egg is fertilized inside the mother. In other animals, such as frogs and fish, the ovum and sperm join outside the mother’s body. CARROL HENDERSON GARY MESZAROS, DEMBINSKY PHOTOASSOCIATES OSTRICH EGG5 March–April 2004 DEPARTMENT OF NATURAL RESOURCES 33 On the bottom of a milkweed leaf hangs from inside the egg DOUGWECHSLER, VIREO a tiny white speck. It is an egg, de posited to make their head BEETLE EGGS by a female monarch butterfl y. swell. Th en they Inside it, things are as busy as push against the eggshell and pop out. McDonald’s at lunchtime. Cells are Some insect eggs serve as time dividing and changing to make up capsules to help the population sur- diff erent body parts. Four days aft er the vive harsh conditions. If the weather egg was laid, a fully formed caterpillar becomes very hot or very cold, the em- JIMSOGAARD will nibble its way out. bryo (developing insect) may enter a S T I N K B U G EGGS 5 Most insects lay eggs. Some eggs are state called diapause, in which develop- long and skinny, like miniature hot dogs. ment stops. It will stay this way until living conditions improve. Some are shaped like footballs. Th ey may be brown, green, yellow, or white. Insects lay their eggs where the Other Invertebrates Invertebrates (animals without GARY C. BAKER, DEMBINSKY PHOTOASSOCIATES young will be able to fi nd food B U T T E R F L Y E GG when they hatch. Some, such as backbones) lay eggs in many places. 5 the monarch, glue their eggs to Pond snails lay their eggs on plants micropyle leaves of specifi c “host” plants under the surface of the water. Spi- wax layer they need to survive. Some wasps ders spin silken cases around their lay eggs inside juicy caterpillars. eggs and hide or carry them. Crayfi sh aeropyles Acorn weevils lay theirs inside carry their eggs in a special holding acorns. Stable fl ies lay eggs in ma- spot under their abdomen. Fresh- chorion nure because their young eat ma- water clams shelter eggs in their gills. nure when they hatch. Have you ever seen a nightcrawler blastoderm Grasshoppers and cockroaches with a fat ring around its body? Th e enclose their eggs in a container ring, called a clitellum, helps a night- yolk called an egg case or ootheca. Th is crawler care for its eggs. Th e clitellum helps protect the eggs from preda- produces a ring of slime that slips off the nightcrawler’s body, gathering tors and from drying out. GARY MESZAROS, DEMBINSKY PHOTOASSOCIATES When it comes time to hatch, eggs. Th en the slime closes and hard- CRAYFISH EGGS 5 5PARTS OF AN INSECT EGG baby insects swallow liquid and air ens into a protective case for the eggs. f u n f a c t You might fi nd these lemon-shaped egg f u n f a c t In some species of bees, ants, See glossary on page 41 for egg cases, which are a little bit smaller than a The eggs of daphnia, a tiny aphids, and wasps, eggs can part definitions. pea, in the soil in your garden. pond invertebrate, can stay develop into new individuals alive for 20 years, waiting for without being fertilized by a male. good hatching conditions. 34 Minnesota Conservation Volunteer March–April 2004 35 embryos soak up the sun’s light rays, and the warmth helps them grow. Minnesota frogs and toads lay their eggs in water. Spring peep- ers lay 800 to 1,000 eggs, each with its own jelly coat. Pickerel frogs encase their eggs in one big jelly blob. American toads lay their eggs in strings that can DOUGWECHSLER, VIREO stretch more than 60 feet long. SALAMANDER EGGS 5 Salamanders are closely related to frogs and toads. Some salaman- f u n f a c t Frogs, toads, salamanders, ders lay eggs in water, and some lay them and fish belong to a group of DOUG WECHSLER, VIREO T O A D E G G S 5 on land. egg-layers called anamniotes. Imagine starting your life in a bowl Most amphibians abandon their eggs Unlike bird and reptile eggs, full of jelly! Th at’s just what would aft er they lay them. But some watch over anamniote eggs have no happen if you were a frog. their eggs. Mudpuppies guard their eggs liquid-filled sac. They don’t A gellike material encases frog and until they hatch. Redback salamanders lay need the liquid because they toad eggs. When the mother fi rst lays eggs among rocks or under a log, then they develop and hatch in ponds the eggs, you can’t see this layer. But curl around the eggs to protect them. and other wet places. when the material soaks up water, it swells to form a thick, clear coat- PARTS OF AN AMPHIBIAN E GG ing. Th e coating keeps the embryos 6 from drying and helps protect them from germs, predators, and bumps. It jelly coat also serves as a greenhouse. Th e dark MIKE DAVIS M U D P U P P Y EGGS 5 animal pole f u n f a c t f u n f a c t Bullfrogs can lay The jelly of some frog eggs contains algae, blastocoele 25,000 eggs in a which might camouflage the eggs so predators single batch. don’t find and eat them. vegetal pole 36 Minnesota Conservation Volunteer 37 In early spring walleyes move to shal- A painted turtle crawls f u n f a c t low, clear water to spawn—lay their out of a pond. She Caviar, a food served as snacks at eggs. A female walleye can produce goes uphill to dry land fancy parties, is the salted eggs of 200,000 eggs or more. Males fertilize and digs a hole. Th en sturgeon or other fish. them aft er they leave her body. Th e she lowers her back pinhead-size eggs fall to the bottom end into the hole and of the lake among stones, where they slowly squeezes out a are protected from round, white egg about SALMON EGGS 6 predators. the size of a ping-pong Aft er a week ball. Plop! When she or two, baby fi sh has laid four to 20 eggs emerge. Th e baby in the hole, she paddles fi sh, called fry, still soil on top of them A.B. SHELDON, DEMBINSKY PHOTO ASSOCIATES SHELDON, DEMBINSKY PHOTO A.B. have a ball of yolk with her hind legs. T U R T L E H A TCHING inside them, so Over the next couple 5 they don’t need to of months, the embry- fun fact eat food for several os inside the eggs grow. When Reptiles and birds belong to a category of days. the baby turtles hatch, they dig animals known as amniotes. Their eggs have a special membrane, called an that en- Other species of their way up to the surface and amnion, closes the embryo in a bag of liquid. This gives fi sh have diff erent crawl down to the pond. the embryo the moisture it needs to develop NATALIE FOBES egg-laying Some snakes lay eggs in properly, making it possible for reptiles and PARTS OF A FISH E GG 6 habits. Some, holes too. Other snakes and birds to lay their eggs in dry places. such as trout, lizards produce eggs that build nests develop and PARTS OF A REPTILE E GG micropyle (closed) for their eggs. hatch inside 6 Bull head and their bodies. shell crappie males T h e n t h e y embryo albumen care for the give birth to amnion chorion chorion eggs until live young. embryo allantois yolk they hatch. This is called yolk sac ovoviviparity. yolk sac yolk 38 March–April 2004 39 shell shell chalaza membranes albumen chorion Bird eggs come in many shapes, colors, textures, and ally the mother) sits amnion sizes. Th e shapes vary greatly, from the round eggs on the eggs to keep of owls, to songbirds’ oval eggs (slightly larger at one them warm.