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The Brain and the Egg

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The Brain and the Egg tissues. Yields per acre are tremen- ing and food? Rayon, along with other dous; the young stems are easy to chemical converts, may also be an end harvest and chip with fairly simple and product of the new forest, helping to light equipment. The young wood is clothe our people in generations to relatively uniform in its characteris- come. Sugar, molasses, and yeasts can tics. Bark and leaves are not considered also be derived from wood. They trash in some products. could help considerably to provide the Maybe our young forest of sprouts world with vital and urgently needed will mass-produce raw material for energy foods and proteins at low cost. mass-produced housing to help shelter Thus, the silage connotation in the our exploding population. What about designation "silage sycamore" may the other two necessities of life—cloth- not be so weird after all. The Brain and the Egg RAYMOND D. SCHAR Mother Nature originally groups of chickens that gradually be- intended the egg of a bird to be used came more and more proficient in exclusively to perpetuate the species. their ability to produce eggs. As the To accomplish this parental instinct, birds were kept confined to smaller the bird was endowed with internal areas, they could no longer forage for organs and glands that responded to their feed. Man became the provider. certain external stimulations that oc- Through trial and error, he learned curred in nature. However, man has that a chicken would lay more eggs altered many of these external stimu- when fed one kind of feed than lants in order to force the domesticated another. By observing that a hen fowl to better serve him. began to produce more eggs in the The domesticated chicken is thought spring of the year as the days became to have originated from the jungle longer, man reasoned correctly that if fowl. This ancestor laid her eggs in he provided artificial light to lengthen the spring and early summer months the day, she would lay more eggs. Pro- as the length of the days increased. tection against the elements and dis- She woxild lay 10 to 12 eggs, incubate eases permitted additional production. them, and brood the chicks, and then All these efforts down through the repeat this process once and occasion- centuries have increased yearly egg ally twice. When man discovered the production from 20 to 30 eggs per hen egg was a good food, he started to to 250 and above in the better pro- domesticate chickens. By taking the ducing strains. But modern man is not eggs from the hen's nest as she laid content. He is intensifying his efforts them, man disrupted her normal to produce an even more prolific reproduction cycle. Since she no chicken. Refined selection principles, longer had to incubate her eggs and crossing and incrossing of families and brood the chicks, her body underwent physiological adjustments that per- RAYMOND D. SCHAR is Coordinator, National mitted her to lay more eggs. Poultry Improvement Plans, Animal Husbandry Through selection, man developed Research Division, Agricultural Research Service. 44 Chicken egg seems minute compared with egg from biggest bird ever known, Madagascar's elephant bird, extinct for centuries. National Geographic Society, which found giant fossil egg, estimates that when fresh it weighed about 20 pounds, equal to 160 hen eggs. strains, fortified diets, and improved time of artificially stimulated ovula- environmental conditions are but a tions to be reliably measured. few of the practices under constant Some of the earliest studies of ovula- study and improvement. In addition, tion led to the conclusion that light is during the last several decades, this probably the main external stimulus never ending search for perfection has to egg production. Since the hen's eye drawn a new group of scientists into picks up the light and sends signals to the picture. These are the physiologists. the brain, it was reasoned that the The physiologists are studying the brain must be the initial biological functions and activities of the organs clock involved in ovulation. It was and glands to find their relationship also discovered that the anterior pitui- to each other and to determine the tary gland, located directly beneath effects of natural and artificially in- the brain, is the organ responsible for duced internal and external stimuli. secretion of the hormones which are Dr. Richard M. Fraps, while a necessary for ovulation. physiologist at the University of Chi- The portion of the brain known as cago, investigated the effect of hor- the hypothalamus apparently is di- mones on feather development in rectly responsible for the initial inter- chickens. In time, he expanded his nal stimulation. This was verified when interest in hormones to include the ovulation was consistently induced by part they played in ovulation. Since the infusion of extracts from the hy- the ovaries of birds produce eggs that pothalamus into the pituitary gland. rnature singly, chickens provided an Conversely, intentional injury to the excellent opportunity for this study. hypothalamus stopped ovulation for A domestic hen will complete an extended period. Also, when cer- more than 200 ovulations a year in tain drugs known to affect the nervous rather definitely established cycles. system were injected into the hen, the This allows a reasonably accurate ovulation cycle was disrupted. How- estimate to be made of the time of ever, if extracts from the hypothalamus natural ovulation, thus permitting the were injected into birds from which the 45 STIMULI saclike membrane which surrounds the single, fully developed yolk. This per- mits the yolk to be released from the ovary. The infundibulum, or upper end of the oviduct, then engulfs this free yolk and starts it on its journey down the hen's reproductive tract. Besides causing maturation and ovu- lation of a yolk, the pituitary-secreted hormones cause the ovary to secrete hormones of its own. These are thought to be estrogens and progestagens and are called feedback hormones. When they reach the hypothalamus, they cause a signal to go to the pituitary which inlubits its hormone produc- tion, completing the cycle. After Dr. Fraps came to work for the U.S. Department of Agriculture at Beltsville, Md., he and his coworkers studied time patterns involved in the intricacies of ovulation. After repeated experiments and observations, they de- termined that release of LH closely re- lates to the onset of darkness in the light/dark cycle of the day (or other light/dark cycle from 22 to 34 hours in length). In most instances, the mem- brane or follicle surrounding the yolk Hormone secretion and flow in hen^s ruptures about 8 hours after the LH ovulation process. When brain is stimu- release. After ovulation, or rupture of lated by light, it signals anterior pituitary the follicle, the yolk spends approxi- gland to release maturation and ovulation hormones. After maturation and release mately 26 to 28 hours in the oviduct, of an ova, the ovary secretes feedback where various glands secrete the albu- hormones which tell brain it is time for men, membranes, and shell that go to pituitary to reduce its hormone produc- make up the complete egg. tion, thus completing ovulation cycle. It was known that under a normal period of daylight a hen will lay an egg on each of two or more consecutive days, skip a day, and then repeat the pituitary gland had been removed, no cycle. So when a hen has produced an ovulation took place. This made it ap- egg on each of 2 consecutive days, she pear that hormone secretion from the is said to have a two-egg sequence; pituitary was necessary for ovulation. production on 3 consecutive days gives There is considerable evidence that her a three-egg sequence, etc. upon receipt of a light-controlled sig- The hen iisually lays the first egg in nal, nerve terminals of the hypothala- a sequence during the first part of the mus discharge a substance called re- Hghted or daylight period. Within 15 leasing factor into the bloodstream. to 45 minutes after she lays this first This is carried to the pituitary where it egg^ ovulation of the second egg takes stimulates that gland into secreting the place. The time lapse between laying maturation and ovulation hormones. an egg and the succeeding ovulation These hormones travel to the ovary becomes less as the number of eggs in where the ovulation, or luteinizing, a sequence increases. Thus, if a hen hormone (LH) causes rupture of the has a two-egg sequence, the time be- 46 tween eggs is about 2S}i hours. If she Exposure to very high intensity light is on a four-egg sequence, the time for a short period of time has produced between eggs is about 26 j^ hours, and inconclusive results for different re- for a six-egg sequence, about 25}^. searchers. Additional studies of light In longer sequences, the lag time for intensity, as well as types of light, are eggs in the middle of the sequence was needed. less than for the first or last eggs. One Other physical environmental fac- researcher, who observed a group of tors thought to affect ovulation include birds for a complete year, recorded lag temperature, moisture, and diet. More times for various hens that laid two- research has been done on the effect of through 13-egg sequences. He noted these factors upon mammals and other that when the sequence was above 10 birds than with chickens. This is true eggs in length, some of the consecutive also of external psychological factors eggs were produced in less than 24- such as group interaction or the pres- hour intervals.
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