GENERAL ZOOLOGY 



dose is received in a short time or distributed over a long period, whether 

 continuous or intermittent. That is, the biological effects of ionizing radia- 

 tions are cumulative. These facts, known for a long time, have assumed great 

 importance for man in recent years. The use of X rays in diagnosis and treat- 

 ment by the medical and dental professions has increased. Now, with 

 experimental detonation of thermonuclear weapons there is unpredictable 

 increase in radioactive fallout in the atmosphere. Radioactive waste from 

 atomic power plants the world over may become an increasing source of con- 

 tamination of water and air. Grave concern is felt, nationally and inter- 

 nationally, over the adverse genetic effects on man, as well as on his 

 domesticated plants and animals. Of the mutations that have been induced 

 in experimental animals and plants, the great majority have been detrimental, 

 many producing death. It is evident that serious attention must be given to 

 the necessity of keeping the total amount of radiation received by every in- 

 dividual below the critical level for genetic damage. The future of the human 

 race is at stake. 



Sex Determination 



Animals are typically dioecious; that is, there are two sexes, which differ 

 essentially in that the males produce microgametes, or spermatozoa, whereas 

 the females produce macrogametes, or ova. In association with this primary 

 distinction between males and females, we have seen that differences exist be- 

 tween the reproductive systems (p. 128). In many animals what are known 

 as secondary sex characters are very conspicuous distinguishing features of 

 the sexes. For example, the gay plumage of many male birds, the vocal 

 differences between the sexes in many vertebrates from frog to man, and in 

 .some mammals the greater growth of hair in the males, as in the lion and 

 man, differentiate the sexes. It is true that in numerous species of vertebrates, 

 especially among lower animals, no such secondary sex characters can be ob- 

 served, although there may be a size difference between the sexes. 



Not all animals are sexually distinct; some are monoecious, or hermaphroditic, 

 and every individual produces both microgametes and macrogametes. Some- 

 times hermaphroditic animals produce first sperm and later eggs, or vice 

 versa, but frequently eggs and sperm are matured at the same time. Almost 

 all plants are monoecious. We see that sexual differentiation between in- 

 dividuals is not by any means a universal attribute of living things. The 

 production of differentiated gametes correlated with the capacity of reproduc- 

 tion is not dependent on the sexual differentiation of individuals. 



The conspicuous differences between the sexes in the higher animals have 

 long excited the curiosity of biologists and led to attempted explanations and 

 hopes for control. Early hypotheses were formulated in terms of the nutritive 

 conditions under which the young developed, in spite of the obvious fact that 

 a litter of pups or kittens, for example, contains both males and females 



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