560 



COLLEGE ZOOLOGY 



Compensatory regulation 



Somewhat resembling regeneration is the 

 compensatory reaction of certain organs as 

 a result of various types of bodily changes. 

 If one kidney is removed, the other kidney 

 increases its activity so as to function for 

 both. If muscular exertion is increased, thus 

 requiring a greater blood supply, the heart 

 increases in size and power. If certain mus- 

 cles are exercised, they increase in size. 



Autotomy 



Certain parts of the body may be sub- 

 jected to injury more than others. When this 

 is true, a definite breaking point may be 

 present and the injured animal automati- 

 cally severs the part at this point. This proc- 

 ess is kno\vn as autotomy. Well-known ex- 

 amples of autotomy are furnished by the 

 crayfish, which may break off injured legs 

 at a point near the base (Fig. 119). Healing 

 and regeneration appear to take place at this 

 point more readily than elsewhere. The star- 

 fish casts off its injured arms near the base 

 at the fourth or fifth ambulacra! ossicle. 



SEXUAL REPRODUCTION 



Sexual reproduction is reproduction by 

 means of gametes. There are two types of 

 gametes that are visibly different and are 

 referred to as eggs and sperms. These usu- 

 ally unite in sexual reproduction, but in 

 some cases eggs may develop normally with- 

 out union with a sperm, as in rotifers, or 

 they may be induced to develop by artificial 

 means, as in the case of sea urchin eggs 

 when the chemical nature of the medium is 

 changed. 



Parthenogenesis 



The development of an egg that has not 

 united with a sperm is called parthenogene- 



sis. In rotifers, the summer eggs develop 

 normally without being fertilized. In aphids, 

 stem mothers that are parthenogenetic and 

 viviparous hatch from eggs that have lived 

 through the winter. Their offspring are wing- 

 less parthenogenetic females. These give 

 rise to wingless parthenogenetic females, but 

 after a time winged females are produced 

 which migrate to other plants. As fall ap- 

 proaches, males and oviparous females ap- 

 pear, mating occurs, and fertilized eggs are 

 laid that remain dormant over winter and 

 give rise to stem mothers in the spring. 



Sperms are stored up in the seminal re- 

 ceptacle of the queen honey bee. She seems 

 to be able to lay fertilized or unfertilized 

 eggs, according to the size of the honey- 

 comb cell in which the individual is to de- 

 velop. Fertilized eggs develop into queens 

 and workers, whereas the unfertilized eggs 

 develop into drones. Many other cases of 

 normal parthenogenesis among inverte- 

 brates could be cited. 



Certain eggs that normally must be pene- 

 trated by a sperm before they will develop 

 can be made to undergo development, at 

 least for a short time, by means of various 

 stimuli; this is known as artificial partheno- 

 genesis. The eggs of many invertebrates 

 have been stimulated to develop by chang- 

 ing the chemical constitution of the me- 

 dium. Pricking, shaking, and raising the 

 temperature may stimulate development. 

 Frogs and rabbits are reported to have been 

 reared from such eggs; they had a mother 

 but no father. 



Recently, parthenogenetic turkeys (Fig. 

 400) have become less rare as a result of 

 U.S. Department of Agriculture studies of 

 natural parthenogenesis in turkey eggs. The 

 exact cause of the parthenogenetic develop- 

 ment is not known. But Dr. M.W. Olsen, 

 investigating this problem, thinks that the 

 incidence of parthenogenesis has been in- 

 creased by selection; he also suspects that a 

 vaccination program in some way enters 

 into the picture. 



