396 - Multicellular Animals, Especially Man 



quently fraternal twins do not tend to re- 

 semble each other any more than other 

 brothers and sisters. On rare occasions, in 

 man and other mammals, incomplete embry- 

 onic fission may occur, producing grossly 

 abnormal offspring, such as "Siamese twins," 



it is difficult to draw a sharp line between 

 accidental regeneration and normal repro- 

 duction by multiple fission. 



In higher animals, such as Mollusca, Ar- 

 thropoda, and Vertebra ta, the power of re- 

 generation is more limited. Many of these 



or individuals with a supernumerary head or animals can regenerate parts of the body, 



limbs. such as a limb that has been lost or injured; 



Budding, which occurs in some flatworms but the limb cannot regenerate a whole ani- 



and Annelida, and in many sponges, Coelen- mal. Moreover, if the whole body be divided 



terata, and Tunicata, is well exemplified by into two or more parts, all the parts will die. 



the case of Hydra. In Hydra the bud origi- Among vertebrates, fish can regenerate fins 



nates as a local accumulation of germ cells. and tails; and salamanders may regenerate 



These cells soon develop into a small but 

 perfect offspring, which sooner or later be- 

 comes detached from the parent (Fig. 3-12). 

 In many other Coelenterata, however, the de- 

 veloping buds remain attached to the parent, 

 forming colonies in which considerable differ- 

 entiation occurs among the associated indi- 

 viduals (Fig. 21-19). 



Regeneration. In many primitive animals 

 such as Hydra, even a small piece of the body 

 can regenerate into a whole animal, and 

 some lower forms fragment so readily that 



tails, legs, and jaws. An adidt frog cannot re- 

 store a lost leg, but a metamorphosing tad- 

 pole can. Adult reptiles, birds, and mammals 

 cannot replace lost limbs, although lizards 

 (but not other reptiles) can regenerate the 

 tail. In mammals, the regeneration of exter- 

 nal parts is limited to the growth of skin 

 and connective tissue over wounds; but some 

 of the internal organs — especially the liver 

 and some other glands — have considerable 

 powers of regeneration. 



TEST QUESTIONS 



1. Provide an example in each case: (a) tem- 

 porary gonads; (b) a monoecious animal; 



(c) external fertilization; (d) mutual insemi- 9. 

 nation. 



2. Some animals produce many eggs, others 

 very few. Explain. 



3. Identify, locate, and give the connections ot: 10. 

 (a) seminiferous tubules: (b) interstitial tis- 

 sue; (c) inguinal canal; (d) epididymis; (e) 



vas deferens: (t) urethra. 11. 



4. Make a labeled diagram to show all the parts 



and connections of the human male repro- 12. 

 due the tract. 



5. Discuss the functions of the seminal vesicles, 13. 

 the prostate, and the glands ot Cowper. 



6. Make a labeled diagram to show the parts 



and connections of the human female repro- 14. 

 ductive tract. Specify the function or func- 15. 

 tions of each part. 



7. Distinguish between amplexus and copula- 

 tion. 16. 



8. Identify, locate, and state the functions of: 



(a) the Graafian follicles; (b) the corpus 

 luteum; (c) the ostium. 



Identify and describe in terms of the human 

 species: (a) ovulation: (b) fertilization: (i) 

 implantation; (d) the decidual reaction: (e) 

 the growth of the fetus: (1) delivery. 

 Distinguish between ovipary and vivipary. 

 Which is more prevalent among vetrebrates? 

 Explain. 



How does the amnion differ from the allan- 

 tois as to its origin in the embryo? 

 What is the amniotic fluid and why is it 

 important? 



Explain the functional importance of the 

 allantois: (a) in reptiles and birds; (b) in 

 mammals. 



What is the placenta? 



How is the placenta related to: (a) the chori- 

 onic layer of the amniotic fold: (b) the allan- 

 tois; (c) the tissues of the uterus? 

 What are the chorionic villi and why are 

 they important? 



