138 GENERAL CONCEPTS 



and the lack ol oxygen will all produce similar defects in mice if applied 

 at comjKirable times in development. Such observations have led to the 

 concept of critical periods in development, periods in which the develop- 

 ment of a certain (Mgan or organ system is occurring rapidly and hence 

 is most susceptible to interference. 



The property of reproduction, which we regard as one of the out- 

 standing characteristics of living things, involves a great many complex 

 and interdependent processes: the elaboration of hormones which regu- 

 late the development of gonads, secondary sex structures and the pro- 

 duction of gametes in the parents; behavior patterns which bring the 

 parents together and have them release their gametes at such a time and 

 in such a place as to make their fusion probable; the union of male 

 and female pronuclei followed by cleavage, gastrulation and morpho- 

 genesis; and devices for the care and protection of the developing young. 

 Our descriptive knowledge of these phenomena is extensive but our 

 understanding of the fundamental mechanisms involved in each of these 

 processes is rudimentary. This is a fertile field for further investigation. 



Questions 



1. What are the advantages and disadvantages of asexual and sexual reproduction in 

 animals? 



2. What is accomplished by the process of meiosis? 



3. Compare mitosis and meiosis. 



4. Contrast spermatogenesis and oogenesis. 



5. What is meant by the terms haploid, diploid, gamete, zygote, synapsis and tetrad? 



6. Define and give an example of (a) hermaphroditism and (b) parthenogenesis. 



7. What is accomplished by the process of fertilization? Contrast external fertilization 

 with internal fertilization. What are the advantages of each? 



8. Define and give an example of (a) oviparous and (b) viviparous animals. 



9. Discuss the effect of the amount and distribution of the yolk on (a) the cleavage 

 pattern of the egg and (b) gastrulation in the embryo. 



10. Distinguish between an enterocoele and a schizocoele. In what animals are these 

 found? 



11. What is the evidence that the primitive streak of the chick is homologous to the dorsal 

 lip of the blastopore of the frog? 



12. Compare the adaptations for the protection and nourishment of the embryo during 

 development in the shark, frog, chick and man. 



13. Compare the current theories as to the factors which regulate development. 



14. Define: organizer, chorda-mesoderm, differentiation, adaptive enzyme. 



Supplementary Reading 



An interesting account of mating instincts in animals is presented in W. P. Pycraft's 

 The Courtship of Animals. Research on the dynamics of development is summarized in 

 Roberts Rugh's Experimental Embryology and in L. G. Earth's Embryology. Detailed 

 accounts of vertebrate development are found in B. M. Patten's Embryology of the Chick 

 and Embryology of the Pig. Comparative vertebrate embryology is well presented and 

 illustrated in Emil Witschi's Development of Vertebrates. 



