256 REPRODUCTION 



2. In some invertebrates parthenogenesis forms the only 

 method of reproduction, and the individuals formed are all 

 females. But in other species where fertilization occasionally 

 takes place the parthenogenetic eggs become either males or 

 females. 



3. In man of all twins born there are two kinds. Those that 

 are developed from two different eggs, each having its own cho- 

 rion and developing its own placenta. These are designated as 

 false twins and in sex may be either male or female, or male 

 and female. In true or identical twins, the two are developed 

 from a single ovum and are included in a single chorion. They 

 are either both males or both females. 



4. In some insects the determination of sex is correlated with 

 a visible difference in the chromosomes of the spermatozoon. 

 In some cases one-half of the spermatozoa contain an unpaired 

 or accessory chromosome. Those showing this structure pro- 

 duce females, while all others give rise to males. In other cases 

 all the spermatozoa exhibit the same number of chromosomes, 

 but one-half of them contain a large "idiochromosome," the" 

 other half a small one. The former give rise to females, the 

 latter to males. Sex, therefore, seems to depend upon which 

 of the sex-determining structures in the two cells predominates 

 after union. It may, moreover, be said that the production of 

 sex is self-regulating, insomuch that a scarcity of the sex of 

 one kind tends to a production of individuals of that sex. 



QUESTIONS ON CHAPTER XIV 



What is reproduction? 



How many methods are there? 



Describe the asexual method. 



Why does a growing mass of protoplasm divide? 



What is the essential fact of sexual reproduction? 



Describe various stages in the development of sexual reproduction. 



What are the primary and secondary sexual characters? 



Describe the development of a spermatozoon. 



What difference is there between the sexual elements? 



What is the rate of movement of a spermatozoon? 



What is semen? 



What is meant by the maturation of the ovum? 



What is a Graafian follicle? 



Describe ovulation. 



How does the ovum reach the uterus? 



What is fertilization? Where does it take place? 



Describe the process and development of menstruation. 



