REPRODUCTION AND EARLY DEVELOPMENT. IOI 



of small, highly motile cells, with little protoplasm out- 

 side the nucleus (Fig. 8, 4). These cells are known as 

 sperm. 



It is the case in many of the lower organisms that the 

 same individual may be both male and female; that is, 

 reproduce ova and sperm. Usually, in such cases, the 

 two kinds of cells do not ripen at the same time, although 

 they may do so. 



FIG. 19. A diagram illustrating the maturing of the male and female cells, and 

 their union (fertilization) to form an embryo. A, the female cell (ovum); B, the male 

 cells (sperm) ; C, the fertilized ovum or embryo. The numerals (1-5) indicate successive 

 stages of maturity, c, chromosomes in the nucleus; p. b., polar bodies; s, sperm cell. 



Questions on the Figure. What are the steps in maturing the 

 ovum? What changes take place in the nucleus? Do the polar 

 bodies have anything to do with the later development? Trace the 

 steps in the formation of sperm from the sperm mother cells. What 

 changes take place in the sperm nucleus? What are the differences 

 between the nucleus of the fertilized ovum and that of the first 

 stage, A, i ? 



107. Fertilization. In producing ova and sperm the 

 female and male individuals reproduce their kind; each 

 kind of cell contains the qualities of the species. But 

 ordinarily neither of these cells alone is capable of 

 going on and dividing and making the growth that is 

 necessary to form an adult. They do not behave like 

 young cells. The full power of development comes only 



