AN INTRODUCTION TO ANIMAL BIOLOGY 59 



cells vibrate rapidly and enable the cells to swim in the 

 water into which they are discharged. 



58. Fertilization. Before an egg-cell can develop into 

 a tadpole it must be entered by a sperm-cell. Soon after 

 the egg-cells of a frog are discharged into the water they are 

 approached by the swimming sperm-cells ; and each egg is 

 entered by a sperm-cell. The sperm-cell then becomes a 

 second nucleus, called sperm-nucleus. This nucleus ap- 



FIG. 21. Diagrams to illustrate fertilization of an egg-cell by a sperm-cell. 

 A, e, nucleus of matured egg-cell; s, a sperm-cell ready to enter. B, 

 sperm-cell entered and transformed into sperm-nucleus (s). C, sperm- 

 nucleus and egg-nucleus united, fertilization complete. D, division lead- 

 ing to two-cell stage (compare with Fig. 22, C). The nuclei are represented 

 as having only two chromosomes; but those of most sperm- and egg-cells 

 have more. 



preaches that of the egg-cell, and the two nuclei unite. 

 When this has occurred the egg-cell is said to be fertilized, 

 and the entering of a sperm-cell and its union with the 

 nucleus of the egg-cell is called fertilization (see Fig. 21). 



59. Division of the Egg-Cell. As soon as the egg-cell is 

 fertilized, it prepares for division into two cells. Delicate 

 fibers appear and form a spindle-shaped structure with a 

 starlike structure (aster) at either end (Fig. 22). In the 

 center of this spindle are a number of small bodies known as 

 chromosomes. These are masses of granules such as may be 

 seen in any stained nucleus (Fig. 11). Within a few minutes 

 the changes shown in Figs. A, B, C, D occur. These con- 

 sist first of a division of the chromosomes (Figs. C, D), then 

 a division of the cell-body, and finally the formation of a 



