THE FAMILY OF FROGS 89 



to sticks or leaves of grasses or to other stationary objects 

 in the water. 



3. Mating and Fertilization. While the mother is laying 

 the eggs in the .water, the male frog pours out great 

 numbers of sperm cells over them, also in the water. 

 These sperms can swim, and under the attraction of the 

 eggs one sperm cell unites with each of the eggs. The 

 gelatin is not a living part of the egg, and therefore the 

 sperm must pass through this until it reaches the dark 

 cell within. The head of the sperm containing the nucleus 

 passes into the egg proper and unites completely with the 

 nucleus of the egg. This forms a fertilized egg or embryo 

 with all the powers of developing into a mature frog. 



4. The Early Development. If we were watching one of 

 these eggs with a microscope, we should see, about two 

 hours after fertilization, a little groove passing round the 

 egg and marking it off into two hemispheres. This shows 

 that the one-celled embryo has now become two-celled. 

 A little later it divides again, at right angles to the first 

 division, into four cells. These four cells are equal. Soon 

 other walls appear at right angles to both of those 

 previously formed. These new walls divide each of the 

 four cells into two, one of which (the upper) is consider- 

 ably smaller than the other. Divisions of all the cells 

 continue to take place rapidly. By the end of twenty-four 

 hours there are too many cells to count. 



During this time the embryo, while changing its shape a 

 little, is not larger than it was at the start, yet there are 

 hundreds of cells where there was one at first. It follows 

 then that the cells in dividing have become much smaller. 

 Yet each has a little mass of protoplasm, a nucleus, and 

 all the other parts that a cell is entitled to have. 

 Furthermore, each one of the 2X chromosomes in the 

 nucleus made up of the union of male and female nuclei 

 splits with every division, and thus every nucleus has still 

 2X chromosomes. 



