igS THE FROG chap, xii 



Without it, as we liave seen, the egg is incapable of develop- 

 ment ; after it has taken place, the egg — or more strictly, 

 the oosperm, since it is now an ovum plus a sperm — is 

 potentially a young frog, since if left undisturbed in water, 

 it will in course of time give rise to a tadpole, which in its 

 turn will change into a frog. 



It must be remembered in the first place that the fertilized 

 egg is a single cell, comparable with a blood corpuscle or 

 an epithelial cell. It is, however, peculiar in two respects : 

 first in having its nucleus derived in part from a sperm, so 

 as to contain matter from both the male and the female 

 parent ; and secondly in having its protoplasm distended 

 with yolk-granules to such an extent that instead of being a 

 minute body visible only under the microscope, it is easily 

 visible by the naked eye. The yolk is not equally distributed : 

 on one hemisphere it is less abundant than elsewhere, 

 and it is this more protoplasmic hemisphere which is super- 

 ficially blackened by a layer of pigment, and which always 

 floats upwards in the water when the egg is laid. 



Segmentation of the Oosperm. — Almost directly after 

 being laid and fertilized, the egg undergoes a remarkable 

 change. X furrow appears all around it, as if made with 

 a blunt instrument, and deepening gradually, at last divides 

 the oosperm into two hemisjiheres in contact with one 

 another by their flat faces (Fig. 64, A). The examination 

 of sections shows that this process is preceded by the 

 division of the nucleus into two ; its final result is the 

 separation of the originally one-celled egg into two cells. 

 Now if you refer to Chapter VIII, you will be reminded 

 of the fact that the epithelial cells of the skin multiply by a 

 similar process of simple fission, or di\ ision into two ; the 

 nucleus in each case di\iding first and afterwards the 

 protoplasm. 



