198 THE FROG CHAP, xn 



Without it, as we have seen, the egg is incapable of 

 development ; 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 

 fertilised 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 to the naked 

 eye. The yolk is not equally distributed : on one hemi- 

 sphere it is less abundant than elsewhere, and it is this 

 more protoplasmic hemisphere which is superficially 

 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 fertilised the egg undergoes a remark- 

 able change. A furrow appears all around it, as if made 

 with a blunt instrument, and deepening gradually, at 

 last divides the oosperm into two hemispheres in con- 

 tact 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 division 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 division into two ; the nucleus in each case dividing 

 first, and afterwards the protoplasm. 



