214 SCIENCE FROM AN EASY CHAIR 



and toad, but in the case of hundreds of different kinds 

 of animals of all sorts. Thus we know the history of the 

 growth from the egg in its minutest details in every 

 kind of animal the " cell-lineage " of the tissues of the 

 full-grown animal traced back to the single original 

 egg-cell. 



The egg of animals is always originally a single 

 " cell " that is to say, a minute corpuscle of slimy con- 

 sistence, with a dense capsulated kernel or " nucleus " 

 within it. The kernel or nucleus divides into two, and 

 the cell itself divides ; each of the daughter cells again 

 divides, and so the process continues, until thousands, and 

 in larger animals millions, of cells are the result, as the 

 mass of cells takes up nourishment and increases in 

 volume. When (as is the case in many animals, e.g. 

 starfishes, worms, and mammals) there is only a little 

 granular food-material mixed in with the protoplasm of 

 the egg-cell, that cell is of small size, only the one two- 

 hundredth of an inch in diameter (see Fig. 31). But in 

 the frog there is much granular food-material, and the egg- 

 cell is distended to the size of a rape-seed. When there 

 is still more, as in the bird and many fishes, the egg-cell 

 does not entirely divide as it does in smaller eggs on 

 commencing growth after fertilisation. The protoplasm 

 collects into a disc incompletely separated from the food- 

 material, and it is the disc only which divides into two 

 four, eight, and ever so many more cells. Some of the 

 cells resulting from the division of the disc form the 

 embryo's body, and others spread, as they multiply, all 

 over the rest of the egg-ball from its edges so as to 

 enclose the granular food-material in a sac, called the 

 yelk sac. In the frog, on the contrary, the protoplasm 

 does not separate as a disc : the whole egg-cell or ball 

 divides to form the embryo-cells, and the food granules 

 are included in the substance of the dividing cells. 



