104 A PRIMER OF BIOLOGY 



adequate nutrition and purification of 1 cubic milli- 

 metre of protoplasm, we see at once that the cell 

 A has more than ample area for its nutritive and 

 excretory needs, and may go on growing without 

 detriment, while B has reached the maximum limit 

 in this respect, and must be insufficiently nourished 

 and accumulate waste products should it by any 

 chance increase still further in volume. Let B, how- 

 ever, divide into hemispheres then each half will have 

 a volume of 564 cubic millimetres, while the area of 

 each will be 564 square millimetres + 28f square 

 millimetres (the area of the circular face exposed), 

 i e., 84" square millimetres more than re-establishing 

 the balance on the side of area. 



It is inconceivable, however, that the two new 

 cells arising in this way should be precisely similar 

 in all respects. Apart altogether from differences 

 in the protoplasm, one or other will have an excess 

 Ovum and of waste products or of reserve products, and thus 

 sperm. jhere arise differences between the daughter cells 

 which result from division, both in minute structure 

 and in activity. It is known that the accumulation 

 of reserve nutritive bodies is accompanied by a 

 tendency to sluggishness and non-motility in a cell, 

 and hence there might arise a more massive and non- 

 motile ovum, and a smaller and more active sperm. 

 These cells are the characteristic reproductive cells 

 of the female and the male respectively, both in the 

 plant world and in the animal. It is, of course, not 

 suggested that this is the way in which these reproduc- 

 tive cells arise in higher individuals, though it is 

 possible that some such explanation might account 

 for the original differentiation of cells of different sex. 

 Our next question must be, at what period in the 

 life cycle of the organism are such cells formed ? 



