330 SCIENCE FROM AN EASY CHAIR 



melts and mixes with the substance of the egg-cell. The 

 whole process is easy to watch with a microscope, and I 

 am writing of what I, in common with many others, have 

 actually seen. 



The egg-cell after this process consists really of the 

 substance of two equal cells the egg-cell and the sperm- 

 cell completely fused so as to form a single cell, having 

 a single " nucleus," which has resulted from the fusion of 

 the nucleus of the egg-cell with that of the sperm-cell. 

 Now, and not before, the egg-cell can divide, take up 

 nourishment, and continue to divide and grow, so as to 

 form a constantly increasing mass of young cells, a young 

 animal which gradually assumes the form of a star-fish. 

 All animals, and plants, too, reproduce themselves in this 

 way. When the animal or plant is not aquatic in its 

 habits the sperm-cell and the egg-cell cannot be dis- 

 charged and take their chance of coming into contact 

 with one another outside the parent's body ; the sperm- 

 cells are, in such cases, received into a chamber of the 

 egg-producing parent's body, and there the fusion of the 

 egg-cells with them, one sperm-cell to one egg-cell, takes 

 place. Parthenogenesis then consists in the omission of 

 the fusion of a sperm-cell with the egg-cell. The egg- 

 cell develops, divides again and again, and produces the 

 young animal without the addition to it of a sperm-cell 

 without, in fact, being " fertilised," as it is called. That is 

 what happens in the summer broods of the little plant- 

 lice or aphides (Fig. 57). When, however, the cold weather 

 comes the virgin mothers suddenly produce two kinds of 

 young males as well as females and then the solitary 

 winter egg, which the late autumn females lay to last 

 through winter until spring, is fertilised by a sperm-cell 

 derived from the late produced autumn male (Fig. 56) in 

 the ordinary way. 



Another parthenogenetic animal is the rare little fresh- 



