THE BEGINNING OF DEVELOPMENT 35 



sperm, and we then get a "virgin birth," or partheno- 

 genesis as it is called in science. The most various 

 and unexpected agents may be effective. It is some- 

 times only necessary to prick the tgg with a sharp 

 needle, or to put it into very weak acid ; some marine 

 eggs may be caused to begin developing if the 

 salt-concentration of the water is altered. In all cases 

 the procedures give rather variable results, and we 

 have very little idea why they give any results at all. 

 But the eggs treated in this way, since they have a 

 haploid set of chromosomes, can go on developing 

 quite normally. The adult which arises is smaller 

 than normal, and its cells are smaller than normal, 

 since they adjust their volume to that of the half-sized 

 nuclei. Some eggs normally develop without being 

 fertilized by sperm, i.e. parthenogenetically. This 

 happens, for instance, to many of the eggs of bees, 

 and these parthenogenetic eggs give rise to drones 

 or males, which have only the haploid number of 

 chromosomes. In some species such animals can 

 produce sperm without performing another reduc- 

 tion division, but usually they are sterile. In other 

 cases the ^gg starts developing parthenogenetically, 

 and then succeeds in doubling its chromosome 

 number, so that the diploid condition is restored. 



Development Begins 



The first steps in the development of the tgg are 

 always the same; the egg divides up into smaller 

 and smaller cells without growing at all, till there 

 is a mass of little cells in place of the large single 



