CHAPTER X 



HAPLOIDS 



THE genetic evidence indicates that one complete 

 set of chromosomes at least is required for normal 

 development. A cell with one set of chromosomes 

 is said to be haploid, and an individual made up of such 

 cells is sometimes called a haplont or frequently, by ex- 

 tension, a haploid. The embryological evidence also indi- 

 cates that one set of chromosomes is necessary for de- 

 velopment. It does not follow, however, that the diploid 

 set can be replaced directly by a haploid set without seri- 

 ous consequences, so far as the developmental conditions 

 are involved. 



Eggs that have been incited to develop by artificial 

 agents may develop into embryos whose cells have only 

 one set of chromosomes. Not infrequently, however, the 

 eggs double the number of the chromosomes (by sup- 

 pressing a protoplasmic division) before they begin to 

 develop, and these fare better than the haploids. 



By cutting off a fragment from a sea urchin egg, and 

 fertilizing it with a single sperm, an embryo can be ob- 

 tained with only one set of chromosomes, the paternal 

 set. By constricting the egg of triton immediately after 

 fertilization, Spemann and later Baltzer have sometimes 

 been able to separate a piece of the egg that contains only 

 a single sperm-nucleus (Fig. 83), and one such embryo 

 was carried through by Baltzer to the time of metamor- 

 phosis. 



If frogs' eggs are exposed to X-rays, or to radium for 

 a sufficient time to injure or to destroy the chromosomes, 



