TETRAPLOIDS 107 



reverse may be true. All these situations have been ex- 

 tensively studied, and are familiar to every student of the 

 cell. The occurrence of such cases does not invalidate the 

 general statement, that the number of the chromosomes 

 is constant and characteristic of each species.^ 



In recent years an ever increasing number of cases has 

 been reported in which indiAdduals have suddenly ap- 

 peared that have double the number of chromosomes 

 characteristic of the species. These are the fourfold types, 

 or tetraploids. Other multiple types have also been found, 

 some arising spontaneously, others derived from the 

 tetraploids. "We speak of these collectively as polyploids. 

 Of these polyploids, the fourfold group is in many ways 

 the most interesting. 



In animals there are only three cases of tetraploidy 

 that are certainly known. The parasitic threadworm of 

 the horse, Ascaris, occurs in two types, one with two and 

 one with four chromosomes, respectively. These two 

 varieties are like each other, even as to the size of their 

 cells. The chromosomes of Ascaris are regarded as com- 

 pound and as formed by the union of a number of smaller 

 chromosomes sometimes called chromomeres. In the cells 

 of the embryo that will become body-cells, each chromo- 

 some breaks up into its constituent elements (Fig. 63, a, 

 b, d). These are constant in number or approximately so, 



2 Delia Valle and Hovasse have in recent years denied that the number 

 of chromosomes is constant in different tissue cells. In so far as the conclu- 

 sions are based on an examination of the somatic cells of Amphibia that 

 have a large number of chromosomes difficult to identify vrith accuracy, 

 their results do not suffice to overthrow the overwhelming number of obser- 

 vations on other forms (and even on some Amphibia) where the number 

 of the chromosomes can be accurately determined. 



It is also known that in certain tissues the number of the chromosomes 

 may be doubled or quadrupled, either by failure of cells to divide when the 

 chromosomes divide, or else by the chromosomes breaking up into a con- 

 stant number of parts. These are special cases that do not affect the general 

 situation. 



