THE MITOTIC CYCLE 



of the first division ; a 'monastral' mitosis results where one nucleus is 

 formed containing both daughter sets of chromosomes. Haploid, 

 diploid and tetraploid sea-urchin larvae at the same stage of develop- 

 ment are all of the same size. The individual nuclei and cells are related 

 in size to their chromosome content, and so it follows that a tetraploid 

 larva has about one quarter the number of cells of a haploid one. 

 Clearly some regulative process during development must limit the 

 number of cell divisions so that the standard size of the organism is 

 maintained. 



Among other observations, Boveri compared the size of the inter- 

 phase nuclei of haploid and diploid merogenous plutei of Echinus, and 

 of tetraploid and diploid gastrulae of Strongylocentrotus, and found a 

 simple proportionality between the squares of their mean nuclear 

 diameters. If the nuclei are assumed to be approximately spherical 

 their surface area must be directly proportional to their number of 

 chromosomes. 



Polyploidy in other animals 



In recent years the most extensive investigations on animal poly- 

 ploidy have been those of Fankhauser et alii on the Amphibia, which 

 provide great scope for such research (Fankhauser^^^). Haploid and 

 polyploid individuals not only occur spontaneously but their frequency 

 may be increased experimentally by treatment such as the application 

 of heat or cold to the eggs. In Amphibia, as in echinoderm larvae, 

 polyploid individuals are of the same size as diploid ; but here, haploid 

 ones are somewhat smaller.* Fankhauser makes use of measurements 

 of nuclear size in cells of the tail fin as a means of determining the 

 degree of heteroploidy in his larvae; the area of the flattened epidermal 

 nuclei is found to be proportional to the number of chromosomes. In 

 other amphibian tissues Hertwig (G. and P.)^®* claim that this is 

 more closely related to the volume of the nucleus. From the data of 

 Fankhauser^^^ and Fankhauser and Humphrey^^^ nuclear areas of 

 larval epidermal cells are plotted against their ploidy in Figure 17. 

 It can be seen that the range in nuclear areas increases at each step. 

 Among the Arthropods, there are polyploid races of certain species 

 which reproduce by parthenogenesis. Of these, the best known is the 

 brine shrimp Artemia, which has been studied by Artom^^^ ^^' ^^^ and 

 by Barigozzi.^^^ The bisexual race is diploid, while there are separate 

 parthenogenetic races which are diploid, tetraploid and octoploid 

 respectively. The cytology of their germ cells is described by White. ^^^^ 

 The relationship between chromosomal status and body size seems to 

 vary in Artemia according to Artom,!^' who investigated the ratio of 



* Recent evidence suggests that in mammals, the relationships between degree of ploidy, 

 t he size of the nuclei and of the whole body are more complex. 



54 



