136 



The Nucleus and Cytoplasm in Development 



is in striking contrast to the gigantism of 

 many polyploid plants and some polyploid 

 invertebrates, for instance, tetraploid brine 

 shrimp embryos (Artemia), triploid isopods 

 (Trichoniscus) and bagworm moths (Solen- 

 obia). Triploid and tetraploid silkworms, on 

 the other hand, show the same neutraliza- 

 tion of larger cell size by smaller cell nimi- 

 ber as polyploid amphibian larvae (for a 

 detailed discussion of the literature see 

 Fankhauser, '41, '45a, and '52).* 



In later stages of development triploid 

 larvae, as a rule, grow normally, while the 

 growth of tetraploids and pentaploids is usu- 



Fig. 32. Diagrams of cross sections through a 

 pronephric tubule (above) and the lens epithelium 

 (below) in (a) a haploid larva, (b) a diploid, (c) 

 a pentaploid (Triturus viridescens) . Normal size 

 and structure are maintained with cells of different 

 sizes by adjustment of nmnber and shape of indi- 

 vidual cells. 



ally retarded. Following metamorphosis, 

 triploid T. alpestris continue to grow at the 

 normal pace for three and more years (Fisch- 

 berg, '47a). In the axolotl, adult and sexually 

 mature triploids are of approximately the 

 same size as diploids, while tetraploids are 

 on the average smaller; pentaploids, at one 

 year, lag considerably behind the tetraploids. 

 The final body size of the adult salamander 

 is definitely not determined by the size of 

 the cells present in the individual. It ap- 

 pears, rather, that the total mass of body sub- 

 stance produced is more or less fixed by the 

 genetic constitution of the species and will 

 be reached under favorable conditions ir- 

 respective of the initial mass of the egg or 

 of the size of the cells. 



* More or less pronounced gigantism has recently 

 been observed in partially polyploid frog tadpoles 

 obtained by colchicine or sulfanilamide treatment 

 of eggs ( Jahn, '52) . 



The reduction in cell number shown by 

 polyploid amphibian larvae produces inter- 

 esting effects in the structure of the nervous 

 system and sense organs, for which earlier 

 investigators had claimed an approximate 

 "cell constancy." For instance, in the eye of 

 a tetraploid, the structure of the retina is 

 profoundly affected in all layers and shows 

 fewer but conspicuously large rods and 

 cones. Adjustment in cell number becomes 

 impossible in the case of Mauthner's cells, a 

 single pair of giant ganglion cells in the 

 medulla at the point of entrance of the sev- 

 enth and eighth nerve roots. Tetraploid 

 larvae, like the diploid, still possess two 

 Mauthner's cells which are about twice nor- 

 mal size. A haploid larva, on the other hand, 

 which has a larger number of cells than the 

 diploid, although not nearly twice as many, 

 may form two Mauthner's cells of half nor- 

 mal size on either side (Fankhauser, '52, 

 PI. 1). 



Special conditions are also offered by 

 tubular organs, such as the pronephric tu- 

 bules and ducts, and by flat epithelia like 

 the epidermis or the epithelium of the lens 

 (Fig. 32). In haploid and polyploid larvae 

 the normal structure of such organs can be 

 maintained only if there is an adjustment 

 both in the number and in the shape of the 

 individual cells. In haploids, the cells are 

 more numerous and more nearly cuboidal; in 

 polyploids, the cells are fewer in number and 

 much more flattened than in the diploid. The 

 thickness of the wall of the tubule or of the 

 epithelium remains approximately the same 

 at all levels of cell size (Fankhauser, '45b). 

 The normal character of the development 

 and organization of polyploid embryos and 

 larvae indicates that it makes little or no 

 difference whether the genes are present in 

 double, triple, or even in quintuple dose as 

 long as all genes are multiplied to the same 

 degree. In this respect, genes located on the 

 sex chromosomes occupy a special position 

 since polyploids may possess various com- 

 binations of sex chromosomes that cannot be 

 realized in diploids. In particular, if sex de- 

 termination should depend on a balance be- 

 tween "male-determining" and "female-de- 

 termining" genes that are spread over both 

 sex chromosomes and autosomes, as is the 

 case in Drosophila, this balance might be 

 completely upset. 



At the very beginning of the investigations 

 on sex differentiation in triploid T. viride- 

 scens before and after metamorphosis, it be- 

 came apparent that the development of the 

 ovaries is much more affected than that of 



