The Role of Nucleus and Cytoplasm 



137 



the testes. While the latter show normal size 

 and structure, the former almost always 

 remain small and are characterized by the 

 complete absence of growing oocytes or "aux- 

 ocytes" (Fankhauser, '41; Fankhauser and 

 Watson, '49). At the age of three years, 

 triploid females of T. alpestris still are com- 

 pletely sterile; auxocytes are rarely seen in 

 the ovaries, while triploid males produce 

 mature spermatozoa, although in relatively 

 small numbers (Fischberg, '47a). In triploid 

 axolotls, the young ovaries also show a 

 large-scale degeneration of very young 

 oocytes in early diplotene stages of meiosis 

 and a deficiency of auxocytes. Later on, how- 

 ever, a varying number of oocytes enter the 

 growth phase and eventually develop into 

 matvire eggs that are spawned normally. 



The complete or partial inhibition of the 

 development of the ovaries in triploid sala- 

 manders appeared to find a simple explana- 

 tion when Humphrey ('45) demonstrated, by 

 a study of the sex ratio among the offspring 

 of female axolotls experimentally converted 

 into males, that the female sex is hetero- 

 gametic, with the sex chromosome formula 

 WZ. Since the male sex is homogametic 

 (ZZ), triploid males have the corresponding 

 formula ZZZ and might be expected to be 

 normal. Triploid females, obtained by refrig- 

 eration of eggs, are two types, WZZ and 

 WWZ, neither of which would have the 

 same ratio of W chromosomes to Z chromo- 

 somes or to autosomes as the diploid. A dis- 

 turbance of the balance of sex-determining 

 genes might thus account for the abnormal 

 development of the ovaries. However, when 

 it became possible, through the use of sex- 

 reversed females and their offspring, to ob- 

 tain still another tvpe of triploid females, 

 of the constitution WWW, it was discovered 

 that such females exhibited exactly the same 

 deficiency of ovarian development as the 

 other two types, in spite of the fact that 

 their sex chromosome balance was the same 

 as in WW diploids which are normal fe- 

 males (Humphrey and Fankhauser, '46). It 

 appears that the primary process of sex 

 determination may not be involved at all 

 but that the later differentiation of the ova- 

 ries, which may be controlled bv other 

 genes located on the autosomes, is adversely 

 affected by the triploid constitution of the 

 cells, possibly by a disturbance of gene 

 dosage. 



The depression of the G:rowth rate and vi- 

 ability of higher nolvploids suggests that 

 some basic metabolic processes may be af- 

 fected directly by the multiple gene com- 



plex, or, indirectly, by the larger size of 

 the nuclei and cells. Measurements of the 

 rate of oxygen consumption and of enzyme 

 activity, which are still in a preliminary 

 stage, should offer valuable information on 

 fundamental problems of cell physiology. 



Recently the study of polyploidy has been 

 extended to other vertebrates. Makino and 

 Ozima ('43) found that cold treatment of 

 eggs of the carp inhibits the formation of 

 the second polar body; it is very probable 

 that treated eggs would give rise to triploid 

 embryos. In mammals, the occurrence of 

 polyploidy was demonstrated for the first 

 time by Beatty and Fischberg ('49). Chro- 

 mosome counts in pre-implantation embryos 

 of the mouse, about three and one-half days 

 after copulation, showed that triploid and, 

 more rarely, tetraploid embryos occur with- 

 out treatment. The incidence of spontaneous 

 heteroploidy varied with the strain of mice 

 used; a certain silver strain exhibited a par- 

 ticularly high percentage of triploids (3.8 

 per cent). In this strain, triploid embryos 

 could be identified as late as mid-term (nine 

 and one-half days). A heat shock (45° C. 

 for 5 to 10 minutes), applied to tubal eggs at 

 the estimated time of fertilization, produced 

 16.5 per cent heteroploid embryos, mostlv 

 triploid, as identified at three and one-half 

 days; treatment during the first cleavage 

 mitosis gave 29 per cent tetraploid embryos 

 (cf. Beatty, '51, for review and list of other 

 references). 



In the rabbit, insemination of females 

 with sperm suspended in colchicine solu- 

 tion produced two individuals that were 

 complex chromosomal mosaics, partly di- 

 ploid, partly polvploid or aneuploid (Hagg- 

 auist and Bane, '50a and b; Melander, '50). 

 Corresponding experiments with pigs pro- 

 dviced one heteroploid individual (Haggquist 

 and Bane, '51; Melander, '51). The inter- 

 pretation of this case is complicated by the 

 fact that the normal, diploid chromosome 

 number seems to vary between different races 

 of pigs. It is interesting that the heteroploid 

 rabbits and pig were reported to be larger 

 than the controls. 



Aneuploidv (Unbalanced Chromosome Com- 

 plements). Hvperdiploid embryos with from 

 one to four extra chromosomes occur spon- 

 taneouslv with verv low frenuency in vari- 

 ous species of amphibians. They can now 

 be obtained on a large scale in the axolotl by 

 breeding triploid females with dinloid males 

 TFankhauser and Humnhrev. '50). At the 

 first meiotic division of the triploid eggs, the 

 third set of chromosomes is distributed at 



