The Role of Nucleus and Cytoplasm 



135 



Two haploid sets of maternal chromosomes 

 are thus left in the egg which combine with 

 the haploid set brought in by the sperm to 

 form a triploid zygote.* In the axolotl, a 

 considerable number of tetraploids and, very 

 rarely, hexaploids and heptaploids, with six 

 or seven sets of chromosomes, respectively, 

 are found among the offspring of triploid fe- 

 males (Table 7). 



and, in R. pipicns, in the pattern of deeply 

 and lightly pigmented epidermis cells in the 

 young embryo, making an early tentative 

 identification of the polyploid individuals 

 possible. 



The early development of most triploid, 

 tetraploid, and pentaploid embryos is normal 

 in appearance and rate, as far as the observa- 

 tions go. Gastrulation, neui'ulation, and or- 



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Fig. 31. Absence of gigantism in polyploid salamander larvae {Triturus viridescens) , in spite of increase 

 in cell size. From left to right: diploid, triploid, tetraploid, pentaploid. Below each larva are groups of gland 

 cells ("Leydig cells") from epidermis of tailfin. 



In polyploid embryos the size of the nu- 

 clei and cells increases approximately in 

 proportion to the number of chromosomes, 

 up to about the pentaploid level, but changes 

 more slowly from there on (Fig. 30). In the 

 axolotl, a more reliable measure of the degree 

 of polyploidy is to be found in the number 

 of nucleoli per nucleus, the maximum num- 

 ber of which always corresponds to the num- 

 ber of chromosome sets present, since each 

 nucleolus is formed at a particvilar point of 

 a certain chromosome of each haploid set. 

 In the intact living larva the larger cell size 

 expresses itself in the pattern of melanophores 



* Treatment of frog's eggs with mitotic inhibitors 

 such as colchicine and sulfanilamide produces em- 

 bryos that are partly polyploid, partly diploid. 



gan formation are thus surprisingly inde- 

 pendent of the size of the cells which, in 

 pentaploids, are about two and one-half 

 times as large as in the diploids. The two 

 heptaploid axolotl embryos recorded to date 

 were abnormal when first observed but con- 

 tinued development to the time of hatching; 

 the upper limit of tolerance for increased 

 chromosome number and cell size, while 

 not known definitely, has probably been 

 reached or surpassed at this level. 



The body size of the young larvae at the 

 time of hatching is remarkably constant re- 

 gardless of chromosome number and cell 

 size (Fig. 31). The increase in the size of 

 the cells is compensated, at each level, by a 

 corresponding decrease in their number. This 



