138 



The Nucleus and Cytoplasm in Development 



random between the poles of the spindle. 

 Eggs are produced with chromosome numbers 

 ranging from the haploid (14) to almost the 

 diploid (28). This is shown by the chromo- 

 some numbers found in the tailtips of the 

 resulting embryos, which range from 28 to 

 41, with a maximum frequency at 33. In 

 about 4 per cent of the triploid eggs, one of 



gills, liver, and other organs, accompanied 

 or followed by accumulation of fluid vmder 

 the skin, in the heart or branchial regions, 

 and in the body cavity. This syndrome is in 

 most aspects typically different from that 

 shown by haploid embryos. Chromosomal 

 imbalance, regardless of the exact number 

 of chromosomes added, seems to have a gen- 



Fig. 33. a, A haploid/diploid, white/dark mosaic axolotl from a cross between a white (recessive) female 

 and a heterozygous dark male, at age of six months. Eye, gills, and limbs are smaller on haploid side (after 

 Humphrey and Fankhauser, '43). 



b and c. Two possible explanations of origin of this mosaic, b, "Partial fertilization"; egg nucleus carrying 

 factor for white first divided alone; left-hand cell produced haploid, white side of body; in right-hand cell, 

 the other descendant of the egg nucleus fused with a sperm nucleus carrying the gene for dark and gave rise 

 to the diploid, dark side. 



c, Dispermy; one sperm nucleus carrying gene for white divided independently and furnished the nuclei 

 for the haploid, white side of the body; a second sperm nucleus with gene for dark fused with the egg 

 nucleus giving rise to the diploid nuclei of the right, dark side. 



the meiotic divisions is suppressed so that 

 tetraploid embryos are produced. 



In contrast to most balanced polyploids, 

 aneuploids have a greatly reduced viability 

 and are usually abnormal in appearance. 

 The addition of as few as one or two chro- 

 mosomes to the diploid complement disturbs 

 the genie balance sufficiently to make normal 

 development a rare event. Of 377 embryos 

 with known aneuploid chromosome number 

 only thirteen lived for periods of three 

 months or more. The great majority showed 

 typical abnormalities at an early stage. Most 

 commonly the circulation was either not 

 established at all or remained subnormal, 

 with frequent stasis and hemorrhages in the 



eral effect on the development and main- 

 tenance of the circulation which in turn 

 creates the fluid imbalance. As might be 

 expected, hypertriploid or near-tetraploid 

 larvae are more viable and normal, since the 

 unbalance created by the addition of single 

 chromosomes to a multiple complement is 

 less severe. 



None of the more viable hyperdiploids 

 have reached sexual maturity so far. How- 

 ever, offspring have been obtained from non- 

 viable embryos by transplantation of gonad 

 preprimordia to normal diploid embryos in 

 the tail-bud stage. 



Chromosome Mosaics. In most of the species 

 of amphibians investigated, embryos appear, 



