The Role of Nucleus and Cytoplasm 



139 



either spontaneously or in various experi- 

 ments (polyspermy, parthenogenesis, cold or 

 heat treatment of eggs), which exhibit two 

 or sometimes three different chromosome 

 numbers in different regions which may vary 

 in extent from a few cells to one-half of the 

 body. Most frequent are haploid/diploid and 

 haploid/triploid embryos, but several other 

 combinations have been observed. 



The haploid side of a mosaic generally 

 shows a much more normal development 

 than may be seen in completely haploid 

 animals (compare Fig. 33a with Fig. 29c). 

 Since mosaics possess a normal circulation, 

 with both hapoid and diploid blood cells cir- 

 culating through the haploid organs, the bet- 

 ter performance of the latter may possibly 

 be explained on this basis. However, parabio- 



Fig. 34. Development of dispermic sea urchin egg. a. Fusion of both sperm nuclei with egg nucleus, each 

 containing a haploid set of 18 chromosomes; both sperm asters have divided, h. Random distribution of 54 

 chromosomes among spindles connecting the four poles of the mitotic figure, c. Anaphase; chromosomes from 

 two adjacent spindles move toward each pole, d. Telophase; division of egg into four cells with different 

 numbers of chromosomes, e, "Stereoblastula," final stage of development. Cells of right-hand half disinte- 

 grating. (After Boveri, '07.) 



Haploid/diploid mosaics are easily dis- 

 covered among the offspring of crosses be- 

 tween white and dark axolotls, if the haploid 

 area is derived from the white (recessive) 

 parent. Figure 33« shows an unusually regu- 

 lar lateral mosaic of this type with a sharp 

 line of demarkation almost exactly in the 

 median plane. The organs on the haploid 

 (left) side are smaller than those on the 

 right. Dissection of the animal at the age of 

 eight months further revealed that this in- 

 dividual started out as a gynandromorph, 

 with a testis on the left and an ovary on the 

 right side. However, at the time of the 

 autopsy the ovary had been almost com- 

 pletely transformed into a testis under the 

 influence of the male sex hormone produced 

 by the testis on the haploid side. The origin 

 of this lateral mosaic may be explained in 

 either one of two ways which are described 

 in Figures 336 and c. 



sis between haploid and diploid embryos of 

 T . pyrrhogaster, which also accomplishes a 

 joint circulation, did not result in the ex- 

 pected improvement of the haploid partner, 

 aside from a reduction of the edema (Kaylor, 

 '40). The presence of a partly diploid nervous 

 system may be another factor favoring nor- 

 mal functioning of the haploid side of the 

 body. Finally, the proximity of normal dip- 

 loid tissues may have a stimulating influence 

 on adjacent haploid tissues, as has been 

 shown by Hadorn ('35, '37) in chimaeras 

 composed of halves of diploid and haploid 

 embryos. 



The Effects of Irregular Distribution of 

 Chromosomes in Multipolar Mitosis. In sea 

 urchin eggs that are fertilized by two sper- 

 matozoa, both sperm nuclei fuse with the 

 egg nucleus (Fig. 34a) so that three sets of 

 chromosomes are present at the first division 

 of the egg. In the majority of such eggs, both 



