134 



The Nucleus and Cytoplasm in Development 



drome which appears later and does not af- 

 fect the head region primarily. 



Other possible factors reducing the vi- 

 ability of haploids may be a deficiency 

 in the synthesis of ribose nucleoprotein 

 (Brachet, '44, '47) or of various enzymes, 

 as is suggested by the increased sensitivity 

 of haploid frog embryos to hexenolactone 

 (Briggs, '46). Whether these disturbances in 



twice the normal size, e.g., the nvimber of 

 primary mesenchyme cells was reduced from 

 about 43 to 23. Such gastrulae did not develop 

 beyond the early pluteus stage and were usu- 

 ally abnormal. The presence of some larvae 

 with as few as 9 to 13 mesenchyme cells of 

 particularly large size suggested that occa- 

 sionally a second monaster cycle followed 

 upon the first, producing octoploid embryos. 



"0^^ 





Fig. 30. Drawings of nuclei from epidermis cells of axolotl larvae to show increase in size with increase in 

 chromosome number. The maximum number of nucleoli in each group of nuclei corresponds to the number 

 of chromosome sets present; smaller numbers are caused by fusion of nucleoli, a, Haploid; b, diploid; c. 

 triploid; d, tetraploid; e, pentaploid; /, heptaploid. 



the biochemistry of haploid cells are con- 

 nected with the reduction in the total mass 

 of nuclear material, perhaps the heterochro- 

 matin in particular, or with an upset of gene 

 dosage remains to be investigated. 



Polyploidy. The first observations on the 

 effect of an increase in chromosome number 

 on development were made by Boveri ('05) 

 on eggs of the sea urchin, Paracentrotus 

 lividus, in which the division of the aster 

 during the first cleavage mitosis was sup- 

 pressed by shaking. In the course of the 

 resulting monocentric mitosis the chromo- 

 somes divided normally but, in the absence 

 of a mechanism to separate the daughter 

 chromosomes, they were all incorporated 

 into a single nucleus during telophase. Since 

 the aster often divided normally at the fol- 

 lowing mitosis, cleavage began with a delay 

 and produced a tetraploid gastrula contain- 

 ing about half the usual number of cells of 



A more detailed analysis of the develop- 

 ment of polyploid embryos became possible 

 in various species of amphibians through the 

 discovery that triploid and, more rarely, 

 tetraploid and pentaploid individuals appear 

 spontaneously among normal larvae raised 

 in the laboratory (Table 7). These excep- 

 tional individuals may be easily identified 

 soon after hatching by means of chromosome 

 counts in stained preparations of the ampu- 

 tated tip of the tail. Very probably they owe 

 their origin to accidents at meiosis, usually 

 nonreduction of the chromosome number dur- 

 ing maturation of the egg. Triploid embryos 

 may be produced on a larger scale by treat- 

 ment of freshly fertilized eggs with extreme 

 temperatures. Refrigeration for several hours, 

 or exposure to -|-36° C. for ten minutes, fre- 

 quently suppresses the formation of the sec- 

 ond polar body which normally is not given 

 off until about one hour after fertilization. 



