EXPERIMENTAL ANALYSIS 



large number of agents ; it has recently been shown that they occur 

 even during the normal germination of the bean (Levan and Lotfy^''^). 

 Hughes and Fell^^^ found that when chick tissue cultures were 

 treated with doses of sulphur mustard just sufficient to induce some 

 fragmentation of the chromosomes, the damage to the mitotic spindle 

 and the cleavage mechanism of the cell was then comparatively slight. 

 Mctaphase was often prolonged, but in anaphase those chromosomes 

 attached to the spindle then moved at the usual rate. The abnormalities 

 in subsequent divisions of such cells have not yet been examined. 

 Persistent breakage of a chromosome is in itself not necessarily lethal 

 to a cell; Hughes-Schrader and Ris^'^ have demonstrated this in 

 their work on the effect of X-rays on Hemiptera such as the coccid bug 

 Steatococcus where, thanks to a diffuse attachment to the spindle, 

 chromosome fragments retain their connection thereto. Embryos of this 

 insect were shown to continue their development after X-ray treatment, 

 and many cycles of somatic division occurred with fragmented chro- 

 mosomes. 



The ultimate lethal effects of chromosome breakage in a lineage of 

 cells where centromeres are of the normal type must therefore be due 

 to loss of chromosomal material. Instances are known where early 

 embryonic development is blocked if a chromosome is lacking in the 

 nucleus; Poulson^'^ has shown that eggs of Drosophila without an 

 X-chromosome do not develop normally beyond segmentation. 



A further possibility is that misplaced chromosomal nucleoproteins 

 may exert inhibitory effects within the cytoplasm. This is suggested by 

 the recent observations of Mazia^''^ that development can be blocked 

 when an embryo is treated with the deoxyribonucleic acid of its own 

 species. This was found to occur with eggs both oi^ Asterias and the frog, 

 when their own respective DNA was added to the surrounding medium. 

 Foreign DNA was found to have no effect in either instance. Bracket 

 (private communication), however, has been unable to confirm these 

 observations of Mazia, though inhibitory effects of constituent molecu- 

 lar units of the nucleic acids, acting either before (p 189) or during 

 (p 198) mitosis, have been described by several authors. 



Blockage of the cycle of the deoxynucleoproteins of the cell at a 

 different point is suggested by the work of Marshak;^^^ Marshak and 

 Harting^^" on the effects of d-usnic acid on the fertilized egg of 

 Arbacia. This substance, which is extracted from a lichen, inhibits 

 DN-ase in vitro in the presence of cobalt. In the egg, usnic acid prevents 

 the fusion of male and female pronuclei, and also the cleavage of the 

 cell, when it is added subsequent to the prophase of the first mitosis. 

 Mazia^'* has shown that DN-ase is found within the cytoplasm of the 

 Arbacia egg and so the inhibitory effects of usnic acid may possibly be 

 exerted therein. Although the particular observations of these two 



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