BEAEERS OF HEREDITARY UNITS 215 



to-day some uncertainty concerning the assumption that 

 normal development is to be expected if in addition to 

 one haploid set of chromosomes other chromosomes are 

 also present, because while one set alone might permit 

 normal development, it is by no means certain that if 

 there were one, two, or more additional chromosomes, the 

 balance might not be upset and abnormal development fol- 

 low. On chance distribution alone the isolation of just one 

 set and no more would seem a very remote possibility, 





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Fig. 99. — First division of a hybrid egg showing the elimination of chromosomes at the 

 equation of the spindle, a. The reciprocal cross, b, shows no such elimination. (After Baltzer). 



but if there is to some degree a tendency for a group of 

 daughter chromosomes to move off together as a result 

 of their method of division, there might be a better chance 

 of such a group getting into one of the three or four 

 blastomeres than by chance distribution alone. At pres- 

 ent it is not possible to make any calculation based on such 

 an assumption. While, therefore, Boveri's argument can- 

 not be accepted as demonstrative, yet it has probability 

 in its favor. 



Baltzer has found a different kind of evidence of 

 chromosomal influence. When the eggs of one sea urchin, 



