NO. 584] 



HEREDITY AND ITS MEANING 



465 



four cells of the four-fold type and allowing them to de- 

 velop into larva?. When the four cells of a four-celled 

 stage of a normal embryo are separated, each cell pro- 

 duces a normal dwarf embryo alike in every respect, but 

 the three- or four-celled embryos from double fertilized 

 eggs, when treated in the same manner, never produce 

 normal dwarfs even when the chromosome distribution 

 has been numerically equal. Large numbers of larvae 

 brought into existence through this experiment showed all 

 possible combinations of characters, just as all possible 

 chromosome combinations were found in their nuclei, 

 and from these and other data the conclusion is drawn 

 that "not a certain number, but a certain combination of 

 chromosomes is necessary to normal development, and 

 this clearly points out that chromosomes have different 

 qualities." In other words, the sea urchin has a set of 

 eighteen chromosomes, each chromosome performing at . 

 least some different functions from its neighbors, making 

 it necessary for the whole set to be present in order to 

 Insure normal development. 



In further investigations, Boveri placed sea-urchin eggs 

 which had been normally fertilized and were about to di- 

 vide under pressure. As a result, division of the nucleus 

 took place, but often no division of the cytoplasm. Such 

 eggs on again dividing often formed more than two poles, 

 resulting in inequalities in chromosome distribution and 

 abnormal larval development. Boveri puts upon these 

 cases an interpretation similar to that of the preceding 

 experiments, as the irregular chromosome distribution 

 seems to be all they have in common. 



Morgan comments on Boveri 's experiments as follows: 

 The evidence makes probable the view that the different chromosomes 

 may have somewhat different t'um-t ions ami that normal <1<-v»-]i >{ >un-n t 

 depends on the normal interactions of the materials produced by the 



Artificial parthenogenesis and experiments with enu- 

 cleated eggs have proved that only one set of chromosomes 

 is necessary to normal development of embryos, but it is 



