CHROMATIN — DEVELOPMENT AND HEREDITY 257 



The results of Cohn ('17) make clear the identity of these two 

 methods. 



The apparent absence of marked fertilizing specificity in the 

 eggs and spermatozoa of fishes would permit fertilization under 

 a wider range of conditions than would be possible in sea-urchins. 

 Assuming that the entrance of the spermatozoon into the egg and 

 the movements of the chromosomes during cell division are 

 comparable processes to the extent that they have their cause in 

 changes in surface tension, though varying in complexity and 

 perhaps in degree of specificity, it is conceivable that slight 

 variations in the condition of eggs and sperm which were no 

 hindrance to fertilization might cause a disturbance in the more 

 complex or more specific mechanism of mitosis. This would 

 account for both individual variations within one cross as well 

 as for the different results obtained at different times in the same 

 cross. Examples of the latter are familiar to all who have worked 

 with fish hybrids. 



An illustration of how slight differences in the physical con- 

 dition of germ cells may cause widely different disturbances in 

 development is seen in the varying degrees of extrusion of chro- 

 matin during the early metaphase. It was suggested that a 

 lowering of the surface tension along the newly formed cell wall 

 was responsible for the effect, the amount of chromatin extruded 

 depending upon the extent to which the surface tension was 

 affected. It is reasonable to suppose that variations in the 

 amount and in the identity of the chromatin expelled from the 

 nucleus under such conditions would be followed by extremely 

 diverse results in the nuclear constitution and consequently in 

 the fate of the developing egg. 



The work of Gray ('13) presents some observations which 

 are of interest in connection with the hypothesis that the char- 

 acter of mitosis is a function of the cytoplasm of the egg. By 

 treating normally fertilized eggs of Echinus acutus, one hour 

 after fertilization, with hypertonic salt solutions he was able to 

 cause an elimination of chromatin similar to that obtained by 

 cross-fertilizing eggs of the same species with the sperm of 

 Echinus esculentus. Because this treatment causes changes in 



