310 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



the less compatible the interacting germ cells. As an example of a slight de- 

 gree of disharmony, we may cite the observation of Doncaster and Gray in 

 crosses between different species in echinidae, who found, as the only ab- 

 normality, a vesicle formation on the part of a few chromosomes. 



Disharmonies between the chromatin of egg and spermatozoon can also be 

 produced experimentally. Thus as a result of injury to the chromatin of 

 spermatozoon or ovum, caused by the exposure of these cells to radiations or 

 to certain dyes preceding fertilization, the result of homoiofertilization can 

 be made to resemble that of heterofertilization. Incompatibilities develop 

 here between the precursors of the organismal differentials of the two germ 

 cells, or between the carriers of these precursors. These incompatibilities may 

 be so severe that the injured nucleus no longer participates in the develop- 

 ment and parthenogenesis results, similar to that found in hybridization be- 

 tween different classes. However, in case the injury of the chromatin has 

 been less pronounced, the radiated nuclear substance may cause merely ab- 

 normalities in embryonal development. 



There exist certain critical periods in embryonal life when difficulties in 

 the formation of some tissues and organs are especially prone to arise, as 

 for instance, during the process of gastrulation ; in addition, it is conceivable 

 that a summation of injurious effects takes place gradually as development 

 proceeds. Furthermore, the incompatibility may affect either growth and 

 differentiation or viability, or both jointly, and there may be a parallelism 

 between the retardation in growth and in the abnormalities in differentiation 

 of the embryo as a whole, or of its individual organs, resulting from heterog- 

 enous fertilization. 



The following list of the results of heterofertilization in echinoids, which 

 Tennent gives, may show the gradation of injurious effects in a certain order 

 of animals. 



1. Elimination of no chromosomes and dominance of one species with 

 inactivation of incompatible chromosomes : 



Toxopneustes 9 X Hipponoe $ (different genera) 

 Echinus 9 X Antedon $ (different families?) 

 Strongylocentrotus 9 X Antedon $ (different families?) 



2. Elimination of part of chromosomes and dominance of one species over 

 the other: 



Hipponoe 9 X Toxopneustes $ medium incompatibility (different 



genera) 



Echinus 9 X Sphaerechinus $ (different families) 



Strongylocentrotus 9 X Sphaerechinus $ (different genera) 



3. Elimination of no chromosomes and intermediate plutei : 

 Sphaerechinus 9 X Strongylocentrotus $ (different genera) 

 Sphaerechinus 9 X Arbacia $ : most compatible (different suborders) 



4. Elimination of part of chromosomes and intermediate plutei : 

 Toxopneustes 9 X Hipponoe $ (different genera) 

 Arbacia 9 X Echinus $ (different suborders) 



