Hybridization of Echinoids. 1 9 



first in the fact that their chromosomes have doubled in numbers by 

 reason of a monaster division. 



This classification does not provide a place for individuals that 

 have been obtained by cross-activation, and that have lost some, but 

 not all, of their paternal chromosomes by a process of elimination. 

 It is evident that such forms are not true hybrids, for they do not 

 contain all of the maternal and paternal chromosomes; neither can 

 they be regarded as false hybrids, since they contain some paternal 

 chromosomes. For these individuals I suggest the name ■partial 

 hybrids. It is conceivable that partial hybrids might be fertile, 

 sterile, or misformed and non-viable. Evidence on these points 

 is incomplete. 



Our classification might then be: 



1. True hybrids, all chromosomes retained : a, fertile individuals; 6, sterile individuals; 



c, misformed, non-viable individuals. 



2. Partial hybrids, partial elimination of paternal chromosomes: a, fertile individuals; 



b, sterile individuals; c, misformed, non- viable individuals. 



3. False hybrids, maternal chromatin only retained; parthenogenetic: a, haploid 



nuclei; b, diploid nuclei. 



Some of the misformed, non-viable, true, and partial hybrids, 

 although they do not produce offspring, are potentially fertile, just 

 as are the young, fertile, true hybrids that die before the age of 

 maturity. In both cases failure to produce offspring may be re- 

 garded as accidental. The group of misformed, non-viable indi- 

 viduals in each class will contain sterile forms : 



The whole matter may be stated in another way. It is only by 

 the application of a performance test and by cytological examination 

 that we can determine whether true cross-fertilization has taken place. 

 The performance test lies in the production of fertile offspring. 

 Cytological examination alone will enable us to determine whether 

 any of the chromosomes have been eliminated. If only the maternal 

 chromatin is retained, the individuals obtained from the cross- 

 activated eggs will be false hybrids (pseudonothi). If both sets of 

 chromosomes are retained, we shall have true hybrids (orthonothi), 

 but if these hybrids should be sterile, sterile because of an inability 

 to produce ripe germ-cells, these hybrids can not be regarded as having 

 been formed from "fertilized" eggs. The egg was activated, no 

 internal block prevented the union of the germ nuclei, developmental 

 block did not occur until the time of expected maturity, but full 

 capacity for development and inheritance was not attained. 



Successful cross-activation lies in demonstrable cortical reactions 

 in the egg. Granting that these reactions have taken place, and that 

 internal block or early developmental blocks have been overcome, 

 the cross-activated egg may give rise to sterile, or to misformed, 

 non-viable, true hybrids. Poll (1920) points out the fact that 



