18 Hyhridization of Echinoids. 



If we define fertilization as the process of conjugation of egg and 

 sperm nuclei, and stop there without further qualification, cases in 

 which there is only a temporary fusion of egg and sperm nuclei, fol- 

 lowed by the rejection of the paternal chromatin, might be accepted 

 as examples of fertilization. If, however, we follow this definition to 

 its logical conclusion and insist further that fertilization "can not 

 be regarded as complete until full capacity for development and 

 inheritance is attained by the zygote," or if we define fertiHzation as 

 ** the permanent fusion of two germ-cells, one of paternal and one 

 of maternal origin" (Wilson, 1902), and give to this the necessary 

 physiological qualification, there need be little difficulty in restricting 

 the use of the term cross-fertilization to its proper signification. 



The real difficulty does not lie in the use of the word ''cross," but 

 in the use of the word "fertilization." Suppose that in an attempted 

 cross we get nothing more than activation. The eggs have then been 

 "cross" activated. 



The sperm nucleus having entered the egg, internal block may inter- 

 vene to prevent its union with the egg nucleus, yet the egg may 

 develop thelykaryotically. Or suppose that no internal block to the 

 union of the nuclei exists, yet in succeeding stages some deviation from 

 a successful course of development occurs, such as the elimination of 

 chromosomes in the first or any succeeding cleavage, or a complete 

 developmental block at the time of beginning gastrulation ; in none 

 of the cases has the egg been fertilized. It has been cross-activated, 

 and in the cases mentioned three different results have followed this 

 activation. On the other hand, cross-activation may be followed 

 by a series of reactions whose sum total is perfect fertilization, judged 

 by the standard of attainment of full capacity for development and 

 inheritance. 



For these different results the terms suggested by Giinther Hertwig 

 (1918), with a slight addition that I shall suggest, will be found useful. 



Classification of Hybrids. 



G. Hertwig classifies hybrids as true hybrids or orthonothi and 

 false hybrids or pseudonothi. The true hybrids contain the full 

 complement of maternal and paternal chromatin, while the false 

 hybrids contain the maternal chromatin only, their development 

 being therefore parthenogenetic. The true hybrids may be divided 

 further into fertile individuals, sterile individuals, and misformed, 

 pathological, non-viable individuals. The fertile and sterile indi- 

 viduals correspond to Poll's (1920 and earlier papers) tokonothi and 

 steironothi. For the misformed individuals Hertwig suggests the 

 name dysnothi. 



The false hybrids are of two types, those with haploid nuclei and 

 those with diploid nuclei, those of the second type differing from the 



