SPECIFICITY IN FERTILIZATION 193 



between suborders give strong plutei; however, no one 

 has carried the crosses considered in this section through 

 the metamorphosis, and it is not known whether this 

 is possible. 



The heredity of intergeneric and wider crosses 

 within the order is a matter of great interest, but the 

 discussion would carry us beyond the scope of our 

 subject. There is a strong tendency for such crosses 

 to exhibit a preponderance of maternal characters^ 

 though this is not always the case. Herbst (1909, 

 1912) and Balzer (1910) have shown that such cases 

 are, frequently at least, accompanied by an extrusion 

 of paternal chromatin, and they have thus furnished 

 an explanation of some cases of matroclinal heredity. 

 But other authors have pointed out that the direction 

 of inheritance may be related to reaction of the sea- 

 water (Tennent), or to seasonal conditions (Vernon), 

 or to factors affecting the growth of the germ cells 

 during their period of growth and maturation (Shearer, 

 De Morgan, and Fuchs). These views are not neces- 

 sarily inconsistent, for the external factors may operate 

 by modification of chromosome relations. 



3. Interclass crosses have also been made in echino- 

 derms. In all such cases it has been necessary to 

 create artificial conditions for the fertilization reaction. 

 The first experiments of this kind were performed by 

 Loeb in 1903, who found that the eggs of Strongylo- 

 centrotus purpuratus can be fertilized by the sperm of 

 Asterias (starfish) in the presence of an excess of alkali. 

 Fertilization succeeded best in a solution of 100 c.c. 

 sea-water to which 1.2 c.c. N/io NaOH had been 

 added. Perhaps 50 per cent formed membranes and 



