EFFECTS OF MIXING DIVERSE ORGANISMS 269 



When the parents are somewhat less diverse, the proc- 

 esses are less striking In appearance, though the ultimate 

 result is the same. When sea urchin ova are fertilized by 

 sperms of the crinold (both being echinoderms) , the sperm 

 nucleus unites with the egg nucleus, and Its chromosomes 

 are not ejected nor destroyed (at least not In the early 

 stages of development).^ But the chromosomes from the 

 crinold have no visible effect on the method of develop- 

 ment, though they cause the offspring to die in early stages. 

 So far as they develop the young are exclusively sea urchin 

 in type. 



When the parents are still more nearly related, as when 

 different genera or species of sea urchins are crossed, varied 

 results are produced. Sometimes the offspring are like the 

 mother species exclusively. Sometimes they are like the 

 mother almost completely, but show a few characteristics 

 of the father. And sometimes they are intermediate between 

 the two parental species. These different results occur in 

 different cases when the same two species are crossed.^ 



In judging of these results, certain considerations must 

 be kept in mind. Such crosses have been made mainly in sea 

 urchins. These do not as a rule develop In captivity beyond 

 the free swimming larval stage, so that the characteristics 

 observed are those of the larvae only. In Sphaerechinus the 

 larva has a latticed skeleton (figure 53, A) while in Para- 

 centrotus the skeleton is composed of simple rods (figure 

 ^2, B). If Sphaerechinus ova are fertilized by Paracentro- 

 tus. In some cases all the progeny have a lattice skeleton 

 like that of the mother; in other cases the skeleton is inter- 

 mediate between that of the two parents (figure 53, C). 



Why are such diverse results produced In different cases 

 of the same cross? The cause for this was discovered by 

 Herbst. Under certain conditions the sea urchin ovum be- 

 gins development before the sperm enters. This can be In- 

 duced by treatment with certain chemicals. When such ova 



