HYBRIDS BETWEEN FUNDULUS AND MACKEREL 393 



Only one cross of greater than suborder width has been made 

 within the bounds of the class Echinoidea and that is of sub- 

 class width. McBride and Fuchs have independently crossed a 

 representative of the subclass Regularia with one of the sub- 

 class Irregularia, using Echinus and Echinocardium. The 

 percentage of fertilizations is so low and the larvae are so un- 

 healthy that no definite conclusions were reached by either 

 author. 



By means of chemical aids to insemination, 'crosses' of inter- 

 class width have been made between Echinoidea eggs and the 

 sperm of Astereridea, Holothuroidea, Ophiuroidea and Crinoidea. 

 Though extremely few larvae resulted — and these were decidedly 

 abnormal — they were described as pure material. By the same 

 chemical means Echinoid eggs have been inseminated with the 

 sperm of MoUusca and AnneUda, and occasional stunted larvae 

 have resulted that also are said to be pure materinal. These 

 interphylum ' crosses' mark the extreme limit of heterogeneity in 

 hybridization, if indeed they are hybrid phenomena at all. 

 One step further and we would have crosses of subkingdom value 

 (Protozoa x Metazoa) or even of kingdom value (plants X 

 animals), which would amount to a reductio ad absurdum. 



It must be borne in mind that, in Echinoids, 'crosses' of greater 

 than suborder width are impossible in nature and can be made 

 only by the use of chemical agents that serve to break down the 

 normal incompatibihty of the heterogenous germinal materials, 

 and may well play a role analogous to that of parthenogenetic 

 agents. 



'Crosses' of greater than suborder width are described as being 

 purely maternal, although the criteria for this judgment are ex- 

 tremely doubtful since the embryo and larvae reared are highly 

 pathological. No paternal heredity is to be expected in view of 

 the fact that the foreign chromatin forced into the egg unnaturally, 

 utterly fails to cooperate in cleavage or in more general cell 

 metabolism, but remains an inert mass till gradually eliminated 

 by absorption. An exception appears to exist in the case of the 

 cross between Echinus and Antedon, in which Godlewski claims 

 a normal behavior of the paternal chromosomes, except that 



