412 H. H. NEWMAN 



hearts. To a certain extent the occurrence of the paternal types 

 of chromatophores bears out this interpretation, for in these 

 recovery cases there are few paternal chromatophores and some- 

 times none. When the hindering effect is lost, the hereditary 

 effect is correspondingly lost or diminished. Although the paral- 

 lelism between the occurrence in these hybrids of degree of ab- 

 normality and abundance of green (paternal type) chromato- 

 phores is not exact, it is certain that lack of green chromatophores 

 is accompanied by nearly normal development and that whenever 

 the green chromatophores occur in abundance development is 

 decidedly abnormal. 



The need of better criteria of hybridisation 



What is a hybrid and by what criteria may a true hybrid be 

 distinguished from a pseudohybrid? These questions have 

 forced themselves upon my attention during the present investi- 

 gations and demand an answer. Is it scientifically a vahd pro- 

 cedure to include as hybrids those egg and sperm unions between 

 representatives of different phyla, such as those between echino- 

 derms and molluscs, and those between different classes, such as 

 Echinoidea and Asteroidea? It is admitted by those who have 

 studied this type of union that the sperm chromatin acts as an 

 inert body within the egg and in no way, except as it hinders 

 development, plays a role in development and heredity. ]\Iore- 

 over, there is no initial attraction or reaction between egg and 

 sperm in these cases, the union being brought about by the aid 

 of chemical agents that serve to break down specific incom- 

 patibility between these foreign materials. I would therefore 

 suggest that these and all similar forced unions between diverse 

 germinal products be called what they are, merely mechanical 

 mixtures of foreign protoplasms. I would further suggest as a 

 definition of hybridization, a natural union between diverse germ 

 cells that involves a fertihzation reaction between egg and sperm 

 and in which the sperm chromatin shows evidences of cooperation 

 in cleavage and subsequent development. The acceptance of 

 these distinctions would appear to clear the atmosphere of mis- 



