558 
K. RUGGLES GATKS AND NESTA THOMAS. 
in some respects the supernumerary chrouiosoraes described 
by Wilson in Metapodius. The latter arise in a similar way, 
by the passing oF both idiochromosomes into the same nucleus 
in meiosis, but they are duplicates of the Y-element in sex 
determination, and as such apparently have no effect on the 
external characters of the organism. 
The view is expressed that the initial nuclear difference, in 
having 15 instead of 14 chromosomes, determines the change 
in the external characters in the (E . lata-semilata series 
of mutants. And the fact is emphasised that the chromo- 
some number is a primary character of any organism, 
originating in the fertilised egg, while the extei-nal features 
are all secondary in development. 
A number of different types of meiotic irregularity are 
described in the lata and semilata mutants. These are to 
be looked upon as germinal changes, though for the most part 
their products degenerate. The irregularities studied include 
the division of a chromosome (probably the extra one) on the 
heterotypic spindle, and various other types of fragmentation 
and degeneration of chromosomes, enumerated ou p. 549. 
The fact that lata- like mutations appear sporadically in 
various races, species and interspecific hybrids of Oenothera, 
combining in some cases the hybrid characters of their 
ancestors with those of lata, shows, as does the presence of 
the extra chromosome, that mutations and Mendelian hybrids 
should be contrasted; for they owe their origin to distinct 
causes, the former to a germinal change, the latter to a 
redistribution of the parental characters. 
Royal College of Science. 
Note. — At the request of Sir Ray Lankester I have added 
the following note regarding the definition of mutation and 
the classification of various other processes which may be 
grouped under the general term ^Aariation.’^ Of course, it will 
be understood that there are not hard and fast lines of limita- 
tion between these processes. They probably shade into 
each other just as do many species. Nevertheless, definitions 
