Now that it is accepted that the phenomena observed in genetical experimentation are repercussions of events which have occurred previously in the chromosomes, it is the rule confidently to appeal, whenever possible, to the evidence derived from cytological studies for final explanations of genetical behaviour. This being so, it follows that in those instances in which, for any reason, experimental breeding work is difficult, studies of the chromosomes during the division cycles may be expected to yield significant information concerning the peculiarities of hereditary transmission that might be expected were such genetical investigation undertaken. Further, the larger mammals of economic importance are, under present conditions, unsatisfactory genetical material; they are expensive, and commonly the details of structure and function which they present for examination are not easily or precisely definable and seem to obey no simple rule of inheritance. For these reasons it seems desirable that thorough cytological studies of these forms should be undertaken, for at least these are not costly and they may be expected to provide explanations of the apparently complicated genetic behaviour which these forms exhibit.