Introductory .-Those writers who hold that the sex of an animals organism can be affected by a modification of external conditions during the period of development have of late years supported their case by reference to the parthenogenetic rotifer, Hydatina senta . Upon certain of the facts of reproduction there is a general consensus of opinion among those who have made a study of this species. Briefly summarised it is as follows:- Three kinds of ova are produced by this rotifer, viz: (1) parthenogenetic ova which develop into♀’s. (2) parthenogenetic ova of smaller size which develop into ♂’s; and (3) fertilised eggs which always develop into ♀’s. Any given ♀ lays but one of these three classes of eggs during her lifetime. We may, therefore, distinguish three kinds of ♀’s by the eggs they lay, viz: ( a ) ♀’s which procedure ♀’s parthenogenetically, or thelytokous ♀’s ; ( b ) ♀’s which procedure ♂’s parthenogenetically, or arrenotokous ♀’s ; and ( c ) the layers of fetilised eggs. Both thelytokous and arrenotokous ♀’s can be impregnated has no effect, unless it occurs during the first few hours after hatching. When this takes place such a ♀ produces fertilised eggs instead of the ♂ eggs which it would otherwise have laid. The layers of fertilised eggs instead of the ♂ eggs which it would otherwise have laid. The layers of fertilised eggs are, therefore, arrenotokous ♀’s, which owe their special nature to the circumstance of having been impregnated by the ♂ during the earlier stages of their growth. And in this connection it is interesting to recall Lenssen’s observation that the ♂ egg extrudes a single polar body, whilst the ♀ egg extrudes none.