The Sexual Conditìons of Myzostoma glabrum (F. S. Leuckart;. 305 



although, from the standpoint of recent research, there is, in a morpho- 

 logical sense, little difference between them. 



Granted that the number four be the typical one for the products 

 of both sperm- aud egg-mother-cells, three of these products get lost 

 in auy hermaphrodite fertili sation (in the form of the second polar 

 body and the products of division of the first polar body), while only 

 two are cast away in a parthenogenetic egg-cell. In addition, pro- 

 ducts of two mother-cells are required for the production of one 

 fertilised egg of an hermaphrodite, whereas in parthenogenesis the 

 sanie two cells would give rise to two (self-) fertilised eggs. There 

 is, thus, in parthenogenesis a greater economy, but the advantages 

 of cross-fertilisation are excluded. There must, undoubtedly, be some 

 law, determining which of the two states, the parthenogenetic or the 

 hermaphrodite, shall be the one adopted, if a departure from the 

 dicecious condition become desirable. 



Parthenogenesis endangers the continued existence of the male, 

 its continuity, to a much greater extent than does hermaphroditism. 



Years ago Weismann demonstrated that in certaincases of partheno- 

 genesis the appearance of males tends to become cyclical, and that 

 the interval between any two appearances tends to lengthen , tili 

 ultimately, as in Artemia, the cycle hecomes practically infinite. 



The cycle would appear to be introduced in parthenogenesis at 

 an early period (Weismann), while in hermaphroditism it probably 

 only occurs long after the state has become firmly established. If 

 the view be accepted, that cross-fertilisation is the great factor in 

 producing variations within a species, then in any given case con- 

 tinued existence is the less endangered by the introduction of partheno- 

 genesis in the Proportion that the species has become stable, or is 

 in a condition of stable equilibrium. If unstable, hermaphroditism 

 is the more favourable arrangement of the two, for it diminishes the 

 variability in no degree, unless it be a self-impregnating herma- 

 phroditism, a condition rarely or never actually niet with. The case 

 of Mìjzostoma typically shews, that great variability may co-exist 

 with hermaphroditism, for within the limits of the genus the number 

 of species with closely allied characters and yet difl'ering from one 

 another in some respects is very great'. 



In Wheeler's memoir ^8) on the sexual phases of Myzostoma, 

 io which reference has already been made, besides the description 



' There is also in M. glahrum a great Variation in the coloration. 



Mittheilungen a. d. Zoolog. Statiou zu Neapel. Bd. 13. 20 



