The Sexual Phases of Myzostoma. 
291 
of the hermaplirodite’s existence tbe reproductive System may receive 
less, during another period more nutriment. Hence tbe animal will 
produce first spermatozoa and tben ova. This dichogamy, especially 
in parasitic animals, is assumed to depend also on another factor, 
viz. tbe necessity of producing an immense number of ova and 
spermatozoa, for tbe vicissitudes of parasitic life are so great that 
only a few of tbe many eggs can ever reacb their full development. 
Tberefore tbe animal must begin very early and continue tbrougbout 
life to produce reproductive elements. And altbougb perbaps provided 
witb an abundance of food, mucb of tbis must be utilized in tbe 
processes of growtb and only a small residuum is available for tbe 
production of tbe less expensive reproductive elements, tbe spermatozoa. 
As tbe animal approacbes its adult stature, bowever, and growtb is 
nearly completed, it can spare more material for tbe production of 
tbe more expensive yolk-laden ova. 
Otber autbors are inclined to believe tbat dicbogamy, being ob- 
viously conducive to cross-fertilization, bas tberefore been produced 
by natural selection from cbance variations in tbe time of maturation 
of tbe reproductive cells. Tbe advantages of cross-fertilization appear 
to bave been demonstrated in some cases, but a fact wbicb I bave 
often observed in M. glahrum — viz. tbat tbe eggs are fertilized as 
readily witb tbe spermatozoa of tbe same individual and develop 
as rapidly and normally as eggs fertilized witb tbe spermatozoa of 
otber individuals — leads me to believe tbat tbe above mentioned 
pbysiological causes are tbe fundamental ones, and tbat tbe ad- 
vantages wbicb may in some cases result from dicbogamy are ac- 
cessory. In otber words: cross-fertilization may be tbe consequence 
of pbysiological causes of a nutritive cbaracter, and tbis consequence 
may not be inevitable, as is sbown in some cases [M, glahrum and 
tbe yuccas among plants), wbere tbe functional male and functional 
female stages of tbe same individual coincide or overlap, and tbereby 
admit of self-fertilization. 
In tbis Connection it is, perbaps, admissible to add a few remarks 
on a question concerning wbicb tbere is still considerable difference 
of opinion among zoologists, tbe question as to wbetber berma- 
pbroditism or gonocborism is pbylogenetically tbe 
more primitive condition in tbe Metazoa. Tbe majority of 
zoologists maintain tbat the Metazoa were originally bermaphrodite 
and tbat tbe dioecious forms bave been derived from tbese by tbe 
Suppression of one set of reproductive Organs, tbe female individual 
Mittheilungeu a. d. Zoolog. Station zu Neapel, Bd. 12. 20 
