The Sexual Pliases of Myzostoma. 
291 
of the hermaplirodite's existence the reproductive system may receive 
less, during another period more nutriraent. Hence the animai will 
produce first spermatozoa and then ova. This dichogamy, especially 
in parasitic animals, is assumed to depend also on another factor, 
viz. the necessity of producine an immense number of ova and 
spermatozoa, for the vicissitudes of parasitic life are so great that 
only a few of the many eggs can ever reach their full development. 
Therefore the animai must begin very early and continue throughout 
life to produce reproductive elements. And although perhaps provided 
with an abundance of food, much of this must be utilized in the 
processes of growth and only a small residuum is available for the 
production of the less expensive reproductive elements, the spermatozoa. 
As the animai approaches its adult stature, however, and growth is 
nearly completed, it can spare more material for the production of 
the more expensive yolk-laden ova. 
Other authors are inclined to believe that dichogamy, being ob- 
viously conducive to cross-fertilization, has therefore been produced 
by naturai selection from chance variations in the time of maturation 
of the reproductive cells. The advantages of cross-fertilization appear 
to bave been demonstrated in some cases, but a fact which I bave 
often observed in M. glahrum — viz. that the eggs are fertilized as 
readily with the spermatozoa of the same individuai and develop 
as rapidly and normally as eggs fertilized with the spermatozoa of 
other individuals — leads me to believe that the above mentioned 
physiological causes are the fundamental ones, and that the ad- 
vantages which may in some cases result from dichogamy are ac- 
cessory. In other words: cross-fertilization may be the consequence 
of physiological causes of a nutritive character, and this consequence 
may not be inevitable, as is shown in some cases [M. glahrum and 
the yuccas among plants), where the functional male and functional 
female stages of the same individuai coincide or overlap, and thereby 
admit of self-fertilization. 
In this connection it is, perhaps, admissible to add a few remarks 
on a question concerning which there is stili considerable difference 
of opinion among zoologists, the question as to whether herma- 
phroditism or gonochorism is phylogenetically the 
more primitive condition in the Metazoa. The majority of 
zoologists maintain that the Metazoa were originally hermaphrodite 
and that the dioecious forms bave been derived from these by the 
suppression of one set of reproductive organs, the female individuai 
Mittheilungeü a. d. Zoolog. Station zu Neapel. Bd. 12. 20 
