Vitellocytes have two important functions in cestode embryogenesis: (1) formation
of hard egg-shell (e.g. Pseudophyllidea) or a delicate capsule (e.g. Cyclophyllidea),
and (2) supplying nutritive reserves for the developing embryos. During evolution
any of these two functions can be reduced or intensified in different taxa depending
on the type of their embryonic development, degree of ovoviviparity and life cycles.
Within the Cestoda, there are three monozoic taxa with only one set of genital organs:
Amphilinidea, Gyrocotylidea and Caryophyllidea. In these monozoic taxa and some polyzoic
groups with well developed vitellaria (e.g. Pseudophyllidea, Trypanorhyncha) a single
oocyte [=germocyte] and a large number of vitellocytes (up to 30) are enclosed within
a thick, hardened egg-shell, forming a type of eggs typical for the basic pattern
of Neodermata. Only one type of egg-shell enclosures, the so-called 'heterogeneous
shell-globule vesicle' is common for the above mentioned cestode taxa. Each membrane-bounded
vesicle of mature vitellocytes contains numerous electron-dense shell globules embedded
in a translucent matrix. In free-living Neoophora and Monogenea there are two types
of vesicles with dense granules; the second is considered to be proteinaceous reserve
material. Within the Cestoda, the numbers of vitellocytes per germocyte are reduced
in those taxa forming eggs of the 'Cyclophyllidean-type' (e.g. Cyclophyllidea, Tetraphyllidea,
Pseudophyllidea). This is particularly evident in Cyclophyllidea; for example, in
vitellocytes of Hymenolepis diminuta (Hymenolepididae) there are numerous vitelline
granules of homogeneously electron-dense material; in Catenotaenia pusilla (Catenotaeniidae)
there are three large, homogenous vitelline vesicles, while in Inermicapsifer madagascariensis
(Anoplocephalidae) there is only one large vitelline vesicle, containing homogeneously
electron-dense material, which occupies most of the vitelline cell volume. In this
respect the Tetraphyllidea and Proteocephalidea, in forming eggs that lack a hard
egg-shell, hold an intermediate position. A comparison of interrelationships which
exist among types of vitellocytes, vitellogenesis, types of embryonic development,
ovoviviparity and life cycles indicates parallelisms and analogies in adaptation to
the parasitic way of life in different groups of cestodes. Knowledge on cestode vitellogenesis
may also have an important applied aspect. Vitellocytes, due to their high metabolic
rate, represent a very sensitive target for analysing effect of anthelminthic drugs
upon the egg formation (ovicidal effects); rapid degeneration of vitellocytes is usually
accompanied by a cessation of egg production.