ULTRASTRUCTURE OF DEVELOPING AMPHIBIAN EGG
133
limitations of this instrument in elucidating any further information. The
development of preparation techniques permitting the study of biological
material with the electron microscope has given new impetus to morphological studies of the egg envelopes. These have dealt especially with the
fine structure of the acellular zone. During the last decade, such studies
were carried out by Dollander (1954, 1956), Kemp (1956), Wartenberg and
Gusek (1960), Wartenberg and Schmidt (1961), Wartenberg (1962),
Wischnitzer (1963a, 1964b), Hope et al (1963), and Tokin and Gabaieva
(1963). A good understanding of the development of the acellular zone,
which leads to the formation of the vitelline membrane, has emerged from
these studies.
B. Terminology
A system of classifying the egg envelopes was originally introduced by
Ludwig (1874) and was later adopted by Korschelt and Heider (1902),
Waldeyer (1906), Wilson (1937), Parkes (1952), and Raven (1961). They
speak of primary, secondary, and tertiary envelopes as those formed by the
egg, follicle cells, and oviduct, respectively. Since it has proved difficult to
determine, with any degree of certainty, the site of formation of the
acellular intraovarian layer (see Odor, 1960), the use of this system of
classification is still unjustified. Moreover, as it will be shown later, it is
conceivable that the zona pellucida, in urodeles at least, may be a product
of both the ovum and follicle cells. Because of this, such a classification
system may not have any validity.
A considerable amount of the controversy relative to the organization
of the egg envelopes probably is due to semantics. The interchangeable use
of a host of terms whose definitions were uncertain or were borrowed from
mammalian terminology has contributed to the confusion (see, e.g., Daniel
and Yarwood, 1939, Fig. 2). The definitions of the terms used in this paper
are as follows:
Surface epithelium. The outermost layer of cells surrounding the developing oocyte.
Theca. The middle, connective tissue layer.
Follicle epithelium. The innermost cellular layer.
Homogeneous layer. The acellular stratum, present in urodeles only,
seen in electron micrographs to be just beneath the follicular epithelium. This zone has also been called the outer layer of the zona radiata
(Wartenberg, 1962).
Zona radiata. The thicker, dense acellular stratum lying, in urodeles,
beneath the homogeneous layer and adjacent to the oocyte. This term
was coined by Waldeyer (1870) because of the characteristic radial
striations exhibited when observed with the light microscope. It is
133
limitations of this instrument in elucidating any further information. The
development of preparation techniques permitting the study of biological
material with the electron microscope has given new impetus to morphological studies of the egg envelopes. These have dealt especially with the
fine structure of the acellular zone. During the last decade, such studies
were carried out by Dollander (1954, 1956), Kemp (1956), Wartenberg and
Gusek (1960), Wartenberg and Schmidt (1961), Wartenberg (1962),
Wischnitzer (1963a, 1964b), Hope et al (1963), and Tokin and Gabaieva
(1963). A good understanding of the development of the acellular zone,
which leads to the formation of the vitelline membrane, has emerged from
these studies.
B. Terminology
A system of classifying the egg envelopes was originally introduced by
Ludwig (1874) and was later adopted by Korschelt and Heider (1902),
Waldeyer (1906), Wilson (1937), Parkes (1952), and Raven (1961). They
speak of primary, secondary, and tertiary envelopes as those formed by the
egg, follicle cells, and oviduct, respectively. Since it has proved difficult to
determine, with any degree of certainty, the site of formation of the
acellular intraovarian layer (see Odor, 1960), the use of this system of
classification is still unjustified. Moreover, as it will be shown later, it is
conceivable that the zona pellucida, in urodeles at least, may be a product
of both the ovum and follicle cells. Because of this, such a classification
system may not have any validity.
A considerable amount of the controversy relative to the organization
of the egg envelopes probably is due to semantics. The interchangeable use
of a host of terms whose definitions were uncertain or were borrowed from
mammalian terminology has contributed to the confusion (see, e.g., Daniel
and Yarwood, 1939, Fig. 2). The definitions of the terms used in this paper
are as follows:
Surface epithelium. The outermost layer of cells surrounding the developing oocyte.
Theca. The middle, connective tissue layer.
Follicle epithelium. The innermost cellular layer.
Homogeneous layer. The acellular stratum, present in urodeles only,
seen in electron micrographs to be just beneath the follicular epithelium. This zone has also been called the outer layer of the zona radiata
(Wartenberg, 1962).
Zona radiata. The thicker, dense acellular stratum lying, in urodeles,
beneath the homogeneous layer and adjacent to the oocyte. This term
was coined by Waldeyer (1870) because of the characteristic radial
striations exhibited when observed with the light microscope. It is