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SAUL WISCHNITZER
egg for his investigations. Subsequent experiments with amphibian eggs
also served to open up the field of chemical embryology (Needham, 1931;
Brächet, 1950). This area of study endeavors to find biochemical and
physiological explanations for the processes of embryological development.
These processes take place within a morphological framework, an understanding of which naturally contributes to a better comprehension of the
metabolic activities themselves.
During the last 15 years, two new tools, the phase-contrast and electron
microscopes, have been developed, which have helped to clarify the morphology of amphibian egg. The application of phase-contrast microscopy to
a study of the lampbrush chromosomes has served to reveal their complex
structure (see review by Callan, 1963). Electron microscopy, since the
development of suitable preparation techniques, has made possible the
study of the ultrastructure of the amphibian egg and its surrounding layers.
The results of these studies have clarified many aspects related to morphogenesis during the critical stages of development prior to fertilization. This
article will review, summarize, and evaluate the results obtained from such
studies.
II. Egg Envelopes
The immediate environment of the developing intraovarian oocyte comprises several cellular layers and an acellular zone. There is a basic similarity
of the organization of these envelopes in all vertebrates. This presentation
will deal essentially with amphibians. The general literature on this subject
has been reviewed in recent years by Odor (1960) and Wartenberg (1962).
A. Historical Review
Swammerdam, as early as 1738, reported the presence of membranes
enclosing amphibian eggs. Subsequent studies bearing upon the structure
of the peripheral oocyte layers were reported by Cramer (1848). Thomson
(1859), Waldeyer (1870, 1906), Hertwig (1906), and King (1908) and were
reviewed by Retzius (1912). Since that time there has been a noticeable
decline in the number of light-microscope studies that have a bearing upon
the morphology of egg envelopes. Only those by Fischer (1932), Kemp
(1953), and Gabaieva (1963) have appeared in the literature.
Although there have been differences of interpretation in the literature
as to structural details, it is agreed that three cellular layers gradually
come to enclose the developing oocyte. Inside of these layers, the acellular
zona pellucida is formed, which lies adjacent to the peripheral cytoplasm of
the oocyte.
The relatively few light-microscope investigations of amphibian egg
envelopes carried out during the last half century doubtless reflected the
SAUL WISCHNITZER
egg for his investigations. Subsequent experiments with amphibian eggs
also served to open up the field of chemical embryology (Needham, 1931;
Brächet, 1950). This area of study endeavors to find biochemical and
physiological explanations for the processes of embryological development.
These processes take place within a morphological framework, an understanding of which naturally contributes to a better comprehension of the
metabolic activities themselves.
During the last 15 years, two new tools, the phase-contrast and electron
microscopes, have been developed, which have helped to clarify the morphology of amphibian egg. The application of phase-contrast microscopy to
a study of the lampbrush chromosomes has served to reveal their complex
structure (see review by Callan, 1963). Electron microscopy, since the
development of suitable preparation techniques, has made possible the
study of the ultrastructure of the amphibian egg and its surrounding layers.
The results of these studies have clarified many aspects related to morphogenesis during the critical stages of development prior to fertilization. This
article will review, summarize, and evaluate the results obtained from such
studies.
II. Egg Envelopes
The immediate environment of the developing intraovarian oocyte comprises several cellular layers and an acellular zone. There is a basic similarity
of the organization of these envelopes in all vertebrates. This presentation
will deal essentially with amphibians. The general literature on this subject
has been reviewed in recent years by Odor (1960) and Wartenberg (1962).
A. Historical Review
Swammerdam, as early as 1738, reported the presence of membranes
enclosing amphibian eggs. Subsequent studies bearing upon the structure
of the peripheral oocyte layers were reported by Cramer (1848). Thomson
(1859), Waldeyer (1870, 1906), Hertwig (1906), and King (1908) and were
reviewed by Retzius (1912). Since that time there has been a noticeable
decline in the number of light-microscope studies that have a bearing upon
the morphology of egg envelopes. Only those by Fischer (1932), Kemp
(1953), and Gabaieva (1963) have appeared in the literature.
Although there have been differences of interpretation in the literature
as to structural details, it is agreed that three cellular layers gradually
come to enclose the developing oocyte. Inside of these layers, the acellular
zona pellucida is formed, which lies adjacent to the peripheral cytoplasm of
the oocyte.
The relatively few light-microscope investigations of amphibian egg
envelopes carried out during the last half century doubtless reflected the