Difference between revisions of "Book - Outline of Comparative Embryology 1-2"

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=Part One General Embryology=
 
=Part One General Embryology=
 
==Chapter II The Germ-Cell Cycle==
 
==Chapter II The Germ-Cell Cycle==
 +
The ontogeny of an organism includes its entire cycle of development from its earliest beginnings to old age and death. Embryology includes the first part of this cycle. Broadly considered, it may be held to include the development of the germ cells (gametogenesis) in their preparation for fertilization and cleavage, and most text-books treat of these matters. It is our purpose, however, in the present study to give but brief treatment to this phase of the subject, for general introductory courses in zoology commonly include a brief outline of gametogenesis, and a more thorough study deserves more treatment than can be given in a course in embryology.
  
The ontogeny of an organism includes its entire cycle of development
 
from its earliest beginnings to old age and death. Embryology includes
 
the first part of this cycle. Broadly considered, it may be held to include
 
the development of the germ cells (gametogenesis) in their preparation
 
for fertilization and cleavage, and most text-books treat of these matters.
 
It is our purpose, however, in the present study to give but brief treatment to this phase of the subject, for general introductory courses in
 
zoology commonly include a brief outline of gametogenesis, and a more
 
thorough study deserves more treatment than can be given in a course
 
in embryology.
 
  
Embryonic development in its narrower sense may be said to begin
+
Embryonic development in its narrower sense may be said to begin with fertilization of the egg and to consist of four periods. These are: first, cleavage; second, formation of germ layers; third, period of organ development; fourth, period of histological differentiation. It is to be noted that in many animals these four periods are not sharply separated from each other. The principles which are of importance from a comparative standpoint are chiefly illustrated by the first three of these periods.
with fertilization of the egg and to consist of four periods. These are:
 
first, cleavage; second, formation of germ layers; third, period of organ
 
development; fourth, period of histological differentiation. It is to be
 
noted that in many animals these four periods are not sharply separated
 
from each other. The principles which are of importance from a comparative standpoint are chiefly illustrated by the first three of these
 
periods.
 
  
The beginning of the life cycle of every organism is very closely
 
related to the development of the parent, in that the cells from which
 
the new organism comes are early set aside and from then on are to
 
be distinguished from the other cells and organs of the body. Strictly
 
speaking, the embryology of an animal would require us to trace the
 
germ cells from their very first appearance on up through the stages
 
of their development and to trace the formation of the matured sperm
 
and ovum as well as the fertilization, cleavage, germ-layer formation
 
and the subsequent stages, if we were to give a complete account of
 
development. But it has come to be the practice to begin the embryologieal account with fertilization and cleavage, and we shall therefore deal with the germ-cell history only briefly, leaving the cytological details for other more exhaustive treatments.
 
  
Our knowledge of this development is a matter of the last half century, and so voluminous has the accumulated information become on
+
The beginning of the life cycle of every organism is very closely related to the development of the parent, in that the cells from which the new organism comes are early set aside and from then on are to be distinguished from the other cells and organs of the body. Strictly speaking, the embryology of an animal would require us to trace the germ cells from their very first appearance on up through the stages of their development and to trace the formation of the matured sperm and ovum as well as the fertilization, cleavage, germ-layer formation and the subsequent stages, if we were to give a complete account of development. But it has come to be the practice to begin the embryologieal account with fertilization and cleavage, and we shall therefore deal with the germ-cell history only briefly, leaving the cytological details for other more exhaustive treatments.
  
the problems connected with the cell that it now constitutes an entirely
 
8
 
THE GERM-CELL CYCLE 9
 
  
separate division 0" zoological science, namely cytology. The foundations of this science were laid in the last two decades of the nineteenth
+
Our knowledge of this development is a matter of the last half century, and so voluminous has the accumulated information become on the problems connected with the cell that it now constitutes an entirely separate division 0" zoological science, namely cytology. The foundations of this science were laid in the last two decades of the nineteenth century by investigators who sought information concerning earlier and earlier stages of germ-cell development. These studies received a great impetus from the conception of the germ plasm which was published during the early part of the period by Weismann. The organism was thought of as consisting of two more or less opposing portions, a germ plasm (which functions as a hereditary vehicle and is passed on to the next generation as it gives rise to the new individual) and soma or somatoplasm (which is the remainder of the organism’s body and is concerned with its individual well-being). It was thought by the earlier students that certain cells were thus the germ plasm, that is, the germ cells as contrasted with the body cells, while others had no part in the reproductive activities of the organism except in so far as they were necessary for nourishment and support.
century by investigators who sought information concerning earlier and
 
earlier stages of germ-cell development. These studies received a great
 
impetus from the conception of the germ plasm which was published
 
during the early part of the period by Weismann. The organism was
 
thought of as consisting of two more or less opposing portions, a germ
 
plasm (which functions as a hereditary vehicle and is passed on to the
 
next generation as it gives rise to the new individual) and soma or
 
somatoplasm (which is the remainder of the organism’s body and is
 
concerned with its individual well-being). It was thought by the earlier
 
students that certain cells were thus the germ plasm, that is, the germ
 
cells as contrasted with the body cells, while others had no part in the
 
reproductive activities of the organism except in so far as they were
 
necessary for nourishment and support.
 
  
This conception has been largely responsible for the attempts, now
 
of many years’ standing, to trace the origin of the germ cells back to
 
the early blastomeres, that is, earlier than the period in which they
 
can be recognized as part of the reproductive organ, the gonad. These
 
attempts were productive of successful results in quite a number of
 
cases in both vertebrates and invertebrates where it was found that
 
the antecedents of the primordial germ cells could be recognized even
 
in early cleavage divisions. In other cases, however, it has not been
 
found possible to trace the history of the primordial germ cells back of
 
their first appearance in the gonads. The origin of the primordial germ
 
cells is an embryological problem of considerable importance in itself
 
aside from its relation to the germ-plasm doctrine, and as such it is
 
discussed in detail in a later chapter in this book. (See Part Two,
 
(‘hapter II.)
 
  
Partly as a result of the inconclusive data from the study of the
+
This conception has been largely responsible for the attempts, now of many years’ standing, to trace the origin of the germ cells back to the early blastomeres, that is, earlier than the period in which they can be recognized as part of the reproductive organ, the gonad. These attempts were productive of successful results in quite a number of cases in both vertebrates and invertebrates where it was found that the antecedents of the primordial germ cells could be recognized even in early cleavage divisions. In other cases, however, it has not been found possible to trace the history of the primordial germ cells back of their first appearance in the gonads. The origin of the primordial germ cells is an embryological problem of considerable importance in itself aside from its relation to the germ-plasm doctrine, and as such it is discussed in detail in a later chapter in this book. (See Part Two, (‘hapter II.)
origin of the primordial germ cells in‘ the animal kingdom as a whole,
 
but more especially as a result of the work of recent years on heredity in
 
which the importance of the chromosome has been made manifest, a
 
new conception of the germ plasm has now showed itself to be more
 
acceptable to many students of these problems than that of Weismann.
 
According to this new view, each cell contains material which corresponds to both the germ plasm and soma, the chromatin representing
 
the former and the cytoplasm the latter. This interpretation explains
 
many facts which had previously proven difficult to understand and
 
enables us to regard the origin of germ cells as a problem of embryology
 
rather than as one whose chief interest is in relation to the transmission
 
of a “germ plasm.
 
10 THE GERM-CELL CYCLE
 
  
1. Cell Division in Gametogenesis
 
  
Regardless of their source or of the time of their first appearance,
+
Partly as a result of the inconclusive data from the study of the origin of the primordial germ cells in‘ the animal kingdom as a whole, but more especially as a result of the work of recent years on heredity in which the importance of the chromosome has been made manifest, a new conception of the germ plasm has now showed itself to be more acceptable to many students of these problems than that of Weismann. According to this new view, each cell contains material which corresponds to both the germ plasm and soma, the chromatin representing the former and the cytoplasm the latter. This interpretation explains many facts which had previously proven difficult to understand and enables us to regard the origin of germ cells as a problem of embryology rather than as one whose chief interest is in relation to the transmission of a “germ plasm.
there are present in a developing gonad certain cells which are known as
 
primordial germ cells. They remain in a quiescent stage during the
 
early development of the organism, beginning their active development
 
only when the somatic structures are well on the way to their adult
 
condition. Somatic structures are produced by the differentiation of
 
the cytoplasmic portions of the cells (giving rise to the familiar histological distinctions between tissues), and must obviously precede
 
those activities which have for their end the reproduction of the organism. On the other hand, specialization of the nuclear structures involved
 
in the function of mitotic cell division is responsible for reproduction
 
and must wait until the proper development of the somatic structure
 
has been attained.
 
  
a. Multiplication Period. At length the primordial germ cells begin a
+
===1. Cell Division in Gametogenesis===
period of proliferation known as the multiplication period. This period
 
varies in different forms, both as to its beginning and its duration, for
 
it does not necessarily begin at the same stage in the life cycle, nor does
 
it consist of the same number of cell divisions. It is said that in the
 
grasshopper there are eight divisions in this period; thus each primordial germ cell would produce as a result 256 descendants. The cells
 
undergoing the various divisions of the multiplication period are knovsn
 
as gonia; the male cells are sperrnatogonia and the female oiigonia, alike
 
in every fundamental respect.
 
  
b. Growth Period. At the end of the multiplication peziod, that is, with
+
Regardless of their source or of the time of their first appearance, there are present in a developing gonad certain cells which are known as primordial germ cells. They remain in a quiescent stage during the early development of the organism, beginning their active development only when the somatic structures are well on the way to their adult condition. Somatic structures are produced by the differentiation of the cytoplasmic portions of the cells (giving rise to the familiar histological distinctions between tissues), and must obviously precede those activities which have for their end the reproduction of the organism. On the other hand, specialization of the nuclear structures involved in the function of mitotic cell division is responsible for reproduction and must wait until the proper development of the somatic structure has been attained.
the formation of the primary oiigonia or spermatogonia, the active proliferation of cells ceases and there follows a growth period. During this
 
period without further division the cells increase in size and store up
 
nutrient material, deutoplasm, to furnish them with energy for the
 
further activities they are to undergo. In the case of the oogonia this
 
material is the yolk, often present in large amount to furnish food for
 
the embryo until it has become at least in part able to obtain sustenance from its surroundings. The amount and distribution of this yolk
 
material is a very important factor in determining the type of cleavage
 
and future development of the embryo, for cell division becomes increasingly difiicult with the accumulation of inert yolk material. (This point
 
should be kept in mind, for frequent use must be made of it in the study
 
of cleavage types.) These cells are new primary spermatocytes or
 
ooeytes.
 
  
c. Maturation Period. Two maturation divisions now succeed each
 
other, usually with some degree of rapidity. They constitute the matuMATURATION PERIOD 11
 
  
ration period during which nuclear events of the utmost importance
+
a. Multiplication Period. At length the primordial germ cells begin a period of proliferation known as the multiplication period. This period varies in different forms, both as to its beginning and its duration, for it does not necessarily begin at the same stage in the life cycle, nor does it consist of the same number of cell divisions. It is said that in the grasshopper there are eight divisions in this period; thus each primordial germ cell would produce as a result 256 descendants. The cells undergoing the various divisions of the multiplication period are knovsn as gonia; the male cells are sperrnatogonia and the female oiigonia, alike in every fundamental respect.
from the standpoint of the future organism occur. In oogenesis as well
 
  
‘go
 
’__, .\‘ 3% Multiplication Period .3, "_ _.
 
.— ~. 2' ° x ‘s
 
99 4? ' ‘
 
e 9» « o°
 
Q1, Gonia
 
'1 “ '1 \\ ’v x‘ , \\
 
I \ I \ \ X \
 
  
Growth Period ' CD
+
b. Growth Period. At the end of the multiplication peziod, that is, with the formation of the primary oiigonia or spermatogonia, the active proliferation of cells ceases and there follows a growth period. During this period without further division the cells increase in size and store up nutrient material, deutoplasm, to furnish them with energy for the further activities they are to undergo. In the case of the oogonia this material is the yolk, often present in large amount to furnish food for the embryo until it has become at least in part able to obtain sustenance from its surroundings. The amount and distribution of this yolk material is a very important factor in determining the type of cleavage and future development of the embryo, for cell division becomes increasingly difiicult with the accumulation of inert yolk material. (This point should be kept in mind, for frequent use must be made of it in the study of cleavage types.) These cells are new primary spermatocytes or ooeytes.
 +
 
 +
 
 +
c. Maturation Period. Two maturation divisions now succeed each other, usually with some degree of rapidity. They constitute the maturation period during which nuclear events of the utmost importance from the standpoint of the future organism occur. In oogenesis as well as spermatogenesis there are two divisions which are similar in all respects except one. In the maturation of male germ cells, the first division results in two second spermatocytes which promptly divide again producing four spermatids from each primary spermatogonium. (In the grasshopper obviously there would be now 1024 descendants from each primordial germ cell.) These cells do not again divide but go through a seriés of changes (spermiogenesis) by which they become metamorphosed into functional spermatozoa with the characteristic structure of spermatozoa; these changes are cytoplasmic in character. In the developing female germ cells, however, maturation results in the production of one functional egg and three non-functional cells which are its equivalents from the standpoint of nuclear content and which are known as polocytes or polar bodies. This is accomplished as an immediate result of the fact that the first maturation spindle is short and occupies a position near the surface of the large oocyte. Since the division plane cuts through the center of the spindle, a very small cell, the polocyte, is cut off from the larger “egg cell” which is now the second oécyte. In the second maturation division both cells divide, the “egg cell” again unequally producing a matured egg and another polocyte, and the first polocyte two other polocytes. This size dil’ferentiation of the four cells which are descendants of the primary oiigonia allows the normal nuclear events of maturation to go on unmodified, but concentrates the ooplasmie materials, which are necessary to the nourishment of the embryo, in one functional cell.
  
     
 
  
Cytes
 
spermafids Poiocytes o o o
 
I and 4 4 4
 
« Mature Egg 5
 
Spermatoza ‘f
 
\‘\\\O,? ,r”"’
 
Jilgie on-x‘E‘5°"e"
 
/\/‘ \ \ ’ ’ ‘
 
4
 
J Fertilization
 
Zygote ‘I
 
  
 
  
 
fiG. 4. Diagram to illustrate the processes of gametogenesia.
 
fiG. 4. Diagram to illustrate the processes of gametogenesia.
  
as spermatogenesis there are two divisions which are similar in all
 
respects except one. In the maturation of male germ cells, the first
 
12 THE GERM~CELL CYCLE
 
 
division results in two second spermatocytes which promptly divide
 
again producing four spermatids from each primary spermatogonium.
 
(In the grasshopper obviously there would be now 1024 descendants
 
from each primordial germ cell.) These cells do not again divide but go
 
through a seriés of changes (spermiogenesis) by which they become
 
metamorphosed into functional spermatozoa with the characteristic
 
structure of spermatozoa; these changes are cytoplasmic in character.
 
In the developing female germ cells, however, maturation results in the
 
production of one functional egg and three non-functional cells which
 
are its equivalents from the standpoint of nuclear content and which
 
are known as polocytes or polar bodies. This is accomplished as an
 
immediate result of the fact that the first maturation spindle is short
 
and occupies a position near the surface of the large oocyte. Since the
 
division plane cuts through the center of the spindle, a very small cell,
 
the polocyte, is cut off from the larger “egg cell” which is now the
 
second oécyte. In the second maturation division both cells divide, the
 
“egg cell” again unequally producing a matured egg and another polocyte, and the first polocyte two other polocytes. This size dil’ferentiation
 
of the four cells which are descendants of the primary oiigonia allows
 
the normal nuclear events of maturation to go on unmodified, but concentrates the ooplasmie materials, which are necessary to the nourishment of the embryo, in one functional cell.
 
  
Gametogenesis is now completed. Sperm and egg are ready for the
 
next step toward the production of a new organism, namely fertilization.
 
By fertilization is meant the entrance of the sperm into the egg, but the
 
process is very complicated and is completed only with the union of egg
 
and sperm nuclei into a fusion or cleavage nucleus. Fertilization has
 
two functions which are distinctly different: the initiation of cleavage
 
or the development of the zygote, and the restoration of the chromatin
 
material equivalent to that lost in reduction. This complicated process
 
has been the subject of much study during the last two decades, especially from the viewpoint of experimental or physiological embryology,
 
and many fundamental conceptions have resulted from this fascinating
 
phase of embryologieal research.
 
  
2. Nuclear Events in Gametogenesis
+
Gametogenesis is now completed. Sperm and egg are ready for the next step toward the production of a new organism, namely fertilization. By fertilization is meant the entrance of the sperm into the egg, but the process is very complicated and is completed only with the union of egg and sperm nuclei into a fusion or cleavage nucleus. Fertilization has two functions which are distinctly different: the initiation of cleavage or the development of the zygote, and the restoration of the chromatin material equivalent to that lost in reduction. This complicated process has been the subject of much study during the last two decades, especially from the viewpoint of experimental or physiological embryology, and many fundamental conceptions have resulted from this fascinating phase of embryologieal research.
  
The nuclear events during gametogenesis must also be understood,
+
===2. Nuclear Events in Gametogenesis===
at least in outline. Their study has been the especial problem of cytology
 
during the last quarter of a century.
 
  
a. Reduction. It may be stated as a general law that every species of
+
The nuclear events during gametogenesis must also be understood, at least in outline. Their study has been the especial problem of cytology during the last quarter of a century.
organism is characterized by a definite number of chromosomes, and,
 
REDUCTION 13
 
  
with a few exceptions which are not in contradiction to a full and adequate statement of the law, this number occurs in all the cells throughout the bodies of all the members of a particular species. This characteristic number is spoken of as the somatic or diploid number. In the
+
a. Reduction. It may be stated as a general law that every species of organism is characterized by a definite number of chromosomes, and, REDUCTION 13
history of the germ cells all the divisions of the multiplication period
 
are found to have this characteristic number. During the maturation
 
divisions, however, this number is halved, that is, reduction occurs, and
 
the matured gametes have only half the number characteristic of the
 
species. The union of the male and female gametes in fertilization effects
 
a return to the somatic number. The process of reduction is often called
 
meiosis and the divisions meiotic divisions. Commonly, although not at
 
all of necessity, the first division is the reductional or heterotypic as
 
distinguished from the somatic or homcotypic divisions. If the first is
 
hcterotypic the second is homcotypic, and conversely.
 
  
In those cases in which the (lijfcrences between reductional and somatic
+
with a few exceptions which are not in contradiction to a full and adequate statement of the law, this number occurs in all the cells throughout the bodies of all the members of a particular species. This characteristic number is spoken of as the somatic or diploid number. In the history of the germ cells all the divisions of the multiplication period are found to have this characteristic number. During the maturation divisions, however, this number is halved, that is, reduction occurs, and the matured gametes have only half the number characteristic of the species. The union of the male and female gametes in fertilization effects a return to the somatic number. The process of reduction is often called meiosis and the divisions meiotic divisions. Commonly, although not at all of necessity, the first division is the reductional or heterotypic as distinguished from the somatic or homcotypic divisions. If the first is hcterotypic the second is homcotypic, and conversely.
mitoses are most clearly recognizable (that is, in those cases in which
 
tctrads are formed), we may say that the explanation lies in two outstanding facts, namely, the peculiar prophase of the first division with
 
its synaptic pairing of homologous chromosomes, and the entire absence
 
of a prophase to the second division. The case of Ascaris, the roundworm, is one of the best known, and it may serve as an illustration of
 
these processes. The somatic number of chromosomes in the common
 
Ascaris is four. These are not looked upon as four separate and unrelated
 
individual chromosomes, however, but as two pairs, for it has been
 
shown in some animals that one member of each pair came from the
 
male parent and one from the female in the preceding fertilization, and
 
it is very probable that this is always the case. The members of a pair
 
are spoken of as homologous chromosomes. Thus each individual derived
 
from a fertilized egg contains in each of its cells two full sets of homologous chromosomes. This fact is to be contrasted with the well-substantiated observation that the germ eells contain but a single set of chromosomes owing to the process of reduction. It will be recalled that a typical
 
mitotic division, whether in a germ cell or a somatic cell, involves in the
 
prophase a condition which is really the climax of the entire set of
 
mitotic events, namely the splitting of the spireme thread which is to
 
condense to form the metaphase chromosomes. For this reason the metaphase chromosome may be looked upon as consisting of two halves, or
 
chromatids, even if this condition sometimes is not easily seen under
 
the microscope. That is to say, the ordinary chromosome is a. bivalent
 
one, a dyad. With these facts in mind we are now ready to inquire into
 
the nature of synapsis.
 
fiG. 5. Fertilization and cleavage of Ascaris. (Redrawn from Kellicott after Bovcri.)
 
  
A, epermntozoon entering the egg as the second maturation division is taking plan-.
+
In those cases in which the (lijfcrences between reductional and somatic mitoses are most clearly recognizable (that is, in those cases in which tctrads are formed), we may say that the explanation lies in two outstanding facts, namely, the peculiar prophase of the first division with its synaptic pairing of homologous chromosomes, and the entire absence of a prophase to the second division. The case of Ascaris, the roundworm, is one of the best known, and it may serve as an illustration of these processes. The somatic number of chromosomes in the common Ascaris is four. These are not looked upon as four separate and unrelated individual chromosomes, however, but as two pairs, for it has been shown in some animals that one member of each pair came from the male parent and one from the female in the preceding fertilization, and it is very probable that this is always the case. The members of a pair are spoken of as homologous chromosomes. Thus each individual derived from a fertilized egg contains in each of its cells two full sets of homologous chromosomes. This fact is to be contrasted with the well-substantiated observation that the germ eells contain but a single set of chromosomes owing to the process of reduction. It will be recalled that a typical mitotic division, whether in a germ cell or a somatic cell, involves in the prophase a condition which is really the climax of the entire set of mitotic events, namely the splitting of the spireme thread which is to condense to form the metaphase chromosomes. For this reason the metaphase chromosome may be looked upon as consisting of two halves, or chromatids, even if this condition sometimes is not easily seen under the microscope. That is to say, the ordinary chromosome is a. bivalent one, a dyad. With these facts in mind we are now ready to inquire into the nature of synapsis. fiG. 5. Fertilization and cleavage of Ascaris. (Redrawn from Kellicott after Bovcri.)
B. pronuclei going into prophnso and division of sphere and centrosome beginning; C, I),
 
showing further advance toward E, the first cleavage division.
 
SYN APSIS 15
 
  
b. Synopsis. By synapsis is meant the union in the first maturation
+
A, epermntozoon entering the egg as the second maturation division is taking plan-. B. pronuclei going into prophnso and division of sphere and centrosome beginning; C, I), showing further advance toward E, the first cleavage division.
prophase of the two homologous members of each chromosome pair to
 
form 3* Smgle chromosome; although one which has a valence of four, as
 
  
h‘ h’ h‘ h‘
 
fiG. 6. Diagram of tctr-ad formation and subsequent ro(lu('lion. (Suggested by diagrams
 
from Sharp.)
 
  
a. chromosome conditions in spernmtogoniu and at beginning: of first maturation prophuse; I). synopsis in prophnse of first maturation prophnsc (first spernw.tot_vte), 0, tctrad-5
+
b. Synopsis. By synapsis is meant the union in the first maturation prophase of the two homologous members of each chromosome pair to form 3* Smgle chromosome; although one which has a valence of four, as contrasted with the preceding bivalent condition. Since two somatic chromosomes, each consisting of two chromatids, are involved in the formation of this new chromosome, it is called it tetrml. It is evident  that by synapsis a reduction in the number of chromosomes, but not in the amount of chromatin, has been effected. It is the function of the two maturation divisions to complete this reduction in amount by distributing the four component parts of each tetrad to the four different spermatozoa,or, in the case of oogenesis, to the egg and the three polar bodies.
ready for the division; d. metaphnse of first niaturmitm division; 0. unaphuse of d; f.
 
second spcrmntocytes produced by the division of e; g, division of second spermutocytes;
 
h. resulting distribution of chromosomes as they go into the four spermatozoa.
 
  
contrasted with the preceding bivalent condition. Since two somatic
 
chromosomes, each consisting of two chromatids, are involved in the
 
formation of this new chromosome, it is called it tetrml. It is evident
 
16 THE GERM-CELL CYCLE
 
  
that by synapsis a reduction in the number of chromosomes, but not in
 
the amount of chromatin, has been effected. It is the function of the
 
two maturation divisions to complete this reduction in amount by
 
distributing the four component parts of each tetrad to the four different
 
spermatozoa,or, in the case of oogenesis, to the egg and the three polar
 
bodies.
 
  
As a result of synapsis it appears that there are in each first spermatocyte or oocyte the reduced number of chromosomes in the form of
+
fiG. 6. Diagram of tctr-ad formation and subsequent ro(lu('lion. (Suggested by diagrams from Sharp.)
tetrads, or quadrivalent chromosomes. Each tetrad consists of four
 
chromatids, aa/bb’, two from each of the homologous mates, A and B.
 
If, in the succeeding metaphases and anaphases, the bivalent chromosomes (dyads) which are the result of the division of the tetrad are
 
each composed of two chromatids derived from the same one of the
 
constituent synaptic mates, that is, a and a’ in one chromosome and
 
b and b’ in the other, the division is said to be rcductional or heterotypic.
 
In other words, in a reducing division whole chromosomes are separated.
 
In this case the second division would be an equational division for the
 
equivalent half chromosomes would necessarily be separated as in any
 
ordinary mitosis. That is, the bivalent aa’ is now divided into a and a’
 
and bb’ into b and b’, and the maturation with the consequent distribution of the four parts of the tetrad to the four germ cells is completed.
 
If, on the other hand, the bivalent chromosomes resulting from the
 
division of the tetrads consist of half of each synaptic mate, that is
 
a and b in one and a’ and b’ (or of course a and b’ or a’ and b may be
 
linked together), then the first division is equational. The second division would then be reductional and as in the previous case would result
 
in the distribution of the four component parts of the tetrad to the four
 
matured germ cells.
 
  
In these cases in which tetrads are formed there is no resting stage
+
a. chromosome conditions in spernmtogoniu and at beginning: of first maturation prophuse; I). synopsis in prophnse of first maturation prophnsc (first spernw.tot_vte), 0, tctrad-5 ready for the division; d. metaphnse of first niaturmitm division; 0. unaphuse of d; f. second spcrmntocytes produced by the division of e; g, division of second spermutocytes; h. resulting distribution of chromosomes as they go into the four spermatozoa.
preceding the second maturation division, but the dyads without a
 
reorganization arrange themselves on the second spindle. There is a
 
large number of cases, however, in which actual tetrads are not formed
 
owing to the failure of the usual split of the spireme to appear during
 
the early stages of the first division. In this case the first division is
 
always reductional; there is a pause between the first and second
 
divisions during which the chromosomes undergo seine reorganization;
 
and the splitting which was delayed takes place. It is evident, therefore,
 
that precisely the same result is achieved in the two cases, whether or
 
not tetrad formation occurs. That is, there are formed from each spermatocyte or ooeyte of the first order four cells, each with half the number
 
of chromosomes characteristic of the species. These four cells are mature
 
spermatozoa or one functional egg and three polocytes or polar bodies.
 
FERTILIZATION 17
 
  
3. Fertilization
 
  
It has already been pointed out that there are two totally different
+
As a result of synapsis it appears that there are in each first spermatocyte or oocyte the reduced number of chromosomes in the form of tetrads, or quadrivalent chromosomes. Each tetrad consists of four chromatids, aa/bb’, two from each of the homologous mates, A and B. If, in the succeeding metaphases and anaphases, the bivalent chromosomes (dyads) which are the result of the division of the tetrad are each composed of two chromatids derived from the same one of the constituent synaptic mates, that is, a and a’ in one chromosome and b and b’ in the other, the division is said to be rcductional or heterotypic. In other words, in a reducing division whole chromosomes are separated. In this case the second division would be an equational division for the equivalent half chromosomes would necessarily be separated as in any ordinary mitosis. That is, the bivalent aa’ is now divided into a and a’ and bb’ into b and b’, and the maturation with the consequent distribution of the four parts of the tetrad to the four germ cells is completed. If, on the other hand, the bivalent chromosomes resulting from the division of the tetrads consist of half of each synaptic mate, that is a and b in one and a’ and b’ (or of course a and b’ or a’ and b may be linked together), then the first division is equational. The second division would then be reductional and as in the previous case would result in the distribution of the four component parts of the tetrad to the four matured germ cells.
functions served by fertilization, namely, the one concerned with the
 
hereditary mechanism in which the diploid number of chromosomes is
 
restored, and the other which sets into operation those processes leading
 
to cleavage and the further development of the embryo. The first of
 
these functions is accomplished in the conjoining of the male and female
 
pronuclei. The sperm with its reduced number of chromosomes enteis
 
the egg, and its subsequent union with the female pronucleus which also
 
contained the reduced number produces in the zygotic nucleus so formed
 
the full number of chromosomes. The second function is accomplished
 
in a very complicated series of processes of physico—(-hemical nature.
 
Much of the research in experimental embryology of the last two (i(‘('£l.(l(‘s
 
has dealt with the physico-chemical aspects of the activation of the egg,
 
  
+
In these cases in which tetrads are formed there is no resting stage preceding the second maturation division, but the dyads without a reorganization arrange themselves on the second spindle. There is a large number of cases, however, in which actual tetrads are not formed owing to the failure of the usual split of the spireme to appear during the early stages of the first division. In this case the first division is always reductional; there is a pause between the first and second divisions during which the chromosomes undergo seine reorganization; and the splitting which was delayed takes place. It is evident, therefore, that precisely the same result is achieved in the two cases, whether or not tetrad formation occurs. That is, there are formed from each spermatocyte or ooeyte of the first order four cells, each with half the number of chromosomes characteristic of the species. These four cells are mature spermatozoa or one functional egg and three polocytes or polar bodies. FERTILIZATION 17
  
he 7 Entrance of speimatozoon into egg of the starfish Astcruza glaL1u.[7..s. (Redraun
+
===3. Fertilization===
from (‘onkhn after Fol )
 
  
and results have been obtained which have given real insight into the
+
It has already been pointed out that there are two totally different functions served by fertilization, namely, the one concerned with the hereditary mechanism in which the diploid number of chromosomes is restored, and the other which sets into operation those processes leading to cleavage and the further development of the embryo. The first of these functions is accomplished in the conjoining of the male and female pronuclei. The sperm with its reduced number of chromosomes enteis the egg, and its subsequent union with the female pronucleus which also contained the reduced number produces in the zygotic nucleus so formed the full number of chromosomes. The second function is accomplished in a very complicated series of processes of physico—(-hemical nature. Much of the research in experimental embryology of the last two (i(‘('£l.(l(‘s has dealt with the physico-chemical aspects of the activation of the egg,
fundamental nature of the living organism.
 
  
The morphological aspect of fertilization is well known from the studies
 
of Lillie on Nerezs, of Wilson on Toxopneustcs, of Kostaneeki and of
 
Wierzejski on Physa, of Boveri on Ascarzs, and of many others. In most
 
animals the entire sperm enters the egg, but there are many others, as
 
in the sea—urchin and the staifish, in which the tail or at least most of it
 
remains outside. The important elements of the sperm that enter the
 
egg and have a function in fertilization are the head which is equivalent
 
to the nucleus, the central body or the structures derived from it, the
 
acrosome which is derived from the Golgi apparatus of the spermatid,
 
and some chondriosome material. Of course the most important of these
 
is the nucleus, but historically the central body also has been the subject
 
of much discussion in connection with fertilization. The sperm enters
 
rather largely through the activity of the cortical layer of the egg and
 
by a. fairly definite path makes its way toward the egg pronucleus.
 
Immediately upon entrance, however, it rotates so that the middle piece
 
18 THE GERM-CELL CYCLE
 
  
precedes in the advance to the female pronuc‘eus. The sperm aster
+
he 7 Entrance of speimatozoon into egg of the starfish Astcruza glaL1u.[7..s. (Redraun from (‘onkhn after Fol )
appears from this region, and it is the usual condition that it should
 
become the aster of the cleaving egg. It divides and forms the spindle
 
  
u‘;.?
+
and results have been obtained which have given real insight into the fundamental nature of the living organism.
  
-~ "‘  * Ag _
+
The morphological aspect of fertilization is well known from the studies of Lillie on Nerezs, of Wilson on Toxopneustcs, of Kostaneeki and of Wierzejski on Physa, of Boveri on Ascarzs, and of many others. In most animals the entire sperm enters the egg, but there are many others, as in the sea—urchin and the staifish, in which the tail or at least most of it remains outside. The important elements of the sperm that enter the egg and have a function in fertilization are the head which is equivalent to the nucleus, the central body or the structures derived from it, the acrosome which is derived from the Golgi apparatus of the spermatid, and some chondriosome material. Of course the most important of these is the nucleus, but historically the central body also has been the subject of much discussion in connection with fertilization. The sperm enters rather largely through the activity of the cortical layer of the egg and by a. fairly definite path makes its way toward the egg pronucleus. Immediately upon entrance, however, it rotates so that the middle piece 18 THE GERM-CELL CYCLE
\\ 
 
  
8 '1
+
precedes in the advance to the female pronuc‘eus. The sperm aster appears from this region, and it is the usual condition that it should become the aster of the cleaving egg. It divides and forms the spindle between the two approaching pronuclei. It is not the rule that the egg aster contributes to the formation of a cleavage spindle.
  
 
  
fiG 15. Steps in fertilization of the sea ur(-hm, Tow1mc'u.stcs (Redrawn from (‘oiiklin
 
after Wilson )
 
  
a, mature spermatozooii, iii. traiisforination of speimatozooii into male pronucleus,
+
fiG 15. Steps in fertilization of the sea ur(-hm, Tow1mc'u.stcs (Redrawn from (‘oiiklin after Wilson )
{, female pronucleus
 
  
between the two approaching pronuclei. It is not the rule that the egg
+
a, mature spermatozooii, iii. traiisforination of speimatozooii into male pronucleus, {, female pronucleus
aster contributes to the formation of a cleavage spindle.
 
  
The entrance of the sperm occurs at different times with respect to
 
the extrusion of the polar bodies in different eggs. At one extreme is
 
the condition as found in the sea-urchin and in the coelenterates in
 
which both maturation divisions are completed before the sperm can
 
FERTI LIZATION 1 9
 
  
enter. At the other extreme is the Ascaris type in which the sperm enters
+
The entrance of the sperm occurs at different times with respect to the extrusion of the polar bodies in different eggs. At one extreme is the condition as found in the sea-urchin and in the coelenterates in which both maturation divisions are completed before the sperm can  enter. At the other extreme is the Ascaris type in which the sperm enters the egg before either polar body has been extruded; in some cases it may be before the germinal vesicle has been broken down, in others while the first maturation mitosis is in the metaphase stage. Nematodes, fiatworms, molluscs, some annelids and crustaceans belong to the Ascaris type. There are of course intermediate stages between these two extremes.
the egg before either polar body has been extruded; in some cases it may
 
be before the germinal vesicle has been broken down, in others while the
 
first maturation mitosis is in the metaphase stage. Nematodes, fiatworms, molluscs, some annelids and crustaceans belong to the Ascaris
 
type. There are of course intermediate stages between these two extremes.
 
  
The series of processes involved in fertilization may be said to be
+
The series of processes involved in fertilization may be said to be completed when the two pronuclei have closely approached each other and a cleavage spindle is formed between them. There is seldom an actual fusion of the pronuclei as such; rather they lie side by side upon the developing first cleavage spindle, and at the end of this first division the chromosomes of the components are intermingled and are no longer to be distinguished as from the two parents.
completed when the two pronuclei have closely approached each other
 
and a cleavage spindle is formed between them. There is seldom an actual
 
fusion of the pronuclei as such; rather they lie side by side upon the
 
developing first cleavage spindle, and at the end of this first division
 
the chromosomes of the components are intermingled and are no longer
 
to be distinguished as from the two parents.
 
  
We are thus back at the stage from which we started and cleavage is
+
We are thus back at the stage from which we started and cleavage is the next step in ontogeny.
the next step in ontogeny.
 
  
 
===Bibliographic Note===
 
===Bibliographic Note===

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I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

Richards A Outline of Comparative Embryology. (1931)
1931 Richards: Part One General Embryology 1 Historical Development of Embryology | 2 The Germ-Cell Cycle | 3 Egg and Cleavage Types | 4 Holoblastic Types of Cleavage | 5 Meroblastic Types of Cleavage | 6 Types of Blastulae | 7 Endoderm Formation | 8 Mesoderm Formation | 9 Types of Invertebrate Larvae | 10 Formation of the Mammalian Embryo | 11 Egg and Embryonic Membranes | Part Two Embryological Problems 1 The Origin And Development Of Germ Cells | 2 Germ-Layer Theory | 3 The Recapitulation Theory | 4 Asexual Reproduction | 5 Parthenogenesis | 6 Paedogenesis And Neoteny | 7 Polyembryony | 8 The Determination Problem | 9 Ecological Control Of Invertebrate Larval Types

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Part One General Embryology

Chapter II The Germ-Cell Cycle

The ontogeny of an organism includes its entire cycle of development from its earliest beginnings to old age and death. Embryology includes the first part of this cycle. Broadly considered, it may be held to include the development of the germ cells (gametogenesis) in their preparation for fertilization and cleavage, and most text-books treat of these matters. It is our purpose, however, in the present study to give but brief treatment to this phase of the subject, for general introductory courses in zoology commonly include a brief outline of gametogenesis, and a more thorough study deserves more treatment than can be given in a course in embryology.


Embryonic development in its narrower sense may be said to begin with fertilization of the egg and to consist of four periods. These are: first, cleavage; second, formation of germ layers; third, period of organ development; fourth, period of histological differentiation. It is to be noted that in many animals these four periods are not sharply separated from each other. The principles which are of importance from a comparative standpoint are chiefly illustrated by the first three of these periods.


The beginning of the life cycle of every organism is very closely related to the development of the parent, in that the cells from which the new organism comes are early set aside and from then on are to be distinguished from the other cells and organs of the body. Strictly speaking, the embryology of an animal would require us to trace the germ cells from their very first appearance on up through the stages of their development and to trace the formation of the matured sperm and ovum as well as the fertilization, cleavage, germ-layer formation and the subsequent stages, if we were to give a complete account of development. But it has come to be the practice to begin the embryologieal account with fertilization and cleavage, and we shall therefore deal with the germ-cell history only briefly, leaving the cytological details for other more exhaustive treatments.


Our knowledge of this development is a matter of the last half century, and so voluminous has the accumulated information become on the problems connected with the cell that it now constitutes an entirely separate division 0" zoological science, namely cytology. The foundations of this science were laid in the last two decades of the nineteenth century by investigators who sought information concerning earlier and earlier stages of germ-cell development. These studies received a great impetus from the conception of the germ plasm which was published during the early part of the period by Weismann. The organism was thought of as consisting of two more or less opposing portions, a germ plasm (which functions as a hereditary vehicle and is passed on to the next generation as it gives rise to the new individual) and soma or somatoplasm (which is the remainder of the organism’s body and is concerned with its individual well-being). It was thought by the earlier students that certain cells were thus the germ plasm, that is, the germ cells as contrasted with the body cells, while others had no part in the reproductive activities of the organism except in so far as they were necessary for nourishment and support.


This conception has been largely responsible for the attempts, now of many years’ standing, to trace the origin of the germ cells back to the early blastomeres, that is, earlier than the period in which they can be recognized as part of the reproductive organ, the gonad. These attempts were productive of successful results in quite a number of cases in both vertebrates and invertebrates where it was found that the antecedents of the primordial germ cells could be recognized even in early cleavage divisions. In other cases, however, it has not been found possible to trace the history of the primordial germ cells back of their first appearance in the gonads. The origin of the primordial germ cells is an embryological problem of considerable importance in itself aside from its relation to the germ-plasm doctrine, and as such it is discussed in detail in a later chapter in this book. (See Part Two, (‘hapter II.)


Partly as a result of the inconclusive data from the study of the origin of the primordial germ cells in‘ the animal kingdom as a whole, but more especially as a result of the work of recent years on heredity in which the importance of the chromosome has been made manifest, a new conception of the germ plasm has now showed itself to be more acceptable to many students of these problems than that of Weismann. According to this new view, each cell contains material which corresponds to both the germ plasm and soma, the chromatin representing the former and the cytoplasm the latter. This interpretation explains many facts which had previously proven difficult to understand and enables us to regard the origin of germ cells as a problem of embryology rather than as one whose chief interest is in relation to the transmission of a “germ plasm.”

1. Cell Division in Gametogenesis

Regardless of their source or of the time of their first appearance, there are present in a developing gonad certain cells which are known as primordial germ cells. They remain in a quiescent stage during the early development of the organism, beginning their active development only when the somatic structures are well on the way to their adult condition. Somatic structures are produced by the differentiation of the cytoplasmic portions of the cells (giving rise to the familiar histological distinctions between tissues), and must obviously precede those activities which have for their end the reproduction of the organism. On the other hand, specialization of the nuclear structures involved in the function of mitotic cell division is responsible for reproduction and must wait until the proper development of the somatic structure has been attained.


a. Multiplication Period. At length the primordial germ cells begin a period of proliferation known as the multiplication period. This period varies in different forms, both as to its beginning and its duration, for it does not necessarily begin at the same stage in the life cycle, nor does it consist of the same number of cell divisions. It is said that in the grasshopper there are eight divisions in this period; thus each primordial germ cell would produce as a result 256 descendants. The cells undergoing the various divisions of the multiplication period are knovsn as gonia; the male cells are sperrnatogonia and the female oiigonia, alike in every fundamental respect.


b. Growth Period. At the end of the multiplication peziod, that is, with the formation of the primary oiigonia or spermatogonia, the active proliferation of cells ceases and there follows a growth period. During this period without further division the cells increase in size and store up nutrient material, deutoplasm, to furnish them with energy for the further activities they are to undergo. In the case of the oogonia this material is the yolk, often present in large amount to furnish food for the embryo until it has become at least in part able to obtain sustenance from its surroundings. The amount and distribution of this yolk material is a very important factor in determining the type of cleavage and future development of the embryo, for cell division becomes increasingly difiicult with the accumulation of inert yolk material. (This point should be kept in mind, for frequent use must be made of it in the study of cleavage types.) These cells are new primary spermatocytes or ooeytes.


c. Maturation Period. Two maturation divisions now succeed each other, usually with some degree of rapidity. They constitute the maturation period during which nuclear events of the utmost importance from the standpoint of the future organism occur. In oogenesis as well as spermatogenesis there are two divisions which are similar in all respects except one. In the maturation of male germ cells, the first division results in two second spermatocytes which promptly divide again producing four spermatids from each primary spermatogonium. (In the grasshopper obviously there would be now 1024 descendants from each primordial germ cell.) These cells do not again divide but go through a seriés of changes (spermiogenesis) by which they become metamorphosed into functional spermatozoa with the characteristic structure of spermatozoa; these changes are cytoplasmic in character. In the developing female germ cells, however, maturation results in the production of one functional egg and three non-functional cells which are its equivalents from the standpoint of nuclear content and which are known as polocytes or polar bodies. This is accomplished as an immediate result of the fact that the first maturation spindle is short and occupies a position near the surface of the large oocyte. Since the division plane cuts through the center of the spindle, a very small cell, the polocyte, is cut off from the larger “egg cell” which is now the second oécyte. In the second maturation division both cells divide, the “egg cell” again unequally producing a matured egg and another polocyte, and the first polocyte two other polocytes. This size dil’ferentiation of the four cells which are descendants of the primary oiigonia allows the normal nuclear events of maturation to go on unmodified, but concentrates the ooplasmie materials, which are necessary to the nourishment of the embryo, in one functional cell.



fiG. 4. Diagram to illustrate the processes of gametogenesia.


Gametogenesis is now completed. Sperm and egg are ready for the next step toward the production of a new organism, namely fertilization. By fertilization is meant the entrance of the sperm into the egg, but the process is very complicated and is completed only with the union of egg and sperm nuclei into a fusion or cleavage nucleus. Fertilization has two functions which are distinctly different: the initiation of cleavage or the development of the zygote, and the restoration of the chromatin material equivalent to that lost in reduction. This complicated process has been the subject of much study during the last two decades, especially from the viewpoint of experimental or physiological embryology, and many fundamental conceptions have resulted from this fascinating phase of embryologieal research.

2. Nuclear Events in Gametogenesis

The nuclear events during gametogenesis must also be understood, at least in outline. Their study has been the especial problem of cytology during the last quarter of a century.

a. Reduction. It may be stated as a general law that every species of organism is characterized by a definite number of chromosomes, and, REDUCTION 13

with a few exceptions which are not in contradiction to a full and adequate statement of the law, this number occurs in all the cells throughout the bodies of all the members of a particular species. This characteristic number is spoken of as the somatic or diploid number. In the history of the germ cells all the divisions of the multiplication period are found to have this characteristic number. During the maturation divisions, however, this number is halved, that is, reduction occurs, and the matured gametes have only half the number characteristic of the species. The union of the male and female gametes in fertilization effects a return to the somatic number. The process of reduction is often called meiosis and the divisions meiotic divisions. Commonly, although not at all of necessity, the first division is the reductional or heterotypic as distinguished from the somatic or homcotypic divisions. If the first is hcterotypic the second is homcotypic, and conversely.

In those cases in which the (lijfcrences between reductional and somatic mitoses are most clearly recognizable (that is, in those cases in which tctrads are formed), we may say that the explanation lies in two outstanding facts, namely, the peculiar prophase of the first division with its synaptic pairing of homologous chromosomes, and the entire absence of a prophase to the second division. The case of Ascaris, the roundworm, is one of the best known, and it may serve as an illustration of these processes. The somatic number of chromosomes in the common Ascaris is four. These are not looked upon as four separate and unrelated individual chromosomes, however, but as two pairs, for it has been shown in some animals that one member of each pair came from the male parent and one from the female in the preceding fertilization, and it is very probable that this is always the case. The members of a pair are spoken of as homologous chromosomes. Thus each individual derived from a fertilized egg contains in each of its cells two full sets of homologous chromosomes. This fact is to be contrasted with the well-substantiated observation that the germ eells contain but a single set of chromosomes owing to the process of reduction. It will be recalled that a typical mitotic division, whether in a germ cell or a somatic cell, involves in the prophase a condition which is really the climax of the entire set of mitotic events, namely the splitting of the spireme thread which is to condense to form the metaphase chromosomes. For this reason the metaphase chromosome may be looked upon as consisting of two halves, or chromatids, even if this condition sometimes is not easily seen under the microscope. That is to say, the ordinary chromosome is a. bivalent one, a dyad. With these facts in mind we are now ready to inquire into the nature of synapsis. fiG. 5. Fertilization and cleavage of Ascaris. (Redrawn from Kellicott after Bovcri.)

A, epermntozoon entering the egg as the second maturation division is taking plan-. B. pronuclei going into prophnso and division of sphere and centrosome beginning; C, I), showing further advance toward E, the first cleavage division.


b. Synopsis. By synapsis is meant the union in the first maturation prophase of the two homologous members of each chromosome pair to form 3* Smgle chromosome; although one which has a valence of four, as contrasted with the preceding bivalent condition. Since two somatic chromosomes, each consisting of two chromatids, are involved in the formation of this new chromosome, it is called it tetrml. It is evident that by synapsis a reduction in the number of chromosomes, but not in the amount of chromatin, has been effected. It is the function of the two maturation divisions to complete this reduction in amount by distributing the four component parts of each tetrad to the four different spermatozoa,or, in the case of oogenesis, to the egg and the three polar bodies.


fiG. 6. Diagram of tctr-ad formation and subsequent ro(lu('lion. (Suggested by diagrams from Sharp.)

a. chromosome conditions in spernmtogoniu and at beginning: of first maturation prophuse; I). synopsis in prophnse of first maturation prophnsc (first spernw.tot_vte), 0, tctrad-5 ready for the division; d. metaphnse of first niaturmitm division; 0. unaphuse of d; f. second spcrmntocytes produced by the division of e; g, division of second spermutocytes; h. resulting distribution of chromosomes as they go into the four spermatozoa.


As a result of synapsis it appears that there are in each first spermatocyte or oocyte the reduced number of chromosomes in the form of tetrads, or quadrivalent chromosomes. Each tetrad consists of four chromatids, aa/bb’, two from each of the homologous mates, A and B. If, in the succeeding metaphases and anaphases, the bivalent chromosomes (dyads) which are the result of the division of the tetrad are each composed of two chromatids derived from the same one of the constituent synaptic mates, that is, a and a’ in one chromosome and b and b’ in the other, the division is said to be rcductional or heterotypic. In other words, in a reducing division whole chromosomes are separated. In this case the second division would be an equational division for the equivalent half chromosomes would necessarily be separated as in any ordinary mitosis. That is, the bivalent aa’ is now divided into a and a’ and bb’ into b and b’, and the maturation with the consequent distribution of the four parts of the tetrad to the four germ cells is completed. If, on the other hand, the bivalent chromosomes resulting from the division of the tetrads consist of half of each synaptic mate, that is a and b in one and a’ and b’ (or of course a and b’ or a’ and b may be linked together), then the first division is equational. The second division would then be reductional and as in the previous case would result in the distribution of the four component parts of the tetrad to the four matured germ cells.

In these cases in which tetrads are formed there is no resting stage preceding the second maturation division, but the dyads without a reorganization arrange themselves on the second spindle. There is a large number of cases, however, in which actual tetrads are not formed owing to the failure of the usual split of the spireme to appear during the early stages of the first division. In this case the first division is always reductional; there is a pause between the first and second divisions during which the chromosomes undergo seine reorganization; and the splitting which was delayed takes place. It is evident, therefore, that precisely the same result is achieved in the two cases, whether or not tetrad formation occurs. That is, there are formed from each spermatocyte or ooeyte of the first order four cells, each with half the number of chromosomes characteristic of the species. These four cells are mature spermatozoa or one functional egg and three polocytes or polar bodies. FERTILIZATION 17

3. Fertilization

It has already been pointed out that there are two totally different functions served by fertilization, namely, the one concerned with the hereditary mechanism in which the diploid number of chromosomes is restored, and the other which sets into operation those processes leading to cleavage and the further development of the embryo. The first of these functions is accomplished in the conjoining of the male and female pronuclei. The sperm with its reduced number of chromosomes enteis the egg, and its subsequent union with the female pronucleus which also contained the reduced number produces in the zygotic nucleus so formed the full number of chromosomes. The second function is accomplished in a very complicated series of processes of physico—(-hemical nature. Much of the research in experimental embryology of the last two (i(‘('£l.(l(‘s has dealt with the physico-chemical aspects of the activation of the egg,


he 7 Entrance of speimatozoon into egg of the starfish Astcruza glaL1u.[7..s. (Redraun from (‘onkhn after Fol )

and results have been obtained which have given real insight into the fundamental nature of the living organism.

The morphological aspect of fertilization is well known from the studies of Lillie on Nerezs, of Wilson on Toxopneustcs, of Kostaneeki and of Wierzejski on Physa, of Boveri on Ascarzs, and of many others. In most animals the entire sperm enters the egg, but there are many others, as in the sea—urchin and the staifish, in which the tail or at least most of it remains outside. The important elements of the sperm that enter the egg and have a function in fertilization are the head which is equivalent to the nucleus, the central body or the structures derived from it, the acrosome which is derived from the Golgi apparatus of the spermatid, and some chondriosome material. Of course the most important of these is the nucleus, but historically the central body also has been the subject of much discussion in connection with fertilization. The sperm enters rather largely through the activity of the cortical layer of the egg and by a. fairly definite path makes its way toward the egg pronucleus. Immediately upon entrance, however, it rotates so that the middle piece 18 THE GERM-CELL CYCLE

precedes in the advance to the female pronuc‘eus. The sperm aster appears from this region, and it is the usual condition that it should become the aster of the cleaving egg. It divides and forms the spindle between the two approaching pronuclei. It is not the rule that the egg aster contributes to the formation of a cleavage spindle.


fiG 15. Steps in fertilization of the sea ur(-hm, Tow1mc'u.stcs (Redrawn from (‘oiiklin after Wilson )

a, mature spermatozooii, iii. traiisforination of speimatozooii into male pronucleus, {, female pronucleus


The entrance of the sperm occurs at different times with respect to the extrusion of the polar bodies in different eggs. At one extreme is the condition as found in the sea-urchin and in the coelenterates in which both maturation divisions are completed before the sperm can enter. At the other extreme is the Ascaris type in which the sperm enters the egg before either polar body has been extruded; in some cases it may be before the germinal vesicle has been broken down, in others while the first maturation mitosis is in the metaphase stage. Nematodes, fiatworms, molluscs, some annelids and crustaceans belong to the Ascaris type. There are of course intermediate stages between these two extremes.

The series of processes involved in fertilization may be said to be completed when the two pronuclei have closely approached each other and a cleavage spindle is formed between them. There is seldom an actual fusion of the pronuclei as such; rather they lie side by side upon the developing first cleavage spindle, and at the end of this first division the chromosomes of the components are intermingled and are no longer to be distinguished as from the two parents.

We are thus back at the stage from which we started and cleavage is the next step in ontogeny.

Bibliographic Note

Among the more important accounts of the subjects contained in this chapter are the following: Wilson, Sharp, Kellicott, Cowdry. These works are cited in full in the bibliography on page 406.


1931 Richards: Part One General Embryology 1 Historical Development of Embryology | 2 The Germ-Cell Cycle | 3 Egg and Cleavage Types | 4 Holoblastic Types of Cleavage | 5 Meroblastic Types of Cleavage | 6 Types of Blastulae | 7 Endoderm Formation | 8 Mesoderm Formation | 9 Types of Invertebrate Larvae | 10 Formation of the Mammalian Embryo | 11 Egg and Embryonic Membranes | Part Two Embryological Problems 1 The Origin And Development Of Germ Cells | 2 Germ-Layer Theory | 3 The Recapitulation Theory | 4 Asexual Reproduction | 5 Parthenogenesis | 6 Paedogenesis And Neoteny | 7 Polyembryony | 8 The Determination Problem | 9 Ecological Control Of Invertebrate Larval Types


Cite this page: Hill, M.A. (2020, August 10) Embryology Book - Outline of Comparative Embryology 1-2. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Outline_of_Comparative_Embryology_1-2

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