The literature relating to the spermatocytes of insects was reviewed at
some length in my previous paper upon the history of these cells in the
Acrididæ (=17=). It is not my purpose to go over this same ground again
except in so far as increased knowledge makes it necessary. More recent
papers by Montgomery, Wilcox and others will, however, be discussed in
detail. The policy previously announced, of restricting comparisons to
results derived from insects, will again be adhered to. I believe that
the main features of the maturation divisions are essentially the same
in all insects, and I desire to see this belief either well established
or overthrown. If it can be demonstrated that so large a class as the
insects are characterized by a common process, it will be a firm basis
upon which to conduct further comparative studies into more
comprehensive groups. On the contrary, if it is shown that there is no
type, even in the class, then it is useless to seek agreements between
widely removed species.
(_a_) _Nomenclature._
A necessary basis for any comparative work is a common terminology.
Confusion inevitably follows the loose application of names to the
structures compared. This is perhaps unavoidable in the early stages of
an investigation, but should be overcome as soon as possible. There is
surely no reason for continuing uncertainty after terms have received
general acceptance. Believing this, I feel called upon to repeat my
criticisms of Montgomery’s application of the well-accepted terms
“prophase,” “metaphase,” “anaphase,” and “telophase.”
In reply to my previous objection directed against this part of his
work, Montgomery acknowledges the validity of the criticism so far as it
relates to the metaphase, but denies the application to the other
phases, particularly to the anaphase. He alleges in support of his
position that the introduction of an unusual condition, the “synapsis,”
makes it impossible to correlate strictly the stages of the germ-cells
with those of ordinary divisions. Upon this point I must again disagree
with him. It is impossible for any known modification of the prophase to
change the essential character of the anaphase, so as to make it precede
instead of follow the metaphase. This stage marks the movements of the
chromosomes from the equatorial plate to the poles, and terminates when
they are massed around the centrosomes. How can the “synapsis” in the
least affect the duration or character of this process? It is apparent
enough, I think, that Montgomery’s subphases of the “anaphase” do not
belong to this portion of the mitotic cycle at all, but are really
portions of the telophase of the spermatogonia and prophase of the first
spermatocyte. Further, it may be noted that, even were these subphases
properly included in the anaphase, they would belong to the
spermatogonia and not to the spermatocytes.
Montgomery himself seems to be rather uncertain of the position of his
“anaphase.” In the first paper, upon _Euchistus_ (=12=), it was put down
as the anaphase of the first spermatocyte; in his later paper (=14=),
upon _Peripatus_, it is recorded as the anaphase of the spermatogonia.
Still more confusing is his use of the “telophases,” for in the article
upon _Peripatus_ (=14=) it is, in the “Contents,” placed as a substage
of the spermatogonial anaphase, and in the body of the work, page 307,
as the telophase of the spermatocyte! Neither the anaphase nor the
telophase can, by any possible construction of their proper meanings, be
made to apply to the “growth period” of the germ cycle, as Montgomery
insists; they are the last stages of the “division period,” in reality.
The prophase of the first spermatocyte is the initial stage in the
constructive process marking the growth period.
Montgomery’s translocation of the terms makes the “synapsis” occur in
the anaphase. This is manifestly an impossible condition of the
chromatin at this time, and his figures show definitely enough that it
is a prophase, or, at the earliest, a spermatogonial telophase, that
witnesses the contraction of the chromatin. The objection urged in my
earlier paper (=17=) to the use of the term as a designation for the
mere contracted condition of the chromatin cannot apply to Montgomery’s
latest use of it; for he here recognizes the justice of my contention
that it was primarily designed to indicate the fusion of the
spermatogonial chromosome to produce the chromosomes of the
spermatocyte. He states this clearly in the following words: “Moore
(1895) first gave the name ‘synaptic phase’ to that stage in the growth
period of Elasmobranchs when the reduction in the number of chromosomes
takes place. Accordingly, the criterion of the synapsis stage is, first
of all, the combination of univalent chromosomes to form bivalent ones;
whether the chromosomes are then densely grouped or not is of secondary
importance.”
(_b_) _The Spermatocytes of the Locustidæ and Acrididæ._
The formation of the first spermatocyte chromosome gives us an insight
into the later changes undergone by these elements such as cannot be
obtained in any other way. The great importance attaching to this part
of the spermatogonial process renders it desirable to exhaust every
effort in obtaining a knowledge of the actual changes here taking place.
This thought has been held constantly in mind during the progress of
these investigations, and every point of resemblance or of difference
between the various species studied has received careful attention.
Despite variations in details, however, I must state that the essential
features of the maturation divisions are the same in all species of the
Orthoptera examined. It is true that as yet only two families, the
Acrididæ and the Locustidæ, have been worked out in a detailed way, but
the close agreement between these raises a strong presumption in favor
of the general prevalence of the type. The processes of the two families
have already been described in detail, but it will perhaps be well to
call particular attention to some points worthy of mention.
The general appearance of the material derived from the two families is
quite different in sections. Even the hastiest observation will show
this. The spermatocytes of the Locustid testis are much smaller, denser
and more deeply staining than those of the Acrididæ. The relative
quantity of chromatin is greater, so that it is possible by
microscopical examination of a section to tell whether it was prepared
from Locustid or Acridian material.
The transformation from the telophase of the last spermatogonial
division to the prophase of the first spermatocyte is marked by
practically the same changes in both families. It is to be observed,
however, that the derivation of the spireme from the disintegrating
chromosomes of the previous generation is not so clearly indicated in
the Locustid cells, and it was for this reason that in the examination
of _Xiphidium_ I was not able to determine certainly that the accessory
chromosome came over from the spermatogonia into the spermatocytes as a
formed element. Upon this point, as upon others, my later material is
clearer, and I was able to reconcile the appearances in the two
families. In both, unfortunately, it has been found impossible to
determine the exact origin of the first spermatocyte chromosomes.
In connection with the transformation of the chromatin from the
spermatogonial condition to that of the spermatocyte, we must take
notice of that stage which is commonly denominated the “synapsis.” The
evidence afforded by the Orthopteran cells is entirely negative
regarding this. In properly fixed material there is no distortion of the
chromatin in the nucleus at any time. It would, if present, be
particularly easy to observe, as was stated in my previous paper, for
during the entire winter the spermatocytes exist in the spireme stage,
and in a longitudinal section of a follicle all stages may be discerned.
On the other hand, in poorly fixed or hastily prepared material the
synapsis is present, and always in such a form as to indicate its
artificial character. What is here said regarding the synapsis refer to
the appearance commonly thus designated, but, as has already been
stated, such an application of the term does not meet the spirit of the
definition as intended by Moore (=20=). A fusion of the spermatogonial
chromosomes of some sort must certainly occur, but that it is always
marked by a unilateral massing of chromosomes, I deny.
During the prophase the chromatin segments in the cells of _Orchesticus_
and other species of the Locustids are heavier, more granular and denser
than they are in _Hippiscus_. It is to be observed, also, that there is
a greater variation in the size of the elements. This fact is observable
from the earliest appearance of definite segments down through both the
spermatocyte mitoses. This disproportion may be such that one chromosome
will exceed another in the same cell by twenty or thirty times its
volume. We have here, as is pointed out in another place, a strong proof
concerning the individuality of the chromosomes, for in some species it
is possible to distinguish a particular chromosome in all the
spermatocytes. This is strikingly the case in _Anabrus_, where there is
always one chromosome very much larger than any of the others. It
exceeds in size even the accessory chromosome, and might be mistaken for
it were it not for the difference in form. It is, however, typically a
tetrad, and shows the four chromatids, while the accessory chromosome
exhibits the usual spermatogonial condition.
As was indicated under the head of “Observations,” the prophase tetrad
characteristic of _Anasa_ and _Hippiscus_ is again exemplified in the
Locustid cells. So close is the resemblance of the maturation
chromosomes of these various insect cells in their early stages, that I
now regard it as practically established that they are commonly present
in all insect spermatocytes. No more important evidence regarding
chromosome structure and behavior can be obtained than that afforded by
these elements. Particularly are the ring figures of value in the
determination of the sequence of the longitudinal and cross divisions,
and upon this point the material from the two families is equally
convincing and positive in demonstrating that the first spermatocyte
mitosis witnesses a separation along the longitudinal cleft of the
spireme thread.
I should like to emphasize the fact that the chromosomes in both the
Orthopteran families studied have been carefully traced from their
earlier appearance down to the time of their dissolution in the
spermatid through such a gradual series of changes that there can be no
reasonable doubt of the accuracy of the conclusion set forth in these
papers. The Orthopteran material possesses one distinct advantage over
the Hemipteran, in that the point of cross-division is always marked by
the same sort of a protuberance as is to be distinguished in the early
chromatin segments. When the two free ends of the element are brought
around to form a closed ring, the last particle of doubt regarding the
position of the planes of separation marked out for the two spermatocyte
divisions is dispelled.
This diagnostic character seems to be lacking in the chromosomes of the
Hemiptera, and Paulmier, in his work on _Anasa_, depends for his
criteria of orientation upon the relative lengths of the chromosome
axes. Such a feature would be valueless in Orthopteran cells, because,
as has been shown, the chromatids move upon each other in such a way as
to exactly reverse the preexisting relation between the axes. How
applicable this observation may be to conditions in the Hemipteran
cells, I do not know; but, judging from the great resemblance of the
elements in the prophase, it would seem most reasonable to expect a
similarity of the divisions.
Paulmier (=22=) advances the suggestion that in the double-V figures we
may find a structure that will serve to reconcile the divergent accounts
concerning the longitudinal and cross divisions of the tetrads. The only
way in which this might be accomplished would be to suppose that each of
the interspaces represents a longitudinal cleavage of the thread, the
first being at right angles to the second. I have given this suggestion
careful consideration, and find no evidence to support it. The double Vs
are only of rare occurrence, the common element being a straight rod, in
the center of which is a diamond-shaped clear spot representing the two
planes of division laid out for the spermatocyte mitoses. If two
longitudinal divisions occur, one must precede the other considerably
and the resulting halves become mutually repulsive, so that they move
apart and lie in one plane with only a slight connection at the point of
final separation. Moreover, the second cleavage must begin at the
opposite end of the segment and proceed in a reverse direction from the
first. Not only this, but the first spermatocyte mitosis divides the
elements along what is generally conceded to be the longitudinal split,
and this must necessarily succeed the supposititious first longitudinal
cleavage by some time. Without going into a consideration of these
points, I may say that they suggest such deviation from normal processes
that only extensive and accurate observations would make Paulmier’s
suggestion worthy of further consideration.
(_c_) _Formation of the Tetrads._
In my former paper I reviewed the results obtained by Montgomery upon
the Hemiptera, but further notice of his work will now be necessary,
since on almost every important point relating to chromosome structure
he has changed his opinion. His late extensive comparative study upon
the Hemipteran cells, as well as that upon _Peripatus_, will at the same
time receive consideration.
It appears from Montgomery’s account that at the point where the
Orthoptera are least valuable in demonstrating chromosomal relations the
Hemiptera and _Peripatus_ are most convincing. I refer here to the
derivation of the first spermatocyte chromosomes from the chromatin of
the spermatogonia. He claims to have observed the union by pairs of the
secondary spermatogonial chromosomes during the anaphase (his synapsis)
so clearly as to be positive of this fusion. I hope this may be
verified, for it offers a logical explanation of the process of
reduction, and is a confirmation of what has previously been assumed
true without sufficient basis in observed fact, as was suggested in my
paper on _Hippiscus_. This, if established, would also be a strong
support of the theory relating to the constancy of the chromosomes. If
this true synapsis is accomplished at this time, however, it must be
noted that it occurs during the last phase of the final spermatogonial
mitosis, and is not an act of the spermatocyte prophase. But as to the
exact location of this point no contention need be made, for it is
conceivable that the time of its occurrence might vary considerably
without affecting the essential nature of the process.
With regard to such an origin of the first spermatocyte chromosomes,
there is an important difference to be noted between the earlier and
later work of Montgomery, and one which he fails to mention. In his
paper (=12=) upon _Euchistus_ he states the matter as follows: “But in
the post synapsis we do not find seven chromosomes, the definitive
number present in the spermatocyte divisions, but a smaller number;
hence, in the synapsis the true (_i. e._, exactly half) reduction of the
chromosomes does not take place, but the number is reduced to less than
one-half.” This statement is based, he says, upon a most careful and
painstaking enumeration of the chromatic segments in a number of nuclei,
and is unhesitatingly declared correct.
In his later paper, on the contrary, he is just as positive that the
definitive reduction is here accomplished, for he says: “Since then I
have been able to demonstrate that this numerical reduction is effected
in the synapsis by the union into seven pairs of the fourteen
chromosomes, each of the seven bivalent chromosomes (pairs) being
composed of two univalent chromosomes joined end to end.” This statement
is made without adducing any specific proof, as was formerly done. By
what means we are to reconcile these diametrically opposite statements
Montgomery does not say. He, however, insists that he has always known
that the fusion by pairs takes place. How this was to be brought about
under his previous assumption that one of the fourteen spermatogonial
chromosomes became removed from participation in the usual processes of
the cell to form a “chromatin nucleolus,” he fails to state. Until the
confusion is cleared up by corroborative evidence on one side or the
other, a most important part of Montgomery’s work must still be regarded
as uncertain.
Despite his recognition of the fusion of the chromosomes in the synapsis
as the essential feature of this stage, Montgomery is insistent upon the
concentration of the chromatin as its distinguishing characteristic.
Regarding this he says: “McClung considers the appearance of the
synapsis stage as artefacts. It is hardly necessary to reply to this
criticism, since in all _Metazoa_ where the spermatogenesis has been
carefully examined, with the exception of certain _Amphibia_, the dense
massing of the chromosomes (?) in the synapsis stage has been shown to
be a perfectly normal phenomenon.”
Concerning two points in this statement I wish to take exception. First,
as was suggested in my previous paper (=17=), the term synapsis is
usually applied to a condition of the prophase in which the apparently
unsegmented spireme exists. It must be remembered that most
investigators consider that the reduction of the chromosomal number
takes place by the segmentation of a spireme into half the usual number
of segments. In the second place, I must resent the implication that the
work done in this laboratory is not “carefully” conducted. Many
“_Metazoa_” have been examined “carefully,” and in none has the
“synapsis” occurred when the material was well fixed and prepared. It
has, moreover, been found possible to produce the appearance at will.
One case of this kind is sufficient to raise the presumption that it may
not be normal even when constantly found in certain preparations. I have
not, however, absolutely denied the possibility of such an occurrence,
because it is conceivable that from the telophase of the preceding
division the massing of the chromosomes may persist during their
elongation. My contention is that the appearance is not a constant or
necessary condition in “all the _Metazoa_,” and this I have proven.
In rather striking contrast to the work of Montgomery, in which an
effort is made to formulate a typical process for the entire _Metazoa_
from the study of a single order, is that of Wilcox, wherein a general
denial of any apparent system in the maturation divisions of animals is
based practically upon the study of a single species. As was stated in
my former paper, I regard Wilcox entirely in error upon the vital point
of his theory of tetrad formation, not by “forced interpretation” of his
own views, but by an actual examination of the object upon which he
worked. There is no point upon which Orthopteran material affords more
indisputable evidence than upon the occurrence of the longitudinal
division of the chromatin thread in the early prophase. My statement
regarding Wilcox’s position on this subject was in no sense “misdirected
criticism,” but an actual statement of fact; it was not an attempt to
explain away “abundant and evident cases which cannot be made to fit
into the scheme,” but simply the presentation of proof that _one_ case
was wrongly interpreted.
Wilcox claims the distinction of being the first and only investigator
to doubt the hypothesis that longitudinal and cross divisions of the
chromatic thread produce chromosomes of a different character. It is
perhaps well that this is so, in view of the reasoning by which such a
distinction is secured. Upon his own unconfirmed and disputed statement
that there is no longitudinal division of the spireme, Wilcox presumes
to disparage the accepted view of practically all cytologists. The
constructive thought of the last two decades is summarily disposed of by
this author in the following language: “The whole question, therefore,
whether a certain division is longitudinal or transverse loses its
practical significance, since the theoretical interpretation which has
long been placed upon these divisions is shown to be impossible and
absurd!” The showing alluded to consists in the statement that the
chromosomes consist of an indefinite number of granules, which cannot be
expected to arrange themselves in any order, and which, therefore, may
be divided in any way without affecting the results.
Laying aside for a moment the question as to the occurrence of a
longitudinal division, we may well inquire whether the belief that, “In
view of this manner of the formation of the chromosomes (by the
aggregation of the chromomeres), it seems absurd to assume that the
separation of an individual chromosome by one plane could be
quantitative while the separation by another plane was qualitative,” is
well founded. At the basis of such an assumption lies the implication
that any definite arrangement of chromomeres is impossible; for if any
definite order were possible, then the supposed argument against the
longitudinal disposition of the chromomeres would be invalid.
The argument of Wilcox is therefore directed against order in general,
and not against order in any one particular, as he would have it appear.
For it must be admitted that if it is possible for the scattered
chromatic granules of the early prophase to arrange themselves at all
(and this even Wilcox does not deny), it is equally possible for them to
come together in a definite order. That they do this is amply evidenced
by the fact that later they appear in definite groups or chromosomes. It
is to be noted, moreover, that the later investigations tend to suggest
that the apparently unorganized chromatic granules in the first
spermatocyte prophase are really bound together and represent merely a
diffuse condition of the spermatogonial chromosomes.
Wilcox’s chief error, however, is not to be sought in speculative
theories, but rather in his faulty observations. He repeatedly denies
the occurrence of any longitudinal split in the chromatic thread of the
first spermatocyte prophase. That he is mistaken here I am thoroughly
convinced, both from a study of his own object and from investigations
upon many other species of the same family. At the present time, also,
practically every spermatologist is aligned in support of the view
denounced by Wilcox. For a while Wilcox had some backing, but most of
those who advocated only cross divisions of the thread have later been
able to demonstrate the longitudinal cleavage in better prepared
material.
There is general acceptance of the opinion that the chromomeres of the
last secondary spermatogonia appear in a linear arrangement to form what
is commonly known as the “spireme.” Wilcox declared that while in a very
fine condition this thread breaks across into segments, which unite by
pairs to form the chromosomes of the first spermatocyte. The great
majority of other investigators are unanimous in the opinion that this
fine thread, made up of granules, becomes double by the division of each
granule individually, thus producing a double thread. Thus it is that
the two halves of a longitudinally divided chromosome are made
equivalent, not by the sifting apart of preexisting granules, but by the
division of these after they are arranged in a linear series. It need
hardly be mentioned that the formation of the thread has here a reason
for existence which is entirely lacking according to Wilcox’s scheme.
This much space has been devoted to Wilcox’s statements, not because
they present any arguments against the generally accepted views of his
fellow workers, but because he represents a rapidly lessening minority
which is content to work in a very limited field and to resort for the
explanation of diverse results to the very convenient theory that great
differences may be expected in the normal processes of even closely
related forms. One needs only to glance at the work of all insect
spermatologists to see how closely the agreement now is upon the
important points of the process. This accordance of results Wilcox
notes, but interprets in his own way, which may be regarded as not
exactly complimentary to the skill and judgment of his colaborers. “It
is only necessary,” he says, “to refer to any recent publication on the
subject to find examples of this attempt to force the divergent
processes in different species to fit the same formula.” This is
certainly a very easy and convenient way to dispose of the accumulated
observations of the many careful investigators who have come to an
agreement upon the important questions under discussion, but I venture
to think will hardly satisfy any one except its sponsor.
After handing in this article for publication, I fortunately secured a
copy of the paper by R. de Sinéty (=37=) in which the spermatogenesis of
various Orthopteran species is described. I regret that the available
time is so short that I shall not be able to bestow upon this
contribution to insect spermatogenesis the attention it deserves, but I
shall try at least to consider the principal points wherein a difference
exists between the results of de Sinéty and of myself.
It is unfortunate that we have here a further complication of the
problem concerning the character of the two maturation divisions in
insects. At this time it had begun to appear as if there was every
possibility of insect spermatologists coming to an agreement with regard
to the maturation processes. Indeed, with the exception of Wilcox, who
occupies a unique and solitary position in the field, workers upon the
subject are committed to a belief in the occurrence of a cross and a
longitudinal division of the chromosomes in the spermatocyte mitoses.
The sole difference of opinion relates to the sequence of the divisions.
We have now to consider in connection with insects the remaining
possibility in tetrad formation—that of two longitudinal divisions—which
finds an advocate in de Sinéty.
Because of a thorough acquaintance with the forms upon which this author
has worked, I do not hesitate to say that he is entirely mistaken with
regard to the character of the second spermatocyte division. I am
convinced of this because of the fact that in the early period of my
work upon Orthopteran spermatogenesis I was inclined to place just such
an interpretation upon the phenomena encountered in the spermatocytes of
the Acrididæ as does de Sinéty. I soon became convinced, however, that I
was proceeding upon a wrong assumption, and abandoned it in favor of the
one which more extended observation taught me is correct. I hope to
demonstrate here the ground for my plain statement that de Sinéty is in
error upon the question of a double longitudinal division of the
chromatin thread during the formation of the tetrads in insect
spermatocytes.
It is fortunate that our author has properly appreciated the value of
the early prophase in the determination of the structure of the first
spermatocyte chromosomes, for we are here upon common ground, and need
only compare like stages in order to reach our conclusions. As will be
recalled, the statement is made elsewhere in this paper that the typical
chromosome of the first spermatocyte is an approximately straight rod,
split longitudinally, and again cleft in its middle by a second fissure
at right angles to the first. Such an element is represented in figures
15_a_, 17, D and E of my paper upon the Acrididæ, and in figures 7, 9,
11 and 38 of the present one. Although this is extremely common, and, as
the photomicrographs show, undeniably present, de Sinéty does not figure
it at all. The nearest approach to such a structure is found in figure
123_c_, where a cross with two nearly equal arms is represented. My
interpretation of this figure, based upon a great number of careful
observations, is that this represents merely an extension of the shorter
arms at the expense of the longer ones. In support of this, I have
stated that all intermediate stages between a rod with a mere
enlargement at the center and a cross with equal arms could be found.
How, according to de Sinéty’s conception of overlying free elements,
could these structures be explained?
It is not necessary, however, to have these gradations in order to
disprove the theory under discussion. One needs only to carefully
examine one of these crosses to be convinced that the two arms lie in
one plane where they intersect, and are not superimposed one upon the
other as de Sinéty shows in his figure 123. Our author clearly realizes
the importance of the cross, as may be judged by the following
quotation:
“La croix est de toutes ces figures celle dont la genèse peut le plus
facilement donner lieu à des interprétations en sens contraire.—C’est
précisément pour cette raison que nous croyons devoir l’étudier
spécialement au point de vue critique, persuadé que, cette figure une
fois rattachée à une théorie, les autres doivent en suivre le sort.”
It is unfortunate, therefore, that he was not able to trace the
formation of the element in its very early stages and through the
various modifications which connect it with the typical rod already
described.
As the simplest modification of this basic form, we find the one where
it is evident that the change consists merely in a flexure of the rod at
the weak spot in its center. Such forms are shown in figure 14 of my
former paper (=17=) and in figures 8, 9 and 11 of this one, but are not
illustrated by de Sinéty. It occasionally happens that in chromosomes of
this character the halves diverge widely at the center, producing the
double-Vs of Paulmier, as is represented in figure 14 of my paper upon
the Acrididæ (=17=) and in figure 8 of the present one. These structures
are not shown by de Sinéty and would be difficult to explain in
agreement with his conception of the tetrad.
I have consistently placed great reliance upon the frequent ring-shaped
chromosomes in determining the structure of the first spermatocyte
elements, and have no occasion to change my opinion of them since
examining the work of de Sinéty. This investigator joins issue with me
upon my interpretation of these structures, and states his attitude in
the following language:
“McClung fait grand fond, pour appuyer son interprétation, sur une
forme spéciale, la forme en anneau, qui pour lui dérive du bâtonnet
(a′ b′)/(a′ b′’), supposé placé transversalement sur le fuseau, inséré
par son milieu et incurvé en dehors jusqu’ à rapprochement et soudure
de ses extrémités.
“Le chromosome en anneau est en effet très fréquent chez les
acridiens; mais il nous a été possible d’en reconstituer l’histoire,
grâce à des détails qui ne semblent pas s’être rencontrés dans les
figures de McClung. On se souvient que nous avons établi les deux
points suivants en complet désaccord avec la théorie de l’auteur
américain:
“1. Les deux moitiés de l’anneau proviennent de la première division
longitudinale.
“2. L’insertion est terminale.”
With equal emphasis, I must deny that the enclosed space in the ring
represents any plane of division in the chromatin thread; and that the
insertion of the spindle fibers is at any place except at the center of
what would be the typical rod-shaped chromosome were the ring
straightened out. We encounter in de Sinéty’s interpretation of these
rings the very error against which I was careful to caution elsewhere in
this paper, _i. e._, _of regarding the points where the fibers are
attached as the crossed ends of a simple segment_. This mistake de
Sinéty has made, and has thereby vitiated all his conclusions concerning
the structure of the tetrads. It is not necessary to repeat here the
proof which I have brought forward in support of my views. No one, I am
sure, will find difficulty in reducing the various forms of chromosomes
found in the first spermatocytes to the type of a doubly split rod, in
which one plane of division is parallel to the long axis and the other
at right angles to it. The explanation offered by de Sinéty requires us
to conceive a doubly split rod in which one separating space may vary
indefinitely while the other is constant. There is here no common type,
but an infinitely variable one, which differs with every modification of
the interspace between the first pair of chromatids in each chromosome.
As a constructive basis for the foundation of his theory of a double
longitudinal division, de Sinéty uses particularly the chromosomes of
_Œdipoda_ (_Hippiscus_) _miniata_, represented in figures 129 and 130,
concerning which he says:
“Survient le phénomène exceptionnellement important de la seconde
division longitudinale; nous regardons comme un point capital dans
notre travail d’en mettre l’existence hors de doute et pour cela nous
désirons ne faire appel qu’à des images extrêmement claires. Nous
considérons comme telles les fig. 129 et 130 rapprochées l’une de
l’autre.
“Il est de toute évidence que le chromosome _a_, fig. 130, n’est que
le chromosome de même désignation, fig. 129, dont les deux anses
jumelles se sont clivées. De même, le chromosome en forme de boucle,
_c_, fig. 129, dont les deux branches représentent, comme nous l’avons
fait remarquer, deux anses jumelles, se retrouve avec un clivage très
évident en _d_, fig. 123. On pourrait faire les mêmes rapprochements
entre _b_, fig. 105, et _a_, fig. 107; ici, le clivage est moins
avancé, mais les granules sont nettement divisés.”
I am obliged to confess that I have never seen in other species of this
genus any appearances that would incline me to place an interpretation
upon them such as does our author upon these. I would venture to
suggest, on the contrary, that the chromosomes represented in figure 129
have not as yet demonstrated any division, but show merely irregular
spaces between chromosomes. At even an earlier stage (figs. 5, 37, and
38), I have shown the formation of the tetrads by means of simultaneous
cross and longitudinal divisions so clearly that presumed successive
divisions, as represented by de Sinéty, cannot be regarded as occurring.
Finally, I would emphasize the fact mentioned in connection with the
discussion of the cross-shaped chromosomes, that where the elements of
one of these compound chromosomes intersect _they lie in one plane, and
are not superimposed upon each other_, as de Sinéty’s theory demands and
as his figures represent. This was shown clearly in Paulmier’s figures
as well as in my own, and is even more clearly demonstrated, if
possible, in the very long, slender chromosomes of the myriapods, which
I have observed in Mr. Blackman’s preparations. This, and the continuity
of the chromatin in contiguous arms of the cross, is alone sufficient to
disprove de Sinéty’s theory, and, fortunately, is easily demonstrated.
This same fault of de Sinéty’s is encountered, in another form, in his
discussion of the ring figures. He asserts that the halves of the rings
are pulled past each other while they lie in the plane of the spindle
axis. Herein my observations fail entirely to agree with his. The rings
lie in the plane of the equator, and no elements of the mitotic figure
show a lateral displacement of the separating halves equal to the width
of the chromosome when viewed in this plane.
(_d_) _The Spermatocyte Divisions._
I approach a discussion of Montgomery’s conclusions regarding the form
of the chromosomes in the first spermatocyte, and the sequence of their
divisions, with considerable hesitation, because of the difficulty I
experience in appreciating his exact position. This is due, not to any
lack of positive statements on his part, but to the partial
contradictions that result from his frequent changes of opinion. The
most important statement in his first paper upon _Euchistus_ reads as
follows: “From the resting stage of the first spermatocyte to the
formation of the spermatid, there is absolutely no longitudinal division
of the chromosomes. I have studied hundreds of nuclei in these stages,
and at the first with a hope of finding a trace of such a process, but
observation shows that all divisions of the chromatin elements are
transverse divisions.”
This would certainly seem to be as strong a stand as one could take upon
the subject, but in later papers Montgomery assumes with equal assurance
the opposing position, which holds for a longitudinal division.
Regarding this he says: “During the synapsis stage the chromosomes
become split longitudinally, as was first shown by Paulmier (1898, 1899)
for _Anasa_—a process that I had overlooked (!) in my former paper
(1898).” Throughout his later investigations this hypothesis serves as
the basis of all his theories, and the careful longitudinal division of
the thread is assigned an important role in the maturation process. So
far as positive assertions to the contrary are concerned, a general
acceptance of the theoretical importance attaching to this act is to be
supposed.
Notwithstanding this, I find nowhere in his later writings any statement
that he abandons the conception formerly entertained regarding the
non-importance of the longitudinal cleavage. This attitude is indicated
in the following language: “If it can be proved that the mode of
division of a chromosome, _i. e._, the axis of the line of division, is
merely a function of its chromomeres, then it would be of no theoretical
value whether the division be longitudinal (equation) or transverse
(reduction). But it happens that the postulated difference forms one of
the main premises of Weismann’s theoretical superstructure. On account
of the differences observed in different objects in regard to the modes
of division of the chromosomes, it would appear that the differences
have no theoretical value, but that the halving of the mass of chromatin
is the process of importance—the standpoint taken by Hertwig.
“In the two reduction divisions the chromosomes may split by two
longitudinal divisions, by two transverse divisions, by one longitudinal
and one transverse division, or by one division (longitudinal or
transverse) preceded or followed by an elimination division. The facts
show already that there is no general uniformity in the mode of division
of the chromosomes in the reduction mitoses. The long line of
observations on different objects show this to be the case, and
demonstrates that the expected uniformity does not occur.”
Herein lies the essential conclusion of the work upon _Pentatoma_,
which, so far as a specific retraction is concerned, stands yet. If this
be abandoned, then the first work upon the chromatin structure of
_Pentatoma_ is practically discredited, for Montgomery has definitely
retreated from his positions concerning the absence of the “chromatin
nucleolus” in the spermatogonia, the non-occurrence of a longitudinal
cleft in the spireme thread, the lack of an equational division of the
chromatin in the spermatocyte, the origin of the “chromatin nucleolus,”
and the fragmentation of the “chromatin nucleolus.” In addition to these
specifically acknowledged errors, we may infer that Montgomery (=12=)
considers himself at fault in his views upon the production of
chromosomes from the “three to six chromatin loops” by breaking apart in
the prophase, and upon the occurrence of both longitudinal and cross
divisions of ordinary chromosomes in the same mitosis. The observations
recorded in his last paper (=15=) upon the production of the
spermatocyte chromosomes by the end-to-end union of those in the last
spermatogonial division warrant this assumption.
It follows from all this that we may practically disregard Montgomery’s
earlier work upon chromosomal structure and take his views as expressed
in the later papers (=14=, =15=) as representing his opinions upon the
subject. These later theories are largely the result of his
investigations upon _Peripatus_, but they seem to be carried over and
applied to the Hemiptera without essential modifications, and we may
regard this concept as applicable to the forms studied by him.
I called attention in my previous paper to the fact that, by many
investigators, the definitive form of the chromosome is used as the
basis for determining the direction and sequence of the chromosome
divisions. This fact and the danger attending the practice was partly
realized by Montgomery in his work upon _Euchistus_ (=12=), for he
devotes considerable space to a consideration of the prophase segments,
but in determining the character of the second spermatocyte division he
regards only the formed element. With respect to this he says: “And now
a fact may be determined which is of the greatest importance in
estimating the morphological value of the second division of the
chromosomes. While the latter are still parallel to the axis of the
spindle, there may be clearly seen in some cases a transverse
constriction on some of the chromosomes, so that they already acquire a
dumb-bell shape.” This constriction is not correlated with any similar
one on the prophase elements, and is here observed for the first time.
In his paper upon _Peripatus_, however, he definitely supports the
contention that it is only in the prophase of the first spermatocyte
that we can learn the construction of the chromosomes, for he says: “The
early stages in the prophase are of the greatest importance in
determining the exact constitution of the chromosomes of the first
maturation division.... Since, then, as has been shown in another
section of the present paper, the split of the univalent chromosome of
the second spermatocyte is a true longitudinal split, corresponding
perfectly in position with the longitudinal split of the early prophase,
it follows that the univalent chromosome does not become turned upon its
axis to take its place on the equator of the spindle.” Orientation is in
both spermatocytes based, accordingly, upon planes determined in the
prophase. Upon this point Paulmier and Montgomery, as students of
Hemipteran spermatogenesis, are now agreed, and their results correspond
with observations made upon Orthopteran cells.
It is upon the sequence of divisions in the spermatocyte that
differences now exist between these investigators and myself. In my
previous paper I took occasion to elaborate the proof in support of my
position regarding the early occurrences of the longitudinal division in
the Orthopteran spermatocytes. Montgomery follows Paulmier in ascribing
the reduction division to the first spermatocyte, and takes no account
of my results upon _Hippiscus_. The objections that I previously urged
against Paulmier’s conclusions apply equally well to Montgomery’s. Until
the chromosomes are traced in a more detailed way through the prophase
to the metaphase, I shall consider the presumption against the
occurrence of the cross-division in the first spermatocyte mitosis. In
this I believe that I am justified by the definite proof of my position
brought forward in the work upon _Hippiscus_. Here, it may be recalled,
I observed and photographed in the same mitosis all stages of movement
by the chromatids along the plane of the longitudinal split. In
addition, I was able to locate definitely the position of the future
cross-division in the ring figures, so that it is impossible to mistake
the character of the first division in them. These two proofs I consider
incontrovertible so far as they apply to the Orthopteran families
studied.
Paulmier judged the planes of the division by the relative lengths of
the chromosome axes, but, as I pointed out, this is not conclusive
unless it can be shown that they have not shifted, as it is possible for
them to do, during the prophase. The value of the ring figure, which is
formed at such an early stage that it would be impossible for the
shifting of the axis to occur, is here evident.
Montgomery finds these rings in _Peripatus_, and realizes the importance
of their evidence in determining the planes of division, but places his
conclusions upon a much more insecure footing than those founded upon
the Orthopteran cells, because of the criterion used in determining
which point represents the junction of the paired chromosomes. The
diagnostic feature he uses is the linin connection persisting between
the “central ends” of the chromosome, which holds them together until
the “distal fibers” connect with the centrosomes and cause the rupture
of the “central” fiber. Since the whole of his elaborate theory
regarding the continuance of the linin spireme is practically a
theoretical conception with little basis in observed fact, the value of
such proof cannot compare with that furnished by the definitely formed
chromosomes themselves in the Orthopteran cells.
In view of all these facts, I think it must still be held an open
question as to which is the reduction and which the equation division in
the Hemipteran spermatocytes, although it is not to be doubted that the
probability of the first spermatocyte being witness of the reduction
division is much increased when thus interpreted by two independent
observers.
(_e_) _The Accessory Chromosome._
I have already, in another paper (=19=), taken up a comparative study of
the accessory chromosome in different insect spermatocytes, and shall
not be obliged, for that reason, to enter into a very lengthy discussion
of the subject here. The great interest attaching to this structure,
however, compels me to consider the work that has been done since the
manuscript of the earlier article was sent in for publication. This
review will concern, very largely, the investigations of Montgomery upon
a considerable number of Hemipteran species, which are set forth in his
paper under the pretentious title “A Study of the Chromosomes in the
Germ Cells of Metazoa.”
In his first work upon _Euchistus_, Montgomery describes a cell element
under the name “chromatin nucleolus” which corresponded so closely to my
accessory chromosome that I concluded the two structures were identical.
These similarities were, the origin from a spermatogonial chromosome,
the integrity and constancy of staining power and position during the
spermatocyte prophase, and participation in the division act during
metakinesis of a spermatocyte.
Among the numerous changes of opinion recorded by Montgomery in his
latest work, there are several relating to his “chromatin nucleolus”
that materially alter the aspect of the question. Perhaps the most
important of these concerns the origin of the element. I was some time
in determining that the accessory chromosome is a spermatogonial
chromosome which divides in the spermatogonia with the other chromatin
elements and comes over into the first spermatocyte as a formed
structure. The work of Sutton upon the early history of the element in
_Brachystola_, however, was convincing in this respect and confirmed me
in the opinion I had already formed. I therefore gave Montgomery the
credit for this discovery, and set it down as strong confirmation of the
assumption that we were dealing with similar structures in the two
orders of insects.
Upon this point Montgomery now completely reverses himself, and declares
that his “chromatin nucleolus” is not a spermatogonial chromosome, but
may be noted in the earlier generations as a nucleolar structure, which,
however, divides in metakinesis. The most important feature to be noted
in this connection is the fact that the structure does not exist as a
simple element, but is observed as a number of granules, and that this
number varies considerably in different species. These granules fuse
during the “synapsis stage,” as do the chromosomes, to produce in the
spermatocyte half the number of “chromatin nucleoli” that were present
in the spermatogonia. In this respect the “chromatin nucleolus” differs
radically from the accessory chromosome, which has the same valence in
both cell generations. The indefinite number and insignificant size of
Montgomery’s structures are other characters that point to extensive
differences between them and the accessory chromosome.
In his work upon _Peripatus_, Montgomery states that in restudying his
preparations of _Euchistus_ he observes a continuous linin spireme which
involves the “chromatin nucleolus” as well as the chromosomes. Here,
again, there is a difference between the Hemipteran element and the
accessory chromosome; for the latter is entirely free from linin
connections in the prophase and is usually surrounded by a hyaloplasmic
investment.
According to Montgomery, also, his “chromatin nucleolus” usually takes
part in both spermatocyte mitoses. In this respect there exists an
essential difference between his element and that found in the
Orthoptera, for, after extended and most critical studies, I have become
convinced that only one division takes place in the spermatocytes. In
those cases where Montgomery admits but a single division, it is stated
to occur in the first spermatocyte, while in the Orthoptera the
accessory chromosome remains undivided here and is halved in the second
spermatocyte.
If, therefore, Montgomery’s recent observations are correct, it must
follow, I think, that his “chromatin nucleolus” and the accessory
chromosome are different structures. I am free to admit, however, that
his statements are far from convincing. So much dependence is placed
upon the numerical relationships of elements that are admittedly very
minute, and so little corroborative proof is given, that I entertain
serious doubts as to the accuracy of the observations. In this
connection I would suggest a comparison between the figures of the
“chromatin nucleolus” in the first paper upon _Euchistus_ (figs. 55–68)
(=12=) and those in the last one (figs. 1–17) (=15=). The showing here
made would alone be sufficient to raise a question as to the nature of
the “chromatin nucleolus,” and until further evidence is forthcoming the
character of the peculiarly modified chromosomes in the spermatocyte of
the Hemiptera must remain in doubt.
Aside from definite retractions that Montgomery has made regarding his
earlier views on the character of the “chromatin nucleolus,” there are
noticeable different attitudes toward it in his earlier and later works.
Thus, in his lecture at Woods Holl (=13=_a_), we find the following:
“These remarkable ‘nucleolar’ structures which stain like chromatin have
been observed by numerous writers, but as yet no satisfactory
description has been given of their mode of origin. They have been
observed by me in spermatocytes of various insects, in hypodermal and
other cells of _Carpocopsa_, and in follicle cells of the testicles of
_Plethodon_ and _Mus_.” At this early stage of Montgomery’s
investigations it is apparent that he views his “chromatin nucleolus”
primarily as a nucleolus with chromatic origin and characters, but the
fact is equally apparent that he now regards it primarily as a
“chromosome” with nucleolar attributes. This is made evident in his
recent definition, which reads: “The chromatin nucleoli are
morphologically chromosomes, undergoing division in mitosis like the
other chromosomes, but differing from them in the rest stage by
preserving a definite (usually rounded) form.”
What has here been said regarding the “chromatin nucleolus” applies to
those structures in _Euchistus_ and other Hemiptera to which Montgomery
has given the name without qualification. According to his definition,
however, there is present in the cells of _Protenor_ and other species
another form, the “chromosome x.” Not only by inference is this
classification operative, but by direct statement we learn that
Montgomery regards this element as a member of the class of bodies which
he calls “chromatin nucleoli.” In speaking of _Protenor_ chromosomes, he
says: “This is the only case in the Hemiptera where one chromosome
becomes differentiated into a ‘chromatin nucleolus’ for the first time
in the spermatocyte generation.”
The noteworthy thing about this “chromosome x” is the fact that in every
essential detail it corresponds to the accessory chromosome of the
Orthoptera. It is a spermatogonial chromosome that comes over intact
into the spermatocyte; it retains its form and staining power unchanged
through the prophase of the spermatocyte; it divides in only one of the
spermatocyte mitoses; and is a large and conspicuous element of the cell
at all times.
This “chromosome x” agrees just as closely in its description to the
accessory chromosome as do the ordinary ones of the two orders, and, if
Montgomery’s account is correct, there would seem to be no reason for
doubting their identity. In two respects, however, there are differences
between these structures. First, it is to be noted that the “chromosome
x” divides in the first spermatocyte, while the accessory chromosome
undergoes separation in the second spermatocyte. Should Montgomery’s
observations prove correct, it would yet indicate no fundamental
difference in the character of the element, for the result is the same
whether division takes place in the first or second mitosis. In either
event, one-half the spermatozoa are provided with the odd chromosome
while the remaining half are not.
The second point of difference would seem to be a more serious one.
Montgomery states that during the spermatogonial mitosis the “chromosome
x” regularly divides as do all the other chromosomes, _i. e._,
longitudinally. In the spermatocyte mitosis, however, the element is
broken across, and the longitudinal split, which is apparent in the
early stages, disappears and is not utilized in division. We have here
the remarkable occurrence of a chromosome entirely unchanged in its
structure, but merely differing in its surroundings, which, instead of
dividing along the plane marked out for it, as it has done in all
preceding mitoses, breaks across after it is a formed element. An
occurrence of this kind, so different from the usual method of division,
would require strong proof to establish it, and this, in my opinion,
Montgomery has not brought forward.
A criticism of the degeneration theory as advocated by Paulmier and
Montgomery has already been given (=17=), so that it would not be
necessary to consider it here except in so far as it has been modified
since its promulgation. As a rule, Montgomery refers to his “chromatin
nucleoli” throughout his late paper (=15=) as degenerating chromosomes,
but in discussing their function specifically he makes important changes
in this conception. These are stated as follows: “When we find,
accordingly, the mutual apposition of them (true nucleoli) to chromatin
nucleoli, it would be permissible to conclude that the chromatin
nucleoli are chromosomes which are especially concerned with nucleolar
metabolism. And this, I think, would be the correct interpretation. The
chromatin nucleoli are in that sense degenerate that they no longer
behave like the other chromosomes in the rest stages, but they would be
specialized for a metabolic function; and from this point of view they
would certainly seem to be much more than degenerate organs.”
It is difficult to comment upon a contradictory statement like this;
but, fortunately, it is not necessary to do so, since it carries with it
its own refutation. The conception of a chromosome specialized in the
direction of increased metabolic activity as being in the process of
disappearing from the species can hardly be regarded seriously.
Taking everything into consideration, it may be said that Montgomery’s
work upon the Hemiptera has left the subject in a very disturbed
condition, and any prospect of a complete agreement between the
accessory chromosome of the Orthoptera and the “chromatin nucleolus” of
the Hemiptera is made more remote than was previously the case. This, I
think, is largely due to the inferior character of the Hemipteran
material, which has lead to misconception of phenomena that are clearly
marked in Orthopteran cells.
It is gratifying to note that the recent work of de Sinéty (=37=)
practically corroborates the conclusions herein set forth regarding the
history of the accessory chromosome. Aside from failure to observe the
important spireme condition of this element in the first spermatocyte
prophase, de Sinéty describes the same series of processes with scarcely
an exception. His summary contains the following account of the
accessory chromosome:
“Le ‘chromosome accessoire,’ découvert par McClung chez _Xiphidium
fasciatum_, se retrouve chez les locustiens que nous avons étudiés.
Chez _Orphania_, il se divise dans les spermatogonies en deux masses
volumineuses et allongés, que l’on reconnait dans les nucléoles,
également volumineux et allongés, des spermatocytes de premier ordre
en prophase. A la métaphase de la première cinèse, on le trouve situé
excentriquement et plus près de l’un des pôles; _il va tout entier a
l’une des cellules-filles_. Dans celle-ci, il se divise comme un
chromosome ordinaire, d’où il suit que _sur quatre spermatides formant
la descendance d’un spermatocyte, deux se trouvent privilegiees_. Par
ce partage inégal, non réalise dans _Xiphidium fasciatum_, d’après
McClung, le chromosome spécial d’Orphania rappelle celui des
hémiptères.”
A like series of processes is recognized in the Phasmids.
As is elsewhere explained in this paper, the occurrence of two divisions
of the accessory chromosome in _Xiphidium_, which was mentioned as a
possible occurrence in my preliminary paper, is shown not to take place.
While it is much more difficult to demonstrate the undivided condition
of the accessory chromosome in one of the spermatocyte mitoses of
_Xiphidium_ than it is in the cells of _Orchesticus_, _Anabrus_,
_Scudderia_, and _Microcentrum_, I am convinced that it does not differ
from the other Locustids in this respect.
We may therefore feel assured that our knowledge of the morphological
character of the accessory chromosome in the Orthoptera is fairly well
established. This gives us a good base from which to conduct further
comparative studies into other groups, and it is to be hoped that our
knowledge of this element will rapidly increase.
Unfortunately, de Sinéty has chosen to add another name to the already
overburdened list of synonyms, and “chromosome spécial” now takes its
place in the literature of insect spermatogenesis. The reason for adding
this name—
“Il reçu successivement leg noms de ‘accessory chromosome’ (McClung),
‘small chromosome’ (Paulmier), ‘chromatin nucleolus’ (?), ‘chromosome
_x_’ (Montgomery). Nous avons préféré éviter ces appellations, qui
semblent toutes supposer une signification qui n’a jamais été définie
ou s’appuyer sur des caractères plus ou moins secondaires, pour
adopter un nom indifférent, celui de ‘chromosome spécial,’ nous
conformant à l’idée de Wilson, pour qui c’est un ‘extra chromosome,’”
would seem to be at least insufficient, since “accessory chromosome” can
scarcely be regarded as implying any more primary or secondary function
than can “chromosome spécial.”
(_f_) _Individuality of the Chromosomes._
In each of my preceding papers I took the opportunity to point out the
fact that, even were the accessory chromosome of no other value, it
would certainly be worthy of study for the light it throws upon the
question of the individuality of the chromosomes. On this point
Montgomery has much to say in his late paper (=15=). I think it cannot
be questioned that we have here indisputable proof that at least one
chromosome may be identified through all the cell generations of the
testis. While this does not prove that chromosomes are persisting and
independent structures, it does evidence the fact that they may be, and
greatly strengthens the hypothesis that they are.
In addition to the evidence here offered by the accessory chromosome,
there must be noted that derived from a study of spermatocytes in which
there is always present one ordinary chromosome that greatly exceeds the
others in size. Such a condition is found in the cells of _Anabrus_. The
disproportion in size of the elements is here so striking that it would
be impossible to fail in distinguishing the giant chromosome. In each of
the spermatocytes of _Anabrus_ there are therefore two chromosomes which
are plainly recognizable. It may be observed further that the remaining
chromosomes are quite different in size, and it may be possible within
reasonable limits of certainty to pick out one or more other chromosomes
in each cell. Unless this could be done for each element, however, it
would not definitely prove that all the chromosomes are distinct and
recognizable structures. The actual recognition of two elements in each
cell of the same generation and its ancestors or descendants in other
generations goes far, however, to render probable the individuality of
each chromosome.
Beyond this point studies upon the Orthopteran cells will not permit me
to go; but Montgomery has been fortunate enough to find in _Peripatus_
an object in which he considers it possible to demonstrate the
continuity of the chromosomes from one generation to another, and their
fusion by pairs in the early history of the spermatocyte to bring about
the reduced number. This is, in the main, a logical conclusion to my own
work, and I am therefore bound to regard his results as probably
correct. While doing this, however, I recognize that the absolute proof
he brings forward in support of his hypothesis is very slight. I
consider any deductions based upon observations of linin structures as
very insecure, and it is upon these that Montgomery principally relies
to demonstrate his theory. Further observations upon the behavior of the
chromosomes between the spermatogonia and the spermatocytes in objects
favorable for study will be awaited with interest. In the meantime it
must be conceded that the work upon insect spermatogenesis has at least
lent strong support to the theory of the individuality of the
chromosomes in general and has definitely shown that there is such a
thing in some instances.
(_g_) _Nucleoli._
Considerable importance is attached by some investigators to the nuclear
structures, properly called plasmasomes, that occur in the
spermatocytes. It is probable that there are marked differences between
the cells of various species in regard to the occurrence of these
bodies, for in the Orthoptera they either do not appear at all, or, if
present, they are minute and inconspicuous. This fact would tend to
disprove any theory which would attach a fundamental importance to these
structures, such as is conceived for the chromatin. The Orthopteran
cells do not allow any observations which would add to our positive
knowledge of the nucleoli, and I include this brief statement merely for
the negative value it may possess.
(_h_) _Rest Stage._
In his first paper upon _Euchistus_, Montgomery assigns an important and
conspicuous place to the “rest stage” among his numerous subphases
preceding the first spermatocyte mitosis. As a result of his later
comparative work upon the Hemiptera, however, we learn that in certain
families no trace of such a condition of diffusion on the part of the
chromatin is observable, from which we conclude that “accordingly such a
stage would appear to have no broad significance.” It has already been
announced that nothing like a rest stage intervenes between the
spermatogonia and spermatocytes of the Orthoptera, and the work of most
investigators would tend to indicate that it is the exception rather
than the rule. In those cases where such a condition of the nucleus
exists, it would seem to be true that nothing more unusual than an
excessive diffusion of the spermatogonial chromosomes occurs, and this
is of hardly sufficient importance to receive a special designation.
The existence of a rest stage between the first and second spermatocytes
is also negatived by the conditions found in the Orthopteran cells. The
formation of chromosomes in the prophase of the first spermatocyte that
are already prepared for two divisions would _a priori_ render
improbable the intervention of a rest stage here; and the actual
observed persistence of the chromosomes, as such, through the telophase
of the first spermatocyte and through the modified prophase of the
second spermatocyte gives actual proof in support of the view that
commonly prevails regarding the suppression of the second spermatocyte
rest stage.
Observations upon numerous species tend to show that the behavior of the
chromatin during the period between the two spermatocyte mitoses varies
considerably with the species and even within the species itself. The
amount of diffusion would, in some measure, seem to be related to the
form of the chromosomes and to vary correspondingly in those individuals
where the chromosomes are of diverse forms. Thus, where the elements of
the second spermatocyte metaphase appear as short double rods, the
amount of diffusion is slight, and the individual chromosomes may be
distinguished throughout the telophase of the first spermatocyte; but in
those cases where the members of the mitotic figure are much elongated
the diffusion is more extensive and the distinction between elements is
made difficult or impossible. Since these two conditions may prevail in
the same testis, it is probably only a question as to the extent of
elongation on the part of each chromosome. In those cases where the
elements become very much extended the appearance of the resting
condition would be simulated closely, while, on the contrary,
chromosomes consisting of spherical or short cylindrical chromatids
would never give a suggestion of such a stage. In this we may find, I
think, an explanation for those cases in which a rest stage is described
as occurring between the spermatocyte generations.