Paper - Studies in reptilian spermatogenesis 1 (1921)
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Painter TS. Studies in reptilian spermatogenesis. I. The spermatogenesis of lizards. (1921) J Exp. Zool. 34: 281-327.
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Studies In Reptilian Spermatogenesis
I. The Speematogenesis of Lizards
Theophilus S. Painter
Six Text Figures And Four Plates (Forty-Eight Figures)
- Contribution no. 149 from the Department of Zoology, University of Texas, Austin, Texas.
Cytologists are well agreed that our knowledge of the chromosomes, especially of the sex-chromosomes, of vertebrate animals is in a very conflicting and unsatisfactory state. A critical review of the literature will show that, while the spermatogenesis of most of the common vertebrates has been worked upon, with a view of determining if sex-chromosomes were present, much of the work has been fragmentary in character, and various criteria have been used for identifying the sex-chromosomes (such as nucleoli in spireme stages, dimorphism of sperm, or the early movement of one chromosome to one pole in a maturation division). Too frequently conclusions have been based upon a few dividing first spermatocyte cells, without checking up such observations on cells of the following division or without a study of the female chromosome complex. In only a few vertebrates is the somatic or diploid number of chromosomes definitely known. Even in forms where the spermatogenesis has been most carefully and completely worked out, as in the pig or the opossum, there is a wide divergence of opinion as to the conditions existing. Thus Wodsedalek ('13) gives 18 as the diploid number of chromosomes for the male pig, while Hance ('17), using an improved technique, shows that there are 40 chromosomes both in the germinal and the somatic cells of this animal. Jordan ('12) gives 9 as the haploid number of chromosomes for the male opossum, while Hartman ('19), working upon ova, found 12 to be the reduced chromosome number of the female. These two examples are typical of the confusion found at present in this field.
The difficulties which have confronted workers in vertebrate spermatogenesis are well known. The vertebrate germ cells have been, in the past, the most difficult of all tissues to fix properly for chromosome studies. The number of chromosomes in every case so far reported on is relatively large, and there is a very pronounced tendency for the chromosomes to fuse together in the spindles so that accurate counting usually has been impossible. Recently, however, through the investigations of Allen ('16, '19) and Hance ('17), some real advances in technique have been made, so that, with care, much of the difficulty experienced by earlier wTiters may be overcome. With the reinvestigation of the common vertebrates with these new methods, we may confidently expect that the present confusion will be cleared up to a large extent.
The reptiles are the only large class of vertebrates that have not been extensively studied by cytologists with a view of determining the condition of the sex-chromosomes. With the exception of a brief note by Jordan ('14) describing the results of his work on the spermatogenesis of the turtles, Chrysemys marginata and Cistudo Carolina,- the reptiles present a virgin field for the investigator.
The following paper presents the results of an extensive study of the spermatogenesis of a number of lizards made during the past two breeding seasons, and, in addition, a study of dividing embryonic cells, made in order to check up certain conclusions regarding the sex-chromosomes.^ In making this study the author had two main objects in view: First, as we knew nothing about the sex-chromosomes of lizards, it seemed desirable to fill out this chapter of our knowledge. The second consideration was to determine what light, if any, a study of reptihan spermatogenesis would throw on the peculiar conditions found in the spermatogenesis of birds, as reported by Guyer in various works, since the reptiles and birds are closely related phylogenetically.
- 2 A complete account of this work has never been published. Professor Jordan, however, very kindly sent me his manuscript together with the figures, and from time to time I shall refer to the conditions found by him in the turtle. " ^ The main facts concerning the spermatogenesis of Anolis carolinensis were presented before the American Society of Zoologists, December 28, 1919. See Anat. Rec, vol. 17, for abstract of paper.
As the following study will show, certain species of lizards have been found in which the peculiar form of the chromosomes, as well as their small numbers, has allowed an accurate study of the spermatogenesis to be made; in fact, it has been found possible to follow most of the chromosomes from the last spermatogonial division through maturation into the immature spermatids. It must be confessed, however, that no facts have been discovered which throw any light on the unusual conditions reported for birds by Guyer ('16). The fact is, that during maturation the chromosomes of lizards behave in quite an orthodox manner, and differ in no essential respect from the behavior of the chromosomes of such a classic insect as Lygaeus during the corresponding period.
Materials and Methods
Lizards embracing two separate families (Iguanidae and Teiidae) were studied. In the case of the Iguanidae, six species including five different genera are reported on. The family Teiidae is represented by only one species, in the vicinity of Austin, Cnemidophorus gularis.
In one respect the lizards offer very favorable vertebrate material for cytological studies. They have a very definite and short breeding season and in the males of many species the approach of this period is heralded by the appearance of the bright colors (blues and greens usually) which mark this sex during the mating season. This fact makes it easy to obtain and preserve the testes when division stages are most numerous.^
Within a radius of ten miles of Austin, one finds forms characteristic of the high-plateau areas as well as those forms which frequent the valleys and moist areas. Capturing members of some of the species alive is no easy task, as they run with what appears to be lightning-like speed. The author is indebted to two students of the Zoological Department, Mr. Kenneth Cuyler and Mr. Deluz.
While a number of fixing fluids were tried, the author has obtained his best fixation by using cold Fleniniing's solution with urea, after the method of Hance, and Allen's modification of Bouin's fluid. The methods of hardening and dehydration as described by Hance ('17) and by Allen ('19) were followed. Of these two methods the modified Bouin's fluid on the whole gave the better results. However, my experience has been that, for somatic or spermatogonial divisions, the cold Flemming method gives a better separation of the chromosomes.
In all cases the spinal cord of the lizard was cut just behind the skull. The abdomen was then opened, the testes removed, split open, and immediately put into the preser\dng fluid, where the tubules were teased apart. This insured rapid and complete fixation.
The lacertehan testis is made up of convoluted tubules, much the same as have been found in all higher vertebrates. In general the various cell generations may be found as follows : spermatogonia lie on the periphery of the tubule, just within this outer circle one finds the first spermatocj^tes in their various stages, and the second spermatoc^^tes and spermatids surround the lumen of the tubule. Exceptions to this arrangement are numerous, especially in the testes of mature males, but one has no difficulty in distinguishing cells of the various generations, since the size of the cells and the shape of the chromosomes are markedly different in these several stages. Supporting cells are commonly seen in the walls of the tubules, but I have failed to find anything which could be identified as true Sertoh cells.
In all the lizards studied, and this is especiallj^ true of the Iguanidae, the chromosome complex is made up of two sets of bodies strikingly different in size. In figure 1 a di\'iding spermatogonial cell is shown. It will be noted that there is an outer circle of large V-shaped chromosomes, while within the center of the spindle is filled with a number of dot-like chromosomes.
Hamblett, for catching many of the males upon which this study was made. Through their knowledge of the habits of the commoner species, together with their agility, they were able to run down and catch some forms which otherwise I should not have been able to obtain. Due acknowledgment is made to these gentlemen in this place.
The same general arrangement is retained through the maturation divisions and is typical for all Iguanidae studied. For convenience in the description and the discussion, the term 'macrochromosome' has been adopted to designate the large V-shaped bodies, and the term 'micro-chromosome' has been applied to the small dot-like bodies. It should be emphasized, however, that the use of these terms is for convenience in descriptions, and that no physiological or functional difference between the macro- and micro-chromosomes is implied. Indeed, in one species studied, there is really no very sharp line to be drawn between the two sets of bodies, and in every case both types be? have in the same way, except for the sex-chromosomes.
The Iguanidae proved to be the most favorable family for study, and among the species examined Anolis carolinensis and Sceloporus spinosus gave the best preparations.
The local fauna around Austin is rich in genera and species for several families of lizards. ]Material has been preserved and studied for the following species, common local names are indicated :
Anolis carolinensis, Mmerican chameleon'
Holbrookia texana, 'Zebra-tail Uzard'
Crotaphytus collaris, 'Alountain boomer'
Uta ornata, 'Rock Hzard'
Sceloporus spinosus, 'Tree lizard'
Sceloporus undulatus consobrinus. Rare Cneminophorus gularis, 'Race runner'
Phrj^nosoma cornutum, 'Horned toad'
Gerrhonotus liocephalus, * Rare
In the following pages the spermatogenesis of the first seven species will be given. IMuch tune has been spent preserving and studying the testes of the 'horned toad,' but no first-class preparations were obtained. The same may be said also of Gerrhonotus, but only one male of this species was examined, and he was too mature to be favorable for study.
Spermatogenesis of Anolis carolinesis
Sperviatogonial divisions. All of my material was from fully mature testes, so that probably all dividing spermatogonial cells observed would have formed primary spermatocytes. Seen from the equatorial plate view dividing spermatogonia (figs. 1 to' 3) show an outer circle of large V-shaped chromosomes (macro-chromosomes) surrounding the dot-like micro-chromosomes which lie scattered about the center of the spindle.
The macro-chromosomes lie well apart, with very little overlapping and no fusion of the elements. The V's show, typically, no trace of an achromatic bridge between the two arms, although, in figure 1, the 'b' chromosomes seem to indicate this. Repeated counts of spermatogonial division stages, such as are shown in figure 1 and 3, indicate that there are twelve macro-chromosomes in every spindle. (Careful drawings of over thirty cells were made, and each gave this nmiiber; in addition, innumerable other counts have been made, always with the same result.) While the shape of the macro-chromosomes makes it somewhat difficult to pair up synaptic mates, one can usually distinguish three pairs, on the basis of size. Two chromosomes, labeled 'a' (figs. 1 and 2) , are larger than the rest (best seen in fig. 2) . The other macro-chromosomes, labeled 'b,' are decidedly smaller than the rest, while a third pair, labeled 'c,' are slightly larger than 'b,' but smaller than the remaining six chromosomes, which are much the same size and shape.
The micro-chromosomes are small dot-like or, in some views, very short rod-like elements, which lie well separated in the center of spindles. There is some variation in the size of various dots (fig. 1) and, in a general way, one can mate up these chromosomes. INIaking accurate counts of the nmnber of micro-chromosomes is made difficult because one or more frequently lie close to a macro-chromosome, and there is always a chance that one or two elements will be hidden and overlooked. The full number seems to be 22 (fig. 1), although in figures 2 and 3 we find 21 and 20, respectively. Occasionally these small bodies are connected up, more or less, by strands of lightly staining material (fig. 3), which increases the difficulty of counting.
The spermatogonial division is completed in the usual way, both the macro- and^^icro-chromosomes dividing. There is no lagging behind of any elements, so far as I have observed.
First jnaturation division. No especial effort has been made to follow in detail the changes which the chromosomes undergo from the telophase of the spermatogonial division up to the prophase of the first maturation division, although the material is such that this probably could be done with comparative ease. Only a general outline of the sequence of stages is given here.
Following the last spermatogonial division, the young spermatocyte enters the diffuse stage (fig. 4), at which time the nucleus is characterized by the presence of two large deeply staining nucleoli and scattered chromatin knots lying on the linin net-work. No plasmosomes have been found at this time. Apparently the formation of the leptotene threads comes about, first, by an increase in the size of the chromatin knots and a decrease in the size of the nucleoli, and following this, the chromatin knots expand into the filament-like leptotene threads. The nucleoli still persist as deeply staining points in the general mass of chromatin threads, although they are small. No attempt was made to follow the course of events during synapsis, but it has been noted that no contraction stages occur in Anolis or any of the other lizards studied, nor is there any marked polarization of the nucleus following. The diplotene nucleus (fig, 5) is characterized by the thick spireme threads and the presence of two deeply staining elements (marked A'^), which appear to be more or less elongated. As the diplotene threads contract to form the tetrads of the first maturation division, these deeply staining elements form a conspicuous bipartite body, which has a compact form and smooth outline, while the rest of the chromatin elements are still much elongated. Figure 6 shows the condition of such a cell.
During this period just described, no trace of the small microchromosomes has been found. They apparently form spireme threads like the macro-chromosomes, and, indeed, in some cells one finds short strands of chromatin (fig. 5, just above the deeply staining X-elements) which may represent microchromosomes.
The tetrads, which condense into the prophase chromosomes, are for the most part ring-shaped, at least this is true for three of them. These rings subsequently divide so that a V-shaped element goes to each pole, but I have not determined which plane the line of splitting cuts through. Among the rings the conspicuous bipartite X-body can readily be seen.
When the first maturation division spindle is formed, one sees in equatorial plate view (figs. 7 and 8) six large macro-chromosomes and eleven micro-chromosomes. Among the macrochromosomes the small 'b' chromosome can always be identified, the ' c' chromosome is frequently seen, but it not so easy to mark the 'a' chromosome in polar view. The presence of six macrochromosomes in the first maturation division indicates that the twelve spermatogonial chromosomes have united, and the presence of eleven micro-chromosomes (there are twenty-two in the spermatogonia) indicates that the same thing holds true for these smaller bodies.
Side views of the first maturation division show several points of considerable interest (figs. 9 to 14). First of all, one can nearly always identify four chromosomes in such views. The large 'a,' the small 'b,' the intermediate 'c' chromosomes show up in all such views, and in addition a bipartite chromosome is observed (marked X) which has spindle fiber attachments at one pole only of the cell. The micro-chromosomes (figs. 10 to 13) show up as small dumb-bell-shaped bodies, which are doubtless small tetrads.
As the first maturation division goes forward, one finds here and there cells in which the bipartite chromosome (marked A') is passing undivided to one pole. Figures 9 to 14 show six such ceUs. A close study of the chromosomes has shown that it is the same chromosome which shows this movement in the different cells, and, further, that this bipartite element is not half of one of the tetrads which has divided early, the other half remaining in the spindle. This bipartite element is beheved to be the same that has been seen in the earher spireme stages, and so it has been marked X in all cases.
In this place it should be emphasized that this early migration of the X-chromosome is not seen in all side views of the spindles. One may examine several dozen cells without finding the X-element lying far out of the division plane, although, after a little experience, it can usually be recognized. But again one may find a patch of dividing cells in which the X-element will show up conspicuously, as in figures 9 to 14. This probably means either that the X-element goes to one pole a little in advance of the other chromosomes or that typically it moves at the same time the other elements divide, but occasionally may go early to one pole. In either case the infrequency of the phenomenon would be explained.
Aside from the fact that the X-element does not divide, the first maturation division proceeds normally. Late anaphases show five macro-chromosomes at one pole and six at the other (figs. 15 to 17), the extra chromosome being the bipartite Xchromosome.5 Thus the study of anaphase stages bears out the observations that the bipartite element passes to one pole of the cell undivided. Hence half of the second spermatocytes will carry, and half will lack the X-element.
Second maturation division. Following the first spermotocyte division, the chromosomes of the young second spermatocyte enter into a spireme condition. No conspicuous nucleoli have been observed in such resting nuclei. When the chromosomes of the second division condense out of this resting nucleus, they are generally already split in the plane in which they are destined to divide. This precocious splitting (fig. 18) makes it somewhat difficult to make accurate counts in the equatorial plate view, because one does not know whether a given chromatin mass is one chromosome already split or two chromosomes lying side by side. Figures 19 to 21 show three cells in which there are five macro-chromosomes. Figures 19 and 20 are equatorial plate views, while figure 21 is an anaphase stage. Figures 22 to 24 show three cells in which six macro-chromosome elements are present. In equatorial view (figs. 22 and 23) the additional X-element appears either as quadripartite (fig. 22) or as a large bivalent chromosome. An anaphase stage (fig. 24) shows six macro-chromosomes at each pole of the cell. This would indicate that the X-element divides equationally at this time.
- In figures 17 and 21 the cell was viewed from one pole, so that in order to show all the chromosomes, one pole was drawn, and then the cell shifted with the mechanical stage until the other pole was clear of the first.
It will be noted that, in all the equatorial views of several division stages, a number of micro-chromosomes are seen. In figure 19 there are clearly ten elements and a blurred mass, which is part of the faintly staining network found in all spindles. In figure 20 there are eleven micro-chromosomes, one of which is larger than the rest. In figure 21 it is clear that eleven micro-chromosomes are going to each spermatid. Figure 23 again shows eleven micro-chromosomes.
From the foregoing description it will be seen that, in Anolis carolinensis, we have a bipartite chromosome which goes undivided to one pole of the cell in the first maturation division. Its behavior is typical of a so-called sex-chromosome or X-chromosome, and has been so regarded by the author. Later in this paper further evidence for this conclusion will be presented.
It is interesting to note, further, that the chromosomes of Anolis behave in a way typical for insects and other invertebrates. The tetrads are formed and divide in the typical invertebrate manner, and the second spermatocyte division does not show any fusion, or so-called 'double reduction,' such as has been described for birds and some mammals.
The transformation of the spermatids into mature sperm has been followed out, but no description of it will be given here. Giant sperm are occasionally found in Anolis and so-called 'syncytial masses' are common. These will be treated in a later section of this paper.
Spermatogenesis of Sceloporus spinosus
This common tree lizard' has proved to be a very satisfactoryform upon which to work, because the germ cells are large and preserve well with either cold Flemming or modified Bouin; there are relatively few micro-chromosomes to be found in the spindles and they are large, and the X-chromosome element is seen passing undivided to one pole in the first maturation division more often than in any other lizard studied. As a glance at the figures of plate 3 will show, the behavior of the chromosomes in Sceloporus spinosus during maturation is closely similar to what is found in Anolis carolinensis.
Spermatog^onia. Dividing spermatogonia (figs. 25 to 27) show a condition strikingly similar to that of Anolis. In equatorial plate view there are twelve macro-chromosomes, forming a general circle, and ten micro-chromosomes lying scattered over the center of the spindle.
A close study of the macro-chromosomes shows one pair longer than the rest, labeled 'a,' a small pair, labeled 'b,' markedly smaller than the other V's, and in figure 25 a third pair, 'c,' is seen, somewhat larger than 'b.' (Fig. 25 is a cell preserved in cold Flemming, while figs. 26 and 27 are taken from cells preserved in Allen's modification of Bouin.) The six remaining chromosomes are all about the same size. A comparison of figures 25 and 26 with figures 1 to 3, taken from Anolis, shows that, as regards macro-chromosomes, the two forms are similar in number and general size relations. The 'b' chromosome pair of S. spinosus is relatively much smaller than the corresponding 'b' chromosomes of Anolis.
The micro-chromosomes of S. spinosus are ten in number and are comparatively large in size. In addition to the rounded balls one also finds somewhat elongated blunt rods, as shown in the figures.
First maturation division. No detailed study was made of the various phenomena exhibited by the nuclear elements up to the prophase of the first division. A casual examination of the preparations showed that S. spinosus differs in no marked respect from Anolis.
In equatorial plate views of the first maturation spindle there are six macro-chromosomes and, typically, five micro-chromosomes. This shows that there has been a pairing of "all the spermatogonial elements. Considering first the macro-chromosomes, we have little difficulty in identifying the 'a,' 'b,' and 'c' chromosomes, and in addition a bipartite chromosome labeled X. Generally, all the macro-chromosomes, except the X and the 'b' chromosomes, show their tetrad character (figs. 28 and 30). The micro-chromosomes are much larger than similar bodies in Anolis; in fact, in S. spinosus there is less difference in size between the largest micro-chromosome and the 'b' chromosome than between the 'b' element and any of the macrochromosomes, excepting 'c' Frequently one of the microchromosomes lies so that both halves of it may be seen (figs. 28 and 30).
In side views of the first maturation spindle one sees in perhaps a third of all the cases a bipartite chromosome (marked X) passing undivided to one pole of the cell. The size and shape of this X-element varies somewhat in different cells, but its bipartite character is strongly marked in every case (figs. 31 to 33). In other respects the first division goes forward normally. In figure 33 the 'b' chromosome and several of the micro-chromosomes have divided.
On the basis of the first division, one should expect that the secondary spermatocytes would be of two kinds, half with, and half without the X-chromosome.
Second maturation division. The precocious splitting of the chromosomes as they come out of the resting nuclear stage makes a study of second spermatocyte divisions difficult. In favorable cells, however, one may clearly make out either five or six macrochromosomes. Figure 34 shows a cell with five, and figures 35 and 36 show cells with six large chromosomes. The X-element is marked in figure 36.
Spermatogenesis of Sceloporus undulatus var. consohrinus
The spermatogenesis of this species was carried out to the extent of determining the chromosome complex and the behavior of the sex-chromosome. This small lizard is rather rare around Austin. My material was taken from two males.
Text figure 1 Sceloporus undulatus var. consobrinus. A. Spermatogonia] plate. B and D. Equatorial plates of the first maturation division. C and E. Late anaphase stages of the first division.
In text figure 1, A to E, the essential features of the spermatogenesis are shown. Dividing spermatogonia (A) show the typical outer circle of twelve macro-chromosomes, and within this the micro-chromosomes which number about eighteen. A careful examination of the macro-chromosomes shows that we have here essentially the size relations found in Anolis and Sceloporus spinosus. The 'a,' 'b,' and 'c,' chromosomes are labeled. (In text fig. 1, A the size difference between 'b' and 'c' is not well shown. It is more striking in the next division.)
The first spermatocyte cells show (text fig. 1, B and D) six large macro-chromosomes and from eight to eleven microchromosomes. Here, again, we can easily identify the 'b' and 'c' chromosomes, but the ' a' element is not so easily distinguished. A bipartite chromosome is also seen in such a view. No side views of the spindles are shown, but the two late anaphase stages represented in text figure 1, C and E, show what takes place in this division ; the lower pole of each cell shows five macrochromosomes, while the upper pole shows six large bodies. The extra element (marked X) appears sometimes as quadripartite (text fig. 1, C), sometimes as a bivalent chromosome (text fig. 1, E).
No study of dividing second spermatocytes was made. Judging, however, from the anaphase stages of the first division, half of the secondary spermatoctes should carry five and half should carry six chromosomes, the extra element being the X-chromosomes. In this respect, S. undulatus consobrinus is like Anolis and S. spinosus.
Spermatogenesis of Holbrookia iexana
This 'zebra-tair lizard has not proved a very satisfactory form for study, because the testes, in my experience, are difficult to preserve well and the sex-chromosome rarely shows up in side views of the first maturation spindle; that is, it rarely passes to one pole of the cell before the autosomes divide. Spermatogonial division stages are rare in my preparations, but favorable plates (fig. 37) show an outer circle of V-shaped macro-chromosomes, while within the micro-chromosomes are found. A close inspection of figure 37 will show the presence of three pairs of macro-chromosomes which are distinctive because of their size. These are labeled 'a,' 'b,' and 'c,' just as was done for the other lizards studied. The micro-chromosomes do not show up well in figure 37.
In equatorial plate view the first maturation spindles show six macro-chromosomes and about twelve or thirteen microchromosomes (figs. 38 and 39). There is a marked tendency for the tetrads to separate in the plane in which they will be divided later. One of the macro-chromosomes appears as bivalent.
Side views of the first maturation division very rarely show the condition reproduced in figure 40. The presence of a bipartite chromosome at one pole strongly suggests a condition similar to that found in the other lizards studied^ but I am unable to state definitely whether or not we see the true X-element in figure 40. Late anaphase stages show the conditions reproduced in figures 41 to 43. In figure 41 we see a pair of chromosomes projecting out from the upper pole of the cell. Figures 42 and 43 show six large chromosomes at the upper pole and five chromosomes at the lower pole, the additional element at the upper pole being marked X. (Fig. 43 is taken from poorly preserved material.) This condition would indicate that an X-chromosome is present in Holbrookia and that it goes undivided to one pole in the first maturation division just as was found in the other lizards described.
The evidence from the second spermatocyte division bears out the conclusions drawn from the first division. Figure 44 shows a side view of a spindle, in which one sees the precocious splitting of the chromosomes. Figure 45 is an equatorial plate view showing five maero-chromosomes, and figure 47 is a late anaphase showing five macro-chromosomes going to each spermatid. (In figs. 47 and 48 the cells were viewed from -one end, so in order to show both poles the cells were shifted with the mechanical stage after one pole was drawn.) Figure 46 shows a cell with six macro-chromosomes, and in figure 48 we have a late anaphase in which six macro-chromosomes are seen distinctly going into each spermatid.
From this study it is clear that the spermatogenesis of Holbrookia texana differs in no essential respect from what was found for Anolis and Sceloporus.
Spermatogenesis of Uta ornata
The study of the spermatogenesis of this form has been carried out sufficiently to give the chromosome complex and the behavior of the sex-chromosome.
Spermatogonial divisions (text fig. 2, A and B) show the presence of twelve macro-chromosomes and from fifteen to eighteen micro-chromosomes. Among the macro-chromosomes we distinguish a large pair, labeled 'a/ a small pair, 'b,' and the 'c' pair as found in the other lizards of the family Iguanidae (text fig. 2, A). The 'b' chromosomes are relativel}' large in this species.
Text figure 2 Uta ornata. A and B. Dividing spermatogonia. C. Equatorial plate view of a dividing first spermatocyte. D and E. Equatorial plate view of dividing second spermatocytes.
The first maturation spindles show six macro-chromosomes and about eight or nine micro-chromosomes (text fig. 2, C). In side views of the spindles I have not observed the early migration of an X-element to one pole. However, the secondary spermatocytes contain five (text fig. 2, D) and six (text fig. 2, E) macrochromosomes, the extra element being large and represents, no doubt, the X-chromosome. Hence it is very probable that a sex-chromosome is present in Uta ornata and that it behaves like this element does in Anolis.
Spermatogenesis of Crotaphytus collaris
The study of this Uzard was undertaken to determine the chromosome complex, and no attempt was made to trace out the history of the chromosome elements during maturation.
There are twelve macro-chromosomes (text fig. 3, A) in the spermatogonial divisions and a number of micro-chromosomes. Among the macro-chromosomes one recognizes the 'a/ 'b/ and 'c' elements described for other Iguanidae.
Text figure 3 Crotaphytus collaris. A. Dividing spermatogoni C. Equatorial plate of the first maturation division.
The first spermatocytes contain six large macro-chromosomes and around twelve micro-chromosomes (fig. 3, B and C).
The local representative of this family is the common 'racerunner,' Cnemidophorus gularis, which is perhaps the conmionest of all the species of lizards found near Austin. The germ cells of this form, however, have proved very difficult to fix properly, and although cold Flemming and modified Bouin's fluid have been used repeatedly, I have never obtained first-class preparations which w^ould allow me to follow the complete history of the chromosomes through maturation. Since, however, Cnemido phorus represents an entirely different family from the lizards so far described, a fragmentary account of the spermatogenesis will be included here.
Spermatogonial divisions have been studied, but the number of chromosomes involved and the poor fixation made it impossible to determine even the approximate number of these elements.
The early history of the first spermatocytes was followed in some detail, and was found to differ in no way from what had been observed in Anolis carolinensis. Two large nucleoli are seen in the 'diffuse' nucleus; these diminish in size as the leptotene threads appear. In early prophase stages the chromosomes generally appear as rounded masses, and one cannot see the formation of tetrads, as could be done in the family Iguanidae. This is due to the faulty fixation of my material.
In the primary spermatocyte division stages the chromosomes appear as rounded masses of varying size (text fig. 4, A and B) about twenty in number. Of these, thirteen are large, while seven are much smaller. It will be seen from the figures, however, that the large and small chromosomes intergrade in size and we do not have sharp division into macro- and microchromosomes such as was found in the family Iguanidae.
In side views of the first spermatocyte spindles one occasionally finds cells such as are shown in text figure 4, C and D. A bipartite body lies to one side of the spindle, well advanced (text fig. 4, C) toward one pole. This element is very suggestive of the X-element found among the other lizards studied, but I was unable to make counts of anaphase stages, so that this point could not be verified.
In the second spermatocyte division (text fig. 4, E and F) the chromosomes have fused together, so that accurate counting is out of the question. This fusion of elements is, perhaps, similar to the 'double reduction' reported in the second spermatocytes of birds and of some mammals. Cnemidophorus is the only lizard, out of seven species studied, in which I obtained such fusion. The chromatin masses vary in number from five up to ten or more, the variation being due to more or less general fusion of the nineteen or more chromosomes which were handed on from the first spermatocyte. This fusion is the result of poor fixation, which made the spermatogonia! division unworkable.
Text figure 4 Cnimedophorus gularis. A and B. Equatorial plate views of dividing first spermatocytes. C and D. Side views of the first maturation spindle. E and F. Views of the second spermatocyte cells showing fusion of chromosomes.
The Female Chromosome Complex
A study was made of the ovarian tissue of both Anolis carolinensis and Sceloporus spinosus, with a view of determining the chromosome complex of the females of these two species. Very small young females were used, the ovaries being preserved and sectioned. As was to be expected, no oogonial divisions were found, but scattered here and there among the nurse cells were dividing cells. A great number of these were carefully studied, but most of the equatorial plates were too crowded to allow more than approximate counts. Here and there, especially in cells which had been cut in two by the sectioning razor, the chromosomes lay fairly well apart, so that counts could be made. In such cases I found typically fourteen macro-chromosomes, though the counts varied between thirteen and fifteen. In only two or three cases did I find dividing cells in which I was confident that there was no error in my count, and these gave fourteen macro-chromosomes as the female complex.
The evidence for the conclusion that in females of both Anolis and Sceloporus there were fourteen macro-chromosomes was so meager that a study of the dividing somatic cells of embryos was undertaken in order to get further light upon this point.
At the outset the author was aware that chromosome counts made on dividing somatic cells may not be accepted unhesitatingly as proof of the number of chromosomes which will be found in the germ cells of the individual. A nmnber of investigators have shown both for vertebrates and invertebrates^ that the number of somatic chromosomes may vary from that which one would expect from a study of the germ cells. More recent work has indicated, however, that this variation in chromosome number is either due to a fragmentation of one of more of the chromosomes (Hance, '17) or that we have multiples of the haploid number (Holt, '17) due to a longitudinal splitting of the chromosomes. Hance's work is of especial interest, in connection with lizard embryos, since from his study of dividing somatic cells of the pig he was able to show that variations from the diploid number (forty) were due to the fragmentation or breaking of some of the normal chromosomes, and not due to the addition of new chromatin elements.
In Sceloporus spinosus, the form upon which the following work was carried out, the large macro-chromosomes have fairly characteristic shapes (V's of various sorts) and there are relatively few of them. On the basis of the study of spermatogenesis, it was clear that the X-element found in the first maturation division was derived from two medium-sized spermatogonial macro-chromosomes. It was believed that two extra V's could be detected, if they were present in the females.
An excellent review of the literature dealing with this point is given by Hoy (-16).
(If the conclusions drawn from a study of spermatogenesis are correct, then the female of this species should have fourteen macro-chromosomes, that is, two extra chromosomes, which are equal to an additional X-chromosome. See p. 307 for further discussion of this point.)
Before examining the somatic divisions of embryos, the chro mosome complex of the male germ cells should be carefully examined. In the adjacent text figure 5, A and B (also plate 3), we have drawings of two spermatogonial divisions. It will be noted that there are ten large conspicuous chromosomes which have a V shape, two small V's labeled 'b,' and ten small elements, the micro-chromosomes. It is not always easy to distinguish the small 'b' chromosomes from the largest microchromosomes, because occasionally, and this is especially true of somatic divisions, the larger micro-chromosomes also have a V shape. The ten large macro-chromosomes, however, are conspicuous and easily identified, and since the X-element comes from a pair of these, we may disregard the 'b' chromosomes and the micro-chromosomes in the following description. If the X-element has been correctly identified in the spermatogenesis, then the female should show twelve large V-chromosomes and the males ten.
The four embryos, from which the cells shown in text figure 5, C to H, were taken, were all obtained from one female, preserved in the same dish of cold Flemming, carried through dehydration in the same vial, embedded in the same paraffin block side by side, and stained on the same slide. In studying embryos, one individual at a time was followed, camera drawings made of the chromosome elements, and afterwards these were compared and checked up. The number of large V-shaped chromosomes was constant for a given embryo. Occasionally one of the Velements was apparently broken in two, but as the two halves were adjacent, it caused no confusion (text fig. 5, H). I did find, however, some variation in the shape, size, and number of the micro-chromosomes in the same individual. In one cell, for example, the normal number (ten) of micro-chromosomes would be seen, while in an adjoining section a cell would be found where, apparently, several of these micro-chromosomes had joined to form one long thin rod. In no case was it difficult to distinguish between such thin rods and the V-shaped macroehromosomes.
Text figure 5 Sceloporus spinosus. A and B. Spermatogonia! chromosomes. C and E. Somatic chromosomes of first embryo. D. Somatic chromosomes of the second embryo. F and G. Somatic chromosomes of a third embryo. H. Somatic chromosomes of a fourth embryo.
Figures C and E show the characteristic condition of the somatic chromosomes for one embryo. In text figure 5, C, ten large V-shaped macro-chromosomes are seen, together with eleven smaller bodies. Of the latter one is a 'b' chromosome, its mate is not identified. In text figure 5, E, there are also ten large chromosomes, two 'b' chromosomes, and ten or eleven micro-chromosomes (depending on how we interpret the small bipartite bodies).
The somatic chromosomes for a second embryo are shown in text figure 5, D. Here there are ten macro-chromosomes, a pair of 'b' chromosomes, which show with unusual clearness, and ten or eleven micro-chromosomes.
The chromosome complex of a third embryo is shown in text figure 5, F and G. Here one finds at least twelve large V-shaped chromosomes. In the cell shown in text figure 5, F, the 'b' chromosomes may be identified, but instead of the dot-like microchromosomes, one sees some elongated rods representing probably several of these small bodies fused together. The cell shown in text figure 5, G, taken from the same embryo, shows very clearly the twelve large V's, the two 'b' chromosomes, and at least ten micro-chromosomes .
The chromosome complex of a fourth embryo is shown in text figure 5, H. Here again there are twelve large V-shaped chromosomes. Of these, one (marked 's') appears to have broken in two. The 'b' chromosomes are less certainly identified, but are probably as labeled. Elongated rods represent, no doubt, fused micro-chromosomes.
The figures given above are typical for what one finds in each embryo. There is never any difficulty in identifying the large V-shaped chromosomes, but, as was noted for Anolis, there is a tendency for the micro-chromosomes to fuse together in some cells. This is not constant for an individual, however, as a comparison of text figure 5, F and G, will demonstrate.
Many embryos, besides those figured, were studied, the results being consistent throughout. Sceloporus spinosis embrj^os are of two types, one carrying ten large V-shaped macro-chromosomes (and the smaller V-shaped 'b' chromosomes) and the other carrying twelve large V-shaped elements and the 'b' chromosomes. The former are destined to be males, the latter are females.
At the present time the point of most general interest in works dealing with vertebrate spermatogenesis is the question: How is sex determined, what is the condition of the sex-chromosomes? Ever since the sex-chromosomes were discovered in the insects, a number of investigators^ have worked upon nearly all of the common vertebrates wdth a view of determining this question. In some cases a sex-chromosome has been reported, in other cases the results were negative, but it may be said with fairness that in only a few instances has the evidence for either the presence or the absence of the X-chromosome been complete enough to be convincing to one critically disposed toward the sex-chromosome theory. In perhaps the majority of cases the evidence for the existence of an X-chromosome has been based on observations of usually first spermatocyte cells, in which typically a bipartite chromosome was seen lying well towards one pole of the spindle and going presumably undivided to it. Only rarely have such observations been extended to the second maturation and proper check counts made here. , And yet in view of all we know about the irregular way (in point of time) in which vertebrate chromosomes divide, it would seem that we must accept with some reserve those works in which the presence or absence of the sex-chromosome was concluded from a study of the first maturation division alone. It has been a matter of common experience, noted by many observers and found in the lizards, that vertebrate chromosomes are variable in the time at which they di^dde; this may be early or late, each body acting more or less as an independent entity. The result is that while a chromosome lying outside of the equatorial plane may be a sexchromosome, it may also be half of a tetrad which has divided, the other half remaining in the equatorial plane, or it may be simply some chromosome displaced mechanically, either by the preserving fluids and subsequent treatment or by the sectioning razor. When the number of chromosomes in the spindle is large, so that one cannot identify each one of them in side view, then he can never be sure which phenomena he is observing.
- No better review of this subject could be given than is contained in a recent paper by Ethel Brown Harvey ('20). To the papers listed by Mrs. Harvey one should add the recent work of Wodsedalek ('20), dealing with the spermatogenesis of cattle.
Because of the peculiar form of the chromosomes in the family Iguanidae, these lizards have been extremely favorable for chromosome study. The total number of chromosomes in this family is large, but, happily, the macro-chromosomes, to which the X-chromosome plainly belongs, are small in number (twelve in the spermatogonia and six in the first maturation division). In side views of practically every complete maturation spindle, all of the large elements can be readily seen and three or four of them identified by their size and shape. (This applies to the 'a,' 'b,' and 'c' chromosomes and the X-chromosome.) During the course of this study, the author has observed, at one time or another, every one of the chromosomes besides the accessory so displaced as to appear to be passing to one pole of the cell undivided. Repeatedly cells have been found in which one of the tetrads has divided early, with one half of it lying well towards one pole while the other half was still to be seen in the equatorial plane. (This was often true of the small 'b' chromosome.) Such observations emphasize the necessity of checking up conclusions made on the first maturation spindle, in order to be sure that a real sex-chromosome has been found.
There are, of course, five points which should be determined in order to be sure of this. They are: a) the diploid number of chromosomes for the male; b) the haploid number of chromosomes in the first division and their behavior; c) the number of chromosomes present in the second spermatocytes; d) the number of chromosomes going to the spermatids; e) the diploid number of chromosomes for the female. Points 5, c, and d must be known and points a and e are very desirable checks. For lizards it was possible to determine all five points for one species (Sceloporus spinosus) and the first four points for two others (Anolis and Holbrookia). In the remaining species of the family enough points were determined to be sure that they conformed to the family type. The results have been consistent throughout.
In all the lizards studied two conspicuous nucleoli were found in the early gj-owth period of the first spermatocytes. These persisted through the various spireme stages as deeply staining bodies and entered the prophase of the di\dsion as a bipartite chromosome. Side views of the first division spindle frequently showed this bipartite body lying outside of the equatorial plate and well up or quite up to one pole. Careful study of the six large chromosomes of the spindle showed that it was the same body in every case which showed this movement (except for the rare displacement of an autosome) and that it was not half of some tetrad which had divided early. On the other hand, it must be emphasized that this early migration of the bipartite chromosome is not seen in all spindles, and in some species it was not seen at all. This fact led to a very careful study of late anaphase stages of the first division, in order to find out if the X-chromosome really passed undivided to one pole. In every case where chromosome counts could be made, one pole contained one more macro-chromosome than the other, the extra element being a bipartite body similar in size and shape to the X-chromosome (figs. 15, 16, 17, 42, 43, and text fig. 1, C andE). Thisshowed that the phenomena was a constant one, that one pole of the first maturation spindle received one more large chromosome than the other.
A study of the number of chromosomes in the second spermatocytes would seem to offer a simple way of verifying first spermatocyte conclusions, but, unfortunately, this did not prove to be the case with the lizards studied. When the chromosomes enter the second maturation spindle, they are already, in most cases, precociously split, so that in equatorial view it is difficult to make counts. One does not know whether one or two chromosomes are involved. The matter is further complicated by the fact that 'the second spermatocyte cells do not preserve well (due perhaps to poor penetration). Under favorable circumstances, however, either five or six macro-chromosomes were found. Anaphase stages of this division were much clearer for study (because of the chromosome shape) and showed that the spermatids received either five or six large chromosomes (figs. 19 to 24, 34 to 36, and 45 to 48).
Among all members of the family Iguanidae which were studied, there were twelve macro-chromosomes in the spermatogonia, and six of these large chromosomes in the first maturation spindle. This shows that the bipartite body which acts like a typical X-chromosome is derived from two spermatogonia! chromosomes. Since the male is heterozygous as regards sex, the female must be homozygous and have the two X composition. And since the X-chromosome of the male comes from two spermatogonial chromosomes, we should expect to find that the females had two more large chromosomes than the males (X = two spermatogonial chromosomes; 2 X = four spermatogonial chromosomes) .
A study of the female chromosome complex was made for Sceloporus spinosus, and while the results with the ovarian tissue were not altogether satisfactory, they indicated that the female has fourteen large chromosomes (the male of this species has twelve). A study of Sceloporus embryos, however, furnished convincing evidence on this point. As a glance at text figure 5 will show, some embryos show constantly two more large Vshaped chromosomes than the others. The embryos with the two extra chromosomes would have become females without doubt.
With this evidence at hand, we cannot escape the conclusion that a true sex-chromosome has been found in the lizards. This X-chromosome is of the 'double-odd' or the 'double accessory' type; that is, X equals two spermatogonial chromosomes.
The double-odd chromosome has been reported for other vertebrates (man, pig) and for a number of invertebrates. Wilson ('09) has reported this condition for Syromastes marginatus; Morgan ('15) describes it for certain Phylloxerans; Wallace ('05) and Painter ('14) found it in spiders; Goldsmith ('19)
NUMBER OF CHROMOSOMES WHICH FORM SEX-CHROMOSOMES
Rod att. to
Bipartite XY condition XY condition Bipartite ?
Bipartite Bipartite Bipartite Bipartite ?
To one pole 1st To one pole 1st To one pole 1st
To one pole 1st To one pole 1st To one pole 1st To one pole 1st To one pole 1st
To one pole 1st
To one pole 1st 1st reductional 1st reductional To one pole 1st To one pole 2nd To one pole 1st To one pole 1st To one pole 1st To one pole 1st To one pole 1st To one pole 1st
Levy, '15 Swingle, '17 King, '12
Jordan, '14 Guyer, '09, Guyer, '09 Malone, '18 Winiwarter and
Sainmont, '09 Newman and
Patterson, '10 Jordan, '11 Stevens, '11 Bachhuber, '16 Allen, '18 Yocum, '17 Wodsedalek, "16 Wodsedalek, '20 Wodsedalek, '13 Guyer, '10 Winiwarter, '12 Montgomery,
reports it for Cicindelidae; Wieman ('10) found it in certain beetles; Guyer ('10) describes it for man, and Wodsedalek ('13) for the pig.
A point of considerable interest is a comparison of the sexchromosomes, as I have found them in lizards, with similar bodies reported for other vertebrates. In table 1, the following points have been tabulated for all cases of vertebrates where the sex-chromosomes have been seen in maturation spindles: first, the number of spermatogonial chromosomes going to make up the sex-chromosome; second, the appearance of the X-chromosomes in the maturation spindles, and, third, the behavior of the X-chromosome.
A glance at column 3 of this table will show that the first maturation division has been reported as the reductional division in nineteen out of twenty cases (the exception is Yocum's work on the house mouse). In lizards, of course, the X-chromosome does not divide in the first maturation division; hence this is the reduction division, as far as the sex-chromosomes are concerned.
The second column shows, in the vast majority of cases where we are dealing with the X-condition this chromosome is bipartite in form. The exceptions are birds and cases where the X-Y condition prevails. Again, in lizards, we invariably find the bipartite condition.
In the first column it will be noted that most observers describe the X-element as coming from one spermatogonial chromosome, and regard the bilobed form of the first maturation division as a precocious splitting in preparation for the second maturation division. A few workers describe the X-element as coming from two spermatogonial chromosomes (fowl, man, Guyer; and pig, Wodsedalek) , and this is unquestionably the condition found in all the lizards studied. In lizards of the family Iguanidae the origin of the X-element from two spermatogonial chromosomes could be determined with great certainty; there can be no doubt that there are twelve macro-chromosomes in spermatogonia and six in primary spermatocytes. One of these six is the sex-chromosome.
Why should we find such close agreement in the appearance and behavior of the sex-chromosomes in vertebrates, and yet find this striking difference in the origin of the X-element? In this connection, it may be pointed out what while the number of chromosomes to be dealt with in lizards is small (twelve), other vertebrates show a range from seventeen to fifty-six, and in very few instances was any author quite certain of his count. An error of one chromosome is not improbable. The author is of the opinion that when the common vertebrates are restudied with improved technique, it will be found that the bipartite X-element arises from two spermatogonia! chromosomes, and that its bipartite form during the first maturation division is an expression of this bivalency, and not the precocious appearance of the plane where the second maturation spindle will separate these elements.
2. Morphology of chromosomes in lizards
A very striking and interesting feature of the morphology of the chromosomes among lizards is the sharp separation into two size groups, which I have termed, for convenience in description, macro- and micro-chromosomes. This size relation, most conspicuous among the Iguanidae, is constant throughout maturation and in somatic divisions, although, as I have noted several times in this paper, there is some tendency for several microchromosomes to become associated together to form long thin rods.
The macro-chromosomes appear in somatic and spermatogonial divisions typically as equal-armed V- or U-shaped bodies. These are really 'bent-rod' chromosomes, as defined bj^ Robertson ('16, p. 221), since there is typically no achromatic bridge between the two halves. In the first maturation division these macrochromosomes form tetrads (except the X-element) with usually a ring or a double-cross form. In the second division we find bent rods or open V-shaped bodies going to either pole.
The micro-chromosomes appear typically as rounded dots in spermatogonia! and maturation divisions, but in somatic divisions they may be elongated, short rods, or rarely small Vs being formed. These small bodies unite during synapsis, so that bilobed or bipartite masses are found in maturation. All micro-chromosomes divide in a regular fashion and, as far as I can determine, the spermatids all receive the same number of them. Furthermore, their number seems to be constant in different individuals. This has been most carefully studied in Sceloporus, where the testes of four males have been examined (after several fixatives), and in addition the somatic divisions of both male and female embryos. In all favorable cells the number of micro-chromosomes seemed to be ten.^
What significance does this sharp separation into two chromosome groups have? Is this merely a striking incident in the spermatogenesis of lizards, or are the macro- and micro-chromosomes physiologically unlike? May the micro-chromosomes be, perhaps, degenerating chromosomes which have outlived their usefulness in this ancient group?
From the evidence at present at hand, it appears that this separation into large and small chromosomes is merely an interesting incident of lizard spermatogenesis. This conclusion is based on the following observations: First, in the Teiidae, the size demarcation is not so sharp, the two groups grade into each other. Further, the micro-chromosomes behave as true autosomes, fusing in synapsis and dividing in a regular fashion in maturation. There is no evidence to show that their number is variable within the species. It is very probable that in the macro- and microchromosomes we are dealing simply with larger and smaller aggregations of chromatin matter. ^
3. Chromosomes and taxonomy
In the family Iguanidae, where the spermatogenesis of six species of lizards has been studied, we have, as far as the macroor large chromosomes are concerned, a confirmation of the general position taken by McClung ('05, '08) after his extensive studies of orthopteran chromosomes, that there is a definite relation between taxonomy and chromosomes; that the degree of relationship may be recognized in the germ-cells as well as in external characters. Robertson ('16) has recently given extensive confirmation to this view.
It will be understood, however, that there is a considerable chance for error in making micro-chromosome counts, for in addition to a chance hiding under other chromosomes, they tend to disappear on long extraction of the stain.
- In Drosophila several genes have been found to exist in the small dot-like chromosomes.
Among all the Iguanids studied, the number of macro-chromosomes (involving considerably over 90 per cent of the chromatin matter) is always the same, namely, twelve, and, what is more remarkable, these chromosomes show the same size relations. There are always, in the spermatogonia, the pair of large 'a,' the small 'b,' and the intermediate 'c' chromosomes (see figures as follows: Anolis, figs, 1, 2, 3; Sceloporus spinosus, figs. 25, 26, 27; Sceloporus U. consobrinus, text fig. 1, A; Holbrookia texana, fig. 37; Uta ornata, text fig. 2, A and B; Crotophytus text, fig. 3, A). The six remaining chromosomes are too much the same size and shape to allow one to pair up accurately synaptic mates. So similar are the macro-chromosomes, of these six species that, except for the micro-chromosomes, the cell plate of one form might be easily substituted for another without its being detected.
On the other hand, the micro-chromosomes, representing in the aggregate less than 10 per cent of the chromatin, do not show this constancy in size and number The number varies from ten to twenty-two, and even in closely related species, as Sceloporus spinosus and S. U. consobrinus, we have ten and eighteen, respectively, in the germ cells. Why should the macro-chromosome be so constant in size and number and the micro-chromosomes so variable? Two possibilities suggest themselves. One is, that in lizards we are witnessing the formation of a small number of large chromosomes through the fusion of the small bodies with the macro-chromosomes, and that this process has gone on in different degrees in different lizards. The other is that while the mass of the micro-chromosomes remains the same throughout the family, there is a tendency in different species for these bodies to fuse together into compound bodies. The latter possibility receives some support from the observation that in somatic divisions these micro-chromosomes do unite, and further, where we have a small number of micro-chromosomes, as in Sceloporus spinosus, they are relatively large in individual size, while when they are numerous, as in Sceloporus U. consobrinus, they are relatively smaller. (Compare micro-chromosomes of figs. 25, 26, 27 with small bodies in text fig. 1, A.)
Whatever the explanation be for the micro-chromosomes, it is clear that the macro-chromosomes of all the Iguanidae are all strikingly alike, and they give additional evidence for the validity of McClung's generahzation which was based on a study of grasshopper chromosomes.
4. Syncytial masses
A very striking feature of the testes of all lizards is the common occurrence of so-called syncytial masses. Similar bodies have been reported for turtles (Jordan), birds, and mammals.
In lizards these masses are most clearly seen in well-matured individuals where the lumen of the tubule is large. In such cases germ-cells tend to scatter somewhat from the walls of the tubules, and one finds here and .there rounded masses of cytoplasm containing from two to eight or more nuclei, but with no distinguishable cell walls. Typically, one finds two primary spermatocytes thus associated (text fig. 6, A). Occasionally there may be more primary spermatocyte cells. Figure B shows such a syncytial mass, as seen in A, undergoing division. Masses showing secondary spermatocyte cells have from six to eight nuclei, and many spermatids may be joined in this fashion.
The formation of these syncytial masses is probably due to the failure of the cytoplasm to divide after the chromosomes have separated. However, it does not seem to interfere with the normal formation of spermatozoa, for one finds such masses as shown in figure C where the formation of the sperm heads is going on normally. ^^
In passing, attention should be called to interesting theoretical possibilities which such a figure as 6, B, affords. Here are two first spermatocyte spindles lying closely associated side by side. It must be a very rigid division mechanism which would prevent at some time a chromosome from getting into the wrong spindle. This would seem to give a mechanism for variation in chromosome number, and if the sperm thus formed were viable would give interesting genetic possibilities in the resulting zygotes. The widespread occurrence of syncytial masses among mammals and birds where the same condition must exist makes the observation of some interest.
- In this connection it is interesting to note that Jordan has found the same condition in the turtles. He describes two, three, or even four sperm arising from the same cytoplasmic mas^.
Text figure 6 A, B, -and C. Syncytial masses. D and E show giant spermatozoa in process of formation.
5. Giant sperm
An additional feature in lizard spermatogenesis is the occurrence of giant spermatozoa. These are rare, but when found are very conspicuous. In text figure 6, D, two such giant spermatozoa and a spermatozoon of normal size are seen in the process of formation. It will be noted that the giant size affects not only the nucleus, but also the achromatic element. In figure E a giant spermatozoon and one of normal size are seen (the tails are not indicated in the figure). The giant spermatozoa tend to occur in patches.
The author has followed in some detail the spermiogenesis in lizards. It is characterized by the elimination of enormous amounts of cytoplasm. An account of the process will not be given, however, until mitochondrial stains have been employed. At this later time it is hoped that the origin of the giant spermatozoa can be determined.
- This study on the spermatogenesis o\ lizards was undertaken with two points in view: first, to determine if sex-chromosomes were present and, second, to see what light could be thrown on the peculiar condition found by Guyer in the spermatogenesis of birds.
- Seven species of lizards, including two families, were examined and their spermatogenesis reported on in this paper.
- In Anolis carolinensis there are twelve large V-shaped chromosomes (which for convenience have been termed macrochromosomes) and about twenty-two small dot-like chromosomes (called micro-chromosomes) in the spermatogonial cells. In the primary spermatocytes there are six macro-chromosomes and eleven micro-chromosomes. One of these macro-chromosomes is bipartite in form and passes to one pole of the cell undivided. As a result, the second spermatocytes are of two kinds, part with five and part with six macro-chromosomes. The sperm are dimorphic as regards this bipartite chromosome, which has been identified as the sex-chromosome. As far as could be determined, the small micro-chromosomes were equally distributed in both of the maturation divisions.
- The spermatogenesis of Sceloporus spinosus was found to be essentially like that of Anolis. There are twelve macrochromosomes and ten micro-chromosomes in spermatogonia. In first spermatocytes there are six and five of these bodies, respectively. The X-chromosomes are frequently seen passing early to one pole of the cell, undivided. As a result, the sperm are dimorphic as regards the sex-chromosome.
- The spermatogonia of Sceloporus undulatus V. consobrinus show twelve macro- and about eighteen micro-chromosomes. An X-element is present, and behaves as it does in Anolis and and S. spinosus.
- The spermatogenesis of Holbrookia texana is essentially like that of the foregoing forms. There are twelve macro- and twenty-two micro-chromosomes in the spermatogonia. An X-element was found, and the sperm are dimorphic as regards it.
- The spermatogenesis of Uta ornata was onlj^ partly worked out. There are twelve macro- and about eighteen micro-chromosomes in the spermatogonia. While the X-element was not identified in the first maturation division, the second spermatocytes were found to be of two kinds, part with five and part with six macro-chromosomes. Hence this species is like the rest described.
- The chromosome complex of Crotaphytus collaris was determined. There are twelve macro- and from twenty-four to twenty-six micro-chromosomes in the spermatogonia. The primary spermatocytes show a halving of these numbers.
- A fragmentary description of the spermatogenesis of Cnemidophorus gularis is given. This species belongs to a different family from the rest which were worked upon.
- Sex-determination in lizards is of the 'double accessory' type; that is, the X-chromosome is derived from two spermatogonial chromosomes. The males are heteroz3^gous as regards sex.
- A study of ovarian tissue in Sceloporus spinosus indicated that the female was of the 2X condition, for two more macrochromosomes were found present than in the males.
- Dividing somatic cells of Sceloporus embryos show either twelve or fourteen V-shaped macro-chromosomes. The nmnber is constant for the individual; that is, we find always the same nimiber in a given embryo. The embryos with twelve macrochromosomes would have become males and the ones with fourteen macro-chromosomes would have become females, without doubt.
- There is a remarkable constancy in the number and size of the macro-chromosomes of all the species of the family Iguanidae studied. All six species reported on show twelve macrochromosomes in dividing spermatogonia, and in every case at least three pairs of the chromosomes are strikingly alike in size and shape.
- Syncytial masses and giant spermatozoa are commonly found in the testes of all the lizards studied.
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All figures were drawn with the aid of a camera lucida, a B. & L., xV oil immersion, and a no. 15 eye-piece. The microscope was elevated on a stand, so that all figures in this paper represent a magnification of approximately 5000 diameters, which has been reduced one-third in reproduction.
EXPLANATION OF FIGURES
All drawings in this plate were taken from the testis of Anolis carolinensis. 1 to 3 Are spermatogonial equatorial plates. The letters refer to chromosomes which can be identified.
4 A very young first spermatocyte, showing two nucleoli.
5 Diplotene stage, showing X-chromosome.
6 Early prophase of the first division, showing tetrads.
7 and 8 Equatorial plates of the first division.
9 to 12 Side views of the first division spindle, showing the X-chromosome passing undivided to one pole of the cell.
EXPLANATION OF FIGURES
All figures were taken from the testis of Anolis carolinensis.
13 and 14 Side views of the first maturation spindle, showing the X-chromosome.
15 to 17 Late anaphase stages of the first maturation division. In the case of figure 17, the two plates overlapped, so in order to show each end clearly the slide was moved a trifle after the upper pole was drawn.
18 Side view of the second maturation spindle, showing the precocious splitting of the chromosomes.
19 Equatorial plate view of a second spermatocyte cell, showing five large chromosomes.
20 Equatorial plate view of a second spermatocyte cell, showing five large chromosomes.
21 Late anaphase of a second spermatocyte cell, showing that five large chromosomes are going to each pole.
22 and 23 Equatorial plate views of second spermatocyte cells, showing the presence of six large chromosomes.
24 Late anaphase of a second spermatocyte division, showing six large chromosomes going to each pole.
EXPLANATION OF FIGURES
All figures in this plate were taken from the testis of Sceloperus spinosus.
25 to 27 Equatorial plate views of spermatogonial divisions.
28 to 30 Equatorial plate views of the first spermatocyte spindle.
31 to 33 Side views of the first spermatocyte spindles, showing the early migration of the X-chromosome to one pole of the cell.
34 to 36 Equatorial plate views of second spermatocyte spindles, showing either five or six large chromosomes.
THEOPHILUS S. PAINTER
EXPLANATION OF FIGURES
The figures of this plate were drawn from cells found in the testis of Holbrookia texana.
37 Equatorial plate view of a dividing spermatogonia.
38 and 39 Equatorial plate views of first spermatocyte spindles.
40 Side view of a first spermatocyte spindle.
41 Telophase of the first division.
42 and 43 Late anaphases of the first division, showing five chromosomes (large) at one pole and six large chromosomes at the other pole.
44 Side view of a second spermatocyte spindle, showing the precocious splitting of the chromosomes.
45 and 46 Equatorial plate views of the second division, showing, respectively, five and six large chromosomes.
47 Late anaphase of the second division, showing five large chromosomes going to each spermatid.
48 Late anaphase of the second division, showing six large chromosomes going to each spermatid.
Cite this page: Hill, M.A. (2021, September 24) Embryology Paper - Studies in reptilian spermatogenesis 1 (1921). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Studies_in_reptilian_spermatogenesis_1_(1921)
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