Paper - Studies in mammalian spermatogenesis II. The spermatogenesis of man

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Painter TS. Studies in mammalian spermatogenesis II. The spermatogenesis of man. (1923) J Exp. Zool. 37(3): 292- .

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This historic 1923 paper by Painter described development of the human spermatozoa.



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Studies In Mammalian Spermatogenesis II. The Spermatogenesis Of Man

Theophilus S. Painter


Department of Zoology, University of Texas

Four Text Figures And Six Plates (Forty-On-E Figures)

Introduction

The question of the number of chromosomes in man is one that has been of unusual interest to biologists ever since it became known that these small deeply staining bodies are constant in number for a given species of animal. The first attempt to solve this problem wasmade by Hansemann in 1891, and he reached the conclusion that “die Zahl sicher hoher als 24 sei.” Since this early date a number of other investigators, using both germinal and somatic tissues, have attempted to determine the exact number of chromosomes in man. Actual counts have ranged from 16 to 48, and there has been no unanimity of opinion as to the true number. In table 1 on page 312 the results of the several investigations are given in condensed form together with references to the original papers. Without going into the literature in more detail at this place, it may be said that up to the spring of 1921 (when a preliminary report of the present paper was published) two widely different numbers had been put forward as the probable diploid or somatic chromosome number in man. The majority of the investigations, indeed all but one of the modern papers," pointed to 24 as being about the diploid number in the male, the female number being either the same or one or two more, depending upon what type of sex chromosome was regarded as the correct one. (Both an X-Y (Wieman), an X—O (Montgomery), and a‘double X’ 0 (Guyer) type of sex chromosome have been described for the male.) Opposed to this View was the single work of von Winiwarter, who found that the male possessed 47 chromosomes and the female 48, the sex chromosome being of the X—O type.

1 Contribution no. 158 from the Zoological Laboratory, University of Texas, Austin, Texas. 291

The great difliculty which seems to have stood in the way of an earlier solution of this question has been that of obtaining suitable material for cytological study. With the exception of those of von Winiwarter, practically all other works upon the spermatogenesis of man have been made on stale tissue——such as that obtained from executed criminals—where the testes have remained in the body for some time after death. It now appears that this delay is unfavorable for chromosome studies, for when fresh material has been used, the chromosome counts have been approximately the same: Von Winiwarter 47, Painter 48, and Grosser 47 and 48.2 A further hindrance to this work has been in obtaining proper fixation of the material, but within the last decade this difliculty in cytological technique has been largely removed.

It has been the good fortune of the writer to obtain for preservation fresh testicular tissue from three individuals. In a short preliminary paper in Science (May 21, 1921) it was stated that the diploid chromosome number in man is either 46 to 48 and that the sex chromosomes are of the X-Y type. In the present contribution the evidence will be presented upon which these two main conclusions are based. It may be said at once that extended study has shown that 48 is, in all probability, the correct diploid or somatic chromosome number for both the male and female of the white and negro races, and that the sex chromosomes are of the X-Y type.3

“While Grosser observed these numbers in two embryos, he interprets his results otherwise (p. 314).

3 The author exhibited a chart together with full explanations before the Second International Eugenics Congress, held during the latter part of September, 1921, in New York City. Photographic enlargements of the following figures were shown: figs. 1, 2, 4, 5, 9, 18, 21, and 30. In the explanation it was pointed out that both the white man and the negro have 48 chromosomes in spermatogonia and that the sex chromosomes are of the X-Y type. A short abstract of this work was also presented to the American Society of Zoologists at their meeting held in Toronto, December 28 to 31, 1921 (Anat. Rec., vol. 23). MAMMALIAN SPERMATOGENESIS 293

Material and Methods

The material upon which this study is based was obtained from three inmates of the Texas State Insane Asylum, through the interest and cooperation of Dr. T. E. Cook, a physician at that institution. Two of these individuals were negroes and one was a young white man. In all three cases the cause for the removal of the testes was excessive self-abuse coupled with certain phases of insanity which made the removal of the sex glands desirable. Doctor Cook, being interested not only with the problems directly concerned with his profession, but also with the larger questions of ‘the greater medicine,’ biology, placed this valuable material at the disposal of my colleague, Prof. D. B. Casteel. Professor Casteel very generously turned the material over to me for study and for comparison with my results on the opossum. I desire to make due acknowledgments to both Doctor Cook and Professor Casteel, whose foresight and generosity have made this study possible.

The operation for the removal of the testes was made, in all three cases, under local anaesthesia. An hour or two prior to the operation the patients were given hypodermic injections of morphine in order to quiet them. This was followed by local injections of novocain in the operating room. None of the patients exhibited any interest or excitement during the operation, nor did they show any signs of pain except when the vas deferens and the accompanying nerves were cut. One of the negroes went to sleep during the operation.

In operating Doctor Cook carefully dissected away the various membranes surrounding the testis so that the latter was fully exposed before the blood vessels were ligated. Immediately following this the testis was removed from the body, quickly cut into small pieces by means of a razor, and these pieces dropped into the preserving fluid and thoroughly teased apart. The interval between the cutting off of the blood supply and the immersion of the tissue in the fixing fluid was very brief, ranging from perhaps forty seconds up to three or four minutes. Only small portions of a testis were preserved, but this was done quickly and the tubules thoroughly teased apart.


Various fixing fluids were tried, but the present study is based almost wholly upon tissue preserved in a modification of Bouin’s fluid, and to a small extent upon tissue preserved in strong Flemming solution cooled to about 4°C. (Hance, ’17). The modified Bouin’s fluid was used at body temperature and fixation lasted about an hour and a half. (For the modification used see Allen, ’19.) The tissues were subsequently treated in the ways recommended by Allen (’19) and by Hance (’17) for their respective methods. Paraffin was used for embedding and sections cut at from 4 to 11,u. Iron haematoxylin was used for staining.

I wish to emphasize two features of the technique which in my experience are important. The first is the advantage of using thin sections. The nucleus of the human primary spermatocyte is some 10;; in diameter, but sections cut thick enough to include a Whole nucleus are frequently of little service. It is better to cut sections at from 4 to 8p, even though by so doing many spindles of the primary spermatocytes Will be cut in two, and complete spermatogonial plates will be somewhat rarer. One gains much in the sharpness with Which the individual chromosomes stand out in thin sections. A second very important feature is the -staining or, more properly speaking, the destaining of the sections after the use of iron haematoxylin. Differentiation must be carried to the point Where the tetrads are almost transparent. This point, even after considerable experience, is rather hard to determine in uncleared sections, but when once obtained the individual tetrads stand out distinctly, and due to their semitransparent nature, one can focus down through one or more and pick out underlying elements. A good deal of my material which at first sight seemed useless for study has proved valuable when cut verylthin and destained as explained above. VVhen the tetrads have lost most of their color the spermatogonial chromosomes will retain most of the dark stain. As already indicated, the modified Bouin’s fluid was the one which gave the best results, but even with this preservative the fixation is very uneven.

Spermatogenesis

The following description will be based almost entirely upon the germ cells of two individuals, a white man and a negro. The germ cells of the third individual, a negro, have not proved favorable for study, due in part to the lack of division figures and in part to preservation. In the case of the first two individuals mentioned it has been possible to follow the history of the chromosomes through the spermatogonial and first maturation divisions in a thoroughly satisfactory manner. The second spermatocyte divisions have not been sufficiently clear to allow of accurate chromosome counts.

In the present contribution especial attention has been directed towards the number of the chromosomes and the type of sex chromosome in man. The material studied gives very clear-cut evidence on both of these points. Certain other phases of the spermatogenesis, however, are not so clear; it is not improbable that more extensive studies on other mammals will be required before human spermatogenesis will be entirely understood (p. 309).

Spermatogonial Division

Dividing spermatogonia in man, just as in other mammals, are found among the supporting cells which for the most part go to make up the wall of the spermatic tubule. Von Winiwarter found three types of dividing cells in this region: typical dividing spermatogonia, cells much like spermatogonia only considerably smaller (possibly an earlier spermatogonial generation), and giant spermatogonia containing apparently the tetraploid rather than the diploid chromosome number. In my own material, the dwarf spermatogonial divisions are not conspicuous, but three other types of dividing cells are found which may be characterized as follows:

a. Typical spermatogonial divisions (figs. 1 to 8) showing elongated chromosomes, some of which are U-shaped, others of the bent-rod type, and one egg-shaped or round. This is the commonest type of dividing cell found in the tubule wall. One seldom finds more than five or six such cells in division in any one section of a tubule, although the division wave may be followed for some 50 to 80 ,4 in successive sections.

1). The second type of cell may be identified by the balled or rounded condition of the chromosomes (text fig. a, A). Such cells appear to be ready for division, as no nuclear wall can be detected, but no trace of a spindle or any characteristic step of division such as a telophase, for example, has been found. The individual chromosomes appear as almost round and their number and general size relations are about those of true spermatogonial chromosomes. No cells have been found where an accurate count could be made, as these balls tend to form aggregations of two or more and overlapping is common. A further characteristic of these cells in my material is that they occur in extensive patches so that twenty or thirty of them are quite common in one section of a tubule.

c. The third type of cell contains typical spermatogonial chromosomes, but the number is tetraploid rather than diploid (text fig. c, A). This last type of cell is only occasionally found.

Further consideration will be given these last two types of cells in a later section of this paper.

Typical spermatogonial divisions are easily recognized by the elongated shapes of the chromosomes. As an examination of figures 1 to 8 will show, these bodies vary much in size and shape. The larger are usually U-shaped, While the smaller elements are more of the bent—rod type. The number of chromosomes present in the spindle is large and there is both crowding and overlapping in the best preparations studied.

From the standpoint of the present work, the spermatogonial divisions are of interest in two particulars: first, the number of chromosomes and, second, the shapes of the individual elements. Both of these points may be determined more easily in cells where the chromosomes are just entering the equatorial plate. At this time they are not as densely grouped together, and if there is overlapping the elements usually lie at different levels. The disadvantage of this period is that there may be a good deal of foreshortening of the individual chromosomes if they lie tipped on end.


Dividing spermatogonial cells in which an approximate count of the chromosomes can be made are abundant in both the white and negro testes. Such cells usually show clearly forty or more chromosomes and one or two involved areas where the chromo Text fig. a A,shows two cells with ‘balled’ chromosomes. B and D are spermatogonial divisions as seen in two sections from two cells of negro material. C is a. partial spermatogonial plate showing some of the chromosomes with especial clearness.

somes are so packed together that only an estimate can be made of their number. In a small number of cases I have found dividing cells where the chromosomes were so separated that dependable counts could be made. The best of these cases are given in figures 1 to 6. Figures 1 and 2 are from white testes and figures 3 to 6 are taken from negro material. As a careful examination will show, even in these six best cells there is some crowding and overlapping of the chromosomes; frequently, however, the latter is not objectionable, as the chromosomes lie at different levels (a fact which cannot well be shown in the drawings). In all of the cells (figs. 1 to 6) 48 chromosomes have been found. In figure 7 a spermatogonial cell (negro) is shown with only 47 chromosomes; the author was unable to find or identify the small chromosome labeled ‘Y,’ as in figures 1 to 5. And in figure 8 another spermatogonial cell (white) is illustrated where there are apparently 49 chromosomes, but a few of the chromosomes are so bunched as to make this count somewhat doubtful.

Other drawings of spermatogonial cells will be found in text figure a, B to D. These latter are either incomplete or taken from two sections, so that we may not expect the correct number of chromosomes to be shown. They are valuable in indicating, however, in a very clear way, the characteristic forms of many of the chromosomes.

A careful examination of the chromosomes of any of the figures given (fig. 4, for example) will show the reader that there are many pairs which have a similar size and shape (chromosomes labeled ‘a’ in fig. 4) and that these lie separate from one another. In the case of figure 4, the mates are on opposite sides of the equatorial plate. It goes without saying, of course, that such chromosomes are to be regarded as synaptic mates or as ‘homologous chromosomes.’ The point that they may lie widely separated is stressed in order to emphasize the fact that these two similar elements could not have arisen by a precocious splitting of a mother chromosome in preparation for division. (See discussion of Grosser’s work on page 314.) A further fact which is revealed when the figures are carefully studied is that in most of the cells (figs. 1 to 5) one chromosome is noticeably smaller than the rest.


This has been labeled ‘Y’ for reasons which will appear later in this paper. The shape of this Y-chromosome may be round, as in figures 1, 2, and 5, or somewhat elongated, as in figures 3 and 4. That this small element is not a large chromosome viewed on end (as apparently von Winiwarter (’12) interpreted a similar element in his figures 15 and 16) is shown by the fact that on careful focusing up and down it appears as a point with a depth not markedly greater than its diameter.

Both the size relations and the shape of the chromosomes of spermatogonia can be appreciated by an examination of figures 31 to 34,4 and for purposes of comparison the chromosomes of von Winiwarter’s figures 15 and 16 are given in figures 36 and 35. The reader should clearly understand that such an arrangement as figures 31 to 36 is only approximate and that there is considerable chance for error both in tracing the outlines of the chromosomes and in their arrangement. Thus when there are several pairs of chromosomes with much the same size and shape, no one can certainly identify the true synaptic mates. On the other hand, such an arrangement brings out several points of interest. First, in spite of the unavoidable error due to foreshortening and the like, it is apparent that in every cell there are a number of chromosome pairs of similar size and shape. These are doubtless ‘homologous chromosomes.’ Secondly, in three of the cells illustrated (figs. 31, 32, and 34) there is one chromosome, almost or entirely round, which is smaller than any of the other elements. A synaptic mate of the same size and shape has not been observed in any cell. Von Winiwarter figured a similar unpaired element in his figures 15 and 16 (figs. 36 and 35). Furthermore, when one pairs up the larger elements of spermatogonia, one rod-like chromosome is left without a mate of the same size and shape. Of course, one cannot say which of the three rod-like elements in figure 31, for example, is the odd chromosome, but it is evident that there are three of them. These two unpaired elements (the

‘ In making figures 31 to 34 the following procedure was followed. The chro.~ mosomes of a given cell were copied, with the aid of a copying camera lucida, on: separate pieces of cardboard. The chromosomes were then mated up according,to their size and shape. 300 THEOPHILUS s. PAINTER

rod and the ball) are very important, as later evidence in matura tion seems to show that we have here the two components of the X—Y sex-chromosome complex.

A third feature, which both figures 31 to 36 and figures 1 to 8 bring out, is that the number of chromosomes in the white and negro races is the same, namely, 48, and that the individual chromosomes are quite similar in size and general shape. (Compare chromosomes of figs. 34, 35, and 36 (white) with figs. 31, 32, and 33 (negro).) Extensive study of many other spermatogonial plates, not illustrated, has shown no visible differences between the chromosomes of the two races. The point that the number is the same for the two races is one of some interest, as the explanation has been advanced (Guyer, ’14; Morgan, ’14) that the reason von Winiwarter’s counts were so much higher than those of other investigators was because he worked on white testes, while in the other cases negro testes had been used. Wieman (’17) showed this View untenable, and the present work fully substantiates such a conclusion.

The division of the spermatogonia is not attended, so far as has been observed, by any unusual behavior on the part of the «chromosomes.

Primary Spermatocytes

The changes which the chromosomes of man undergo prior to and following synizesis are essentially the same as those found in other vertebrates, as has been shown by Gutherz (’12), von Winiwarter (’12), and others. The nucleoli characteristic of this whole period have been very carefully studied by many observers with a hope of finding in them a clew to the condition of the sex chromosomes, and the observations made have been variously interpreted, either as showing the presence of sex chromosomes of the X—Y type (Wieman), or sex chromosomes of the X-O type (Guyer, Montgomery), or as disproving the visible presence of any sex chromosomes at all (Gutherz). In my own material I have been able to duplicate repeatedly cells such as are figured by Guyer, Gutherz, von VViniwarter, and Wieman. But after an extensive study of the nucleoli, not only in man but in another mammal (opossum), the author thinks that we know too little about their origin and fate to allow us safely to draw any general conclusions from them.

There are three nucleoli in the early growth period. One of these is large and quite round and on prolonged extraction loses its stain and appears to be a typical plasmosome. The other two nucleoli are unequal in size and are somewhat irregular in outline. They retain their stain even after the plasmosome has faded. After synizesis the plasmosome gradually disappears, but there persists usually one, sometimes two, deeply staining bodies of unequal size. The figures given by Gutherz are typical of this period. Here and there, however, one will find a nucleolus made up of two unequal parts (text fig. b, A) more or less connected. A somewhat similar condition was figured by Wieman and is very suggestive of the X-Y chromosome found in the first maturation division, but whether or not this similarity has any significance remains to be proved.

First Maturation Division

The chromosomes of this period are especially well shown in the negro testis, there being no sign of a fusion of elements in the best material, and if the strain is extracted to the point where the chromosomes are semitransparent, each individual in the spindles can be studied with comparative ease (figs. 9 to 18). Looked at from a general standpoint, the first maturation division is characterized by several outstanding features which deserve emphasis here. a) There is a lack of regularity in the time the individual chromosomes divide, and as a result one frequently finds spindles in which one tetrad has divided early and either the two halves are at opposite poles (fig. 21) or, one half is at or near one pole while the other half is still in equatorial plane of the spindle (text fig. b, B). b) There is one element in this division which does not have a typical tetrad form, but is made up of two elements of very unequal size (figs. 15, 17 to 21, and 24). Occasionally this element divides early (figs. 25 and 30), the large component going to one pole, the smaller one to the opposite pole of the cell. c) A third feature, which is probably not to be re302 THEOPHILUS‘ s. PAINTER

garded as a normal condition, is the occasional displacement of a Whole tetrad from the equator of the spindle (fig. 11). This latter condition is, without doubt, due to some cause foreign to the living cell, such as displacement by the razor or displacement

Text fig. b A, a pachytene stage (only few threads are shown), showing the peculiar type of nucleolus which may occasionally be found. B, a primary spermatocyte division, showing the early division of an autosome, one half being well up toward one pole while the other half is still in the equatorial plane of the spindle. C, a late telophase of the first division, showing the heavy fibers which may be found occasionally. D, a primary spermatocyte, showing the early division of both an autosome (only one half shown) and the X—Y sex chromosome.

through difiusion currents set up by the preserving agent or the subsequent treatment of the tissue. In well-preserved and properly destained material there is little difficulty in identifying a displaced tetrad (such asseen in fig. 11) Text fig. c Giant germ cells of man. A, a spermatogonial cell, showing about the tetraploid chromosome number. B, a primary spermatocyte in division, showing about the diploid chromosome number (48). C shows normal and giant germ cell in the pachytene stage.

or the half of such an element (fig. 21 and text fig. b, B). Undoubtedly the irregularity in the time of division of the chromosomes has no fundamental significance, but is a characteristic which the germ cells of man share with those of lizards, opossums, and other higher vertebrates. But especial emphasis is given to the behavior here because it seems very probable that in poorly fixed or overstained material the true nature of these whole or half tetrads would not be recognized, and a more fundamental significance be assigned to them. (For example, that they were sex chromosomes, p. 318.)

Aside from the exceptions noted above, the first maturation division presents no unusual features. In side views of spindles the chromosomes are seen to possess typical tetrad forms of varying shapes and sizes. Even on limited study, one soon comes to recognize certain peculiarly shaped tetrads in different spindles. (This is best shown in figs. 37 to 41.)

It is difficult to determine the exact number of chromosomes in the first maturation division, in spite of the fact that_ there is no fusion of individual elements. Polar views of equatorial plates are of little or no service, from the standpoint of counting, because the large elongated tetrads which typically lie on the outside of the spindle tend to obstruct the view of the smaller bodies lying within. As von Winiwarter has pointed out, the most favorable time for counting is that period in which the chromosomes are just ready to enter the spindle. They are more scattered then and overlapping elements lie at different levels in the cell. In figure 9 a cell in this early stage is shown, and it gives the clearest view of all the chromosomes which I have found. There are twenty-four elements present. Figure 10 is a somewhat later stage and the chromosomes are not so clearly separated, but twenty—four may be identified. Such cells as these (figs. 9 and 10) are extremely rare, and it has only been in some five or six cases that I have been able to make satisfactory counts. These, however, have always given 24 chromosomes—a number which is in exact agreement with von Winiwarter’s count for the same period. (That author’s conclusion is based on a very large number of counts.)

Surprisingly good approximate chromosome counts can be made in side views of spindles where the chromosomes are tipped a little toward the observer or where the spindle has been cut in two by the sectioning knife. Of course, such counts cannot be trusted to give the exact chromosome number, but in spindles which are at all favorable for study one has no difficulty in making out clearly over twenty elements, and in several cases twentyfour chromosomes have been found.

A further attempt was made to check up the chromosome number by a study of the individual shapes of the different elements. It had been noted, in a preliminary study of the spindles, that a number of tetrads could be identified because of their peculiar shapes and size relations. It was hoped that on prolonged study it would be possible to recognize all of the elements in the spindle and hence get at the number in this way, This hope has not been fully realized, as will be seen below, but a number of interesting facts relating to chromosome morphology have been brought to light.

The method of procedure is as follows. Spindles were selected (in material where most of the stain had been extracted) where it was possible to make out clearly some twenty of the chromosomes. Careful drawings were then made of these (figs. 11 to 15). (When a tetrad lay underneath another and hence was hidden, the procedure used was to indicate its shape and size to one side of the spindle and to show its position by means of an arrow. If the spindle was in two successive sections, then the chromosomes of of the second section were represented below those of the first, without a cell wall. When the shape of an element was not clear, its outline is dotted in. The individual elements were copied on separate bits of cardboard by the aid of a copying camera lucida. In this way it was possible to arrange the chromosome as shown in figures 37 to 41.)

In figures 37 to 41 the chromosomes of five spindles (figs. 11 to 15) are lined up. In addition to lining up the chromosomes in the approximate order of their size, an attempt has been made to place similar chromosomes from different cells in the same column. This can be done with fair certainty in the case of only six or seven elements (chromosomes A, B, D, G, K, R, and XY). In the remainder of the cases the order is only a tentative one, for when one is dealing with similarly shaped chromosomes which differ only slightly in size, it becomes impossible to match them up in more than an approximate manner.

While realizing that in making up figures 37 to 41 certain unavoidable errors might be introduced, it is felt that these are not of such a nature as to vitiate several important conclusions. The first of these is that in man the chromosomes have an individuality which expresses itself in the different cells in specific form. After one once learns to know the chromosome ‘R,’ it can be recognized in practically all cells where it is not too much hidden by other elements, or unless it lies almost or quite parallel to the line of vision. The same thing may be said of a few other elements (A, B, D, K, and XY). With regard to the rest of the chromosomes, the attempt to assign them places is only of a very tentative character. But it is not to be doubted that in the course of time the individual chromosomes of man will come to be known quite exactly as we now know the chromosome of insects, such as the grasshopper or the dipteran, Drosophila melanogaster. A second fact brought out by figures 37 to 41 is that chromosome X-Y does not exhibit the typical tetrad form which is characteristic of the other twenty—three chromosomes. It is to a more detailed discussion of the form and behavior of this element that we now turn.

Chromosome X-Y is characterized by the fact that it is made up of two very unequal parts. The larger component (labeled ‘X’ throughout the figures) is a rod-like element of a somewhat varying form which is connected to the small ball—like component (labeled ‘Y’ throughout this study) by a heavy fiber. It is a characteristic of this fiber that it has in it two thickenings which are best seen in figures 19, 21, 25, 26, and 30.

Under ordinary conditions this X-Y chromosome lies near the center of the spindle along with the other small chromosomes, so that it is not readily seen unless the spindle is cut in two so as to expose it to view, or else when it divides earlier than the tetrads. In either of these events it will be recognized at once. The earliest stage in which I have certainly identified the X-Y element (in the first maturation division) is shown in figures 9 and 10. Even at this relatively early period, the unequal components are clearly seen. In figures 19, 20, 21, 24, and 26 its typical form, as observed in spindles, is shown. In figures 17 and 18 (white), and 25 to '30 (negro), also text figure b, D, the behavior of the two components (X- and Y-elements) when they divide early is clearly indicated. The X—component goes to one pole of the cell while the Y-element goes to the other.

When the X-Y chromosomes separate at the same time that the tetrads divide, their distribution cannot be determined, but there is no reason for believing that it would be different from what is seen in the cells represented in figures 24 to 30. I have never observed the late separation of the X-Y components in any of my material as I was able to do in the opossum.

With the exception of figures 17 and 18, all of the cells illustrated in figures 9 to 30 were found in negro testes. The white material was not so favorable for study because of a scarcity of division figures, and also because of the preservation. However, the X-Y chromosome has been found repeatedly; its morphology and behavior are the same in the white man as in the negro (figs. 17 and 18).

The peculiar heavy thread which unites the two components of the X-Y chromosome is a very conspicuous element in many cells. The meaning of the thickening or knots as seen in figures 19, 26, and 30 is unknown, and the nature of the thread is not understood. Occasionally in late telophase stages a heavy fiber with knots has been observed lagging behind in the spindle (text fig. b, C). This would appear to be the same structure which connected the X-Y chromosomes, but this has not been demonstrated.

Second Maturation Division

In none of my material have second spermatocyte divisions been found where an accurate count of the chromosome number was possible. All of the figures so far observed would allow only an approximate sort of count to be made. The point of greatest interest in this division would be to verify the distribution of the X-Y chromosome components. 011 the basis of the first maturation division as I have found it, all second spermatocytes should show 24 chromosomes, but in half the cases there should be included a very small ‘Y’ element, and in the other half of the 308 THEOPI-IILUS s. PAINTER

cases there should be 24 chromosomes without this very small Y-chromosome. It may be added that Von Winiwarter found that secondary spermatocytes showed 23 and 24 chromosomes, respectively. However, since that author did not identify the ‘Y’ component in the first division and because of its small size it may have easily been overlooked in the second division.

Giant Germ Cells in Man

The fact has been known for a long time that in man various sorts of atypical spermatozoa occur in the seminal fluid. Among these types may be mentioned giant spermatozoa (see Broman, ’02, for a review of literature).

Von Winiwarter records the presence in the human testis of spermatogonial cells which contain more than the diploid chromosome number. During the present study the author has found abundant evidence for the existence of such giant germ cells. In text figure c, A, a spermatogonial cell is shown in which there are approximately double the ordinary number of chromosomes (that is, 96 rather than 48). In an examination of primary spermatocyte cells in the various spireme or thread stages, it is not at all uncommon to find cells which are very markedly larger than the average. In text figure c, C, a giant and an ordinary cell are shown as they lay adjacent in the testis. Occasionally one may even find dividing primary spermatocytes which show approximately double the number of chromosomes expected. In text figure c, B, such a cell is figured. There are forty-four elements shown (approximately), while a few elements lying very deep in the cell were omitted from the drawing. There are, however, approximately 48 chromosomes in the cell. It will be noted that the tetrads which lie separated from the rest have normal shapes.

I have been unable to follow, in man, the transformation of such giant germ cells into giant spermatozoa, but in the case of the lizard (Painter, ’21) this has been possible. How such giant germ cells arise is unknown, though presumably a cytoplasmic division fails to take place in young spermatogonia. MAMMALIAN SPERMATOGENESIS 309

The observations recorded above are of considerable theoretical interest and the phenomena recorded deserves further study. In such tetraploid cells there must be four ‘homologous chromosomes.’ How do these behave during synapsis? In such cells as I have studied this seems to take place normally and about forty—eight elements result. In text figure c, B, such tetrads as are clearly outlined appear normal both in size and shape. In this connection the recent work of Belling (’21) on Datura should be mentioned.

The most interesting feature of such giant spermatozoa, however, is the genetic possibilities which they offer. If such spermatozoa are Viable, and there seems no good reason Why they should not be, since they are of normal appearance, then we have a cytological basis for individuals with the triploid number of chromosomes (for a sperm carrying 48 chromosomes uniting with an egg with 24 would give, of course, 72 elements), and for individuals with an unusual combination of sex chromosomes. (Such a giant sperm would carry either 2 X, or an X-Y, or 2 Y-chromosomes.) Bridges (’21) has shown that in Drosophila melanogaster when the XXY condition occurs in triploid flies, intersex or hermaphroditic individuals result. Should a similar chromosome constitution occur in man (and the cytological possibility is given by these giant spermatozoa), then one might expect the same result which Bridges got, namely, hermaphrodites.

Cells with Balled Chromosomes

The author has already figured (text fig. a, A) and described the peculiar condition found in extensive patches of cells in which the chromosomes seem contracted into ball-like masses. How this condition arises or What it results in the author is unable to say, but since I have observed similar phenomena in the testes of the bat and of the striped skunk, it is inferred that this condition is not peculiar to man alone. It is possible that it is a product of poor preservation or a step in the degeneration of sections of a tubule, but whether either of these explanations is the correct one, such cells probably do not form a step in the normal production of spermatozoa.

Discussion

In table 1 a brief review is given of the literature dealing with the number of chromosomes, or, with the sex chromosomes of man, so far as these are known to the author. In the several columns of the table the following points will be found: type of tissue used for study (when given); the observed chromosome number in spermatogonia or somatic cells; the observed haploid or reduced number; the probable diploid number as indicated by the work in question (this is often inferred, thus when the haploid number is reported as 12, the diploid is given as 24, even though the author in question may not have so stated); and the type of sexchromosome described.

From the standpoint of the present paper (chromosome numbers and sex chromosomes), many of the works cited above are of little more than historic interest, either because they were done before the modern era of cytology, or because the material studied was admittedly so poorly preserved as to allow of only a general approximation of the chromosome number. In passing, however, it should be noted that Flemming, in 1882, gave figures of human chromosomes which correspond more closely to what I have observed in my material than the figures of any other workers with the exception of von Winiwarter and Grosser. A

more detailed reference will be made to some of the works cited in the following discussion.

1. Chromosomemcmber in mam

In the foregoing pages evidence has been presented to show that in the dividing spermatogonia of both the white man and the negro 48 chromosomes appear (figs. 1 to 6) and that in primary spermatocytes 24 chromosomes are present. How do these findings agree with those of other investigators?

An examination of table 1 will show that von Winiwarter, and Grosser are the only two investigators whose diploid chromosome MAMMALIAN SPERMATOGENESIS 311

counts approach those recorded in the present WOI‘k.5 Von Winiwarter found 47 chromosomes in spermatogonia and 24: in primary spermatocytes. These results differ from my own by just one spermatogonial chromosome; we both find the same number of elements in the first maturation division.

In order to compare von WiniWarter’s spermatogonial chromosomes With those which I found, figures 35 and 36 were prepared,‘ the chromosomes being those of . figures 16 and 15, respectively, in von Winiwarter’s paper of 1912.6 While such a method of comparison may be open to some objections, it shows clearly that I found one more small rod-like chromosome than von Winiwarter observed.

Were my spermatogonial counts the only evidence upon which the diploid number of 48 chromosomes is founded, one would hardly be in a position to urge it as the correct number, in preference to Von WiniWarter’s number of 47, for when dealing with as involved a chromosome complex as the human spermatogonia present, the ‘probable error’ of counting might well amount to one chromosome. On the other hand, both von Winiwarter and the author agree that the reduced or haploid number is 24, and in primary spermatocytes I have found very strong if not conclusive evidence that the sex chromosome of man is of the X-Y type. If this last is the true sex—chromos0me condition, then the diploid number must be an even one, and just double the haploid number of 24. This agrees with the observed facts (figs. 1 to 6) that there are 48 chromosomes in spermatogonia.

How are the observations of Von Winiwarter and myself to be reconciled with the great bulk of the literature which points to

5 Prof. H. M. Evans has recently called my attention to a foot-note in Babcock and Clausen’s “Genetics in Relation to Agriculture,” ed. 1918, page 538, in which they report that Evans found 48 chromosomes in the spermatogonia of man. They state that this indicates an X-"Y condition in the male, presumably on the basis that if the male has an even number of chromosomes corresponding to that of the female, the X-Y condition would follow. To date no figures or account of his work has been published by Professor Evans.

5 In order to bring the magnification of von Winiwarter’s figures up to that of my own, photographs were made of his figures, and these were subsequently enlarged to the approximate size of my drawings. Following this the individual chromosomes were copied as explained in foot-note 4.


TABLE 1 5.. $3 «a z Z 5; E8 E “ 5 AUTHOR MATERIAL g 8 ;' 3, m

5 as E: *3. EE :5 E ° E“ E‘ Hansemann. . . . . . . . . ‘normal’ tissue 18-40 24 Flemming, H. cornea. 20-28 Flemming, H. cornea 22-28 24 Bardeleben, K. testes 16 8 ‘.7 Bardeleben, K. testes 16 8 ? 16? Bardeleben, K . . . . . .. testes 16 8 16 Wilcox, E. V . . . . . . . . testes 15-19 36 Fick . . . . . . . . . . . . . . . . testes 32 ? 32 ? Moore and Arnold... testes 16 32 Moore and Walker .. testes 16 32 Duesberg . . . . . . . . . .. testes 24 12 24

Guyer, M. F . . . . . . . . . testes 22 12-14 22 X-O

Branca. . . . . . . . . . . . . .. somatic 24 24

Branca. . . . .. . . . . . . . . . 24 12 24

Branca . . . . . . . . . . . . .. 24 12 24

Gutherz, D . . . . . . . . .. testes 12 24 Montgomery, T. H.. testes 12 24 X-O Wieman, H. L . . . . . .. somatic 33-38 34 Wieman, H. L . . . . . . . ‘testes 24 12 24 X-Y Jordan, . . . . . . . . . . . .. testes 12 24 X-O Winiwarter, H. v.... testes 47 24 47 X-O

Referfence

1891, Virch. Arch., vol. 123, p. 356.

1882, Arch. mikr. Anat., vol. 20. 1898, Anat. Anz., vol. 14. 1892, Verh. d. Anat. Gesel. Wien, p. 205. 1897, Arch. Anat. u. Phys. Anat. Abt., p. 193.

1898, Jen. Zeitsch., vol. 24.

1900, Anat. Anz., vol. 17, p. 316.

1905, Arch. Anat. u. Phys. An-at. Abt. sup.

1907, Proc. Roy. Soc. London, vol. 77B.

1906, Univ. Liv. Reports, Vol. 106.

1905, Anat. Anz., vol. 28.

1910, Biol. Bull., vol. 19.

1910, C. R. Assoc. Ana.t., vol. 12.

1911, Bibl. Anat., Vol. 21.

1912, C. R. Assoc. Anat., vol. 14.

1912, Arch. mikr. Ana.t., vol. 79.

1912, Journ. Acad. Nat. Sci. Phila., vol. 15. 1912, Am. Jour. Anat.,

vol. 14.

1917, Am. Jour. Anat., vol. 21.

1914, Car. Pub]. 182. 1914, Arch. f. Biol., vol. 27. MAMMALIAN SPERMATOGENESIS 313

TABLE 1—C'ontinued

F3 9 ' o m ~ [31

5 5 E : z ... Q _ AUTHOR MATERIAL g E S U2 H M E REFERENCE E3 5.8 §3 EE

1) —- D4 E1 Winiwarter, H. v.... testes 24 47 X—O 1921, C. R. Soc. Biol., July. Grosser, . . . . . . . . . . .. somatic 47-48 24 1921, Anat. Anz., vol. 54. Painter, T. . .. testes 46-48 24 46-48 X-Y 1921, Science, N. S., vol. 53, p. 503. Painter, T. . .. testes 48 24 48 X-Y 1921, Rec. 2nd Int. Eugenics Congress. Painter, T. S . . . . . . . . testes 48 24 48 X-Y 1922, Anat. Rec., vol. 22.

24 as the probable diploid number in man? In answer to this it may be pointed out that while von Winiwarter, Grosser, and the author have used fresh tissue as a basis for study, other investigators have almost universally used ‘stale’ tissues which had remained in the body for some time after the death of the individual. While we have no exact data upon this point, it seems possible that the time interval was sufficient to allow a certain amount of necrosis to set in in such tissue, which expressed itself in a fusion of the chromosome elements. This fusion linked with unfavorable preservation may explain why such competent observers as Guyer, Montgomery, and Wieman missed the diploid chromosome number by so far.

A second explanation which suggests itself is that Guyer, Montgomery, and Wieman may have confused primary spermatocyte divisions and spermatogonial divisions. If this were the case, then the numbers found by these authors would be about right for the haploid count (22 to 24). Without seeing the original preparations it would be impossible for the author to say if this second suggestion will hold, but it is certain that Guyer’s figures of spermatogonial chromosomes resemble far more poorly preserved tetrads than they do any spermatogonial chromosomes which have been figured by von Winiwarter or observed and figured by myself.

Grosser, in a recent paper, has reported on the chromosomes which he found in the amnion of young human embryos of unknown sex. In the two clearest mitoses of a 5.5—mm. embryo he found 47 chromosomes, and in the clearest mitosis of a 5.8-mm. embryo he found 48 chromosomes. Other counts from these two individuals ranged from 45 to 54 apparent chromosomes. A third embryo 11 mm. in length gave totally different results. In the amnion of this individual the chromosomes showed a pronounced tendency to occur in pairs, the number of which varied between 20 and 30, with an average of some 24. Grosser regards this pairing of the chromosomes as being due to the early splitting of the elements in preparation for the following mitosis, and he thinks when a higher number is present (47 or 48), it is due to the complete separations of daughter threads. In short, he believes that the true somatic number is about 24 chromosomes as maintained by the majority of workers in this field.

Professor Grosser has very kindly sent me microphotographs of dividing cells of all three of the embryos referred to above, and they certainly bear out in the most striking way the observations recorded by him. The chromosomes in his preparations are better separated and show with greater sharpness than in any of the cells which I have found in my own material. And the apparent pairing of the threads in the 11-mm. embryo is very striking. However, I am unable to accept the conclusions which Grossser has drawn from his observations. In the spermatogonia figured by von Winiwarter and in the present paper, like chromosomes do not occur regularly side by side, as should be the case were they derived from splitting, nor are there four ‘homologous’ chromosomes as would follow from Grosser’s conclusions. Furthermore, there can be no question that the haploid number of chromosomes is very close to, if not exactly, 24 chromosomes. There are dozens of cells in my own material where one can count more than twenty tetrads. For these reasons we are forced to regard Grosser’s embryo (11 mm.) showing the pairing of the chromosomes as having some explanation other than a precocious splitting of the threads. It may be that the salt solution in which this third embryo was kept prior to preservation or perhaps a sudden change in temperature may have been responsible for the apparent union of the threads. The chromosome counts of the other embryos may be taken as a confirmation of the chromosome counts of Von Winiwarter and those in the present paper. The embryo with 47 chromosomes was probably a male in which the small Y chromosome was overlooked.

2. Sex chromosomes in man

The evidence presented in the present paper for the existence of sex chromosomes of the X-Y type in man rests primarily on two sets of observations: first, that in spermatogonia there are two chromosomes which do not have synaptic mates of the same size and shape and, second, and this is perhaps the most convincing evidence, in primary spermatocytes there is one chromosome the two parts of which are unequal in size, and during mitosis these unequal components segregate to opposite poles of the cell.

The small ‘Y’ chromosome can be easily identified in spermatogonial cells in which there is not too much foreshortening of the other somewhat larger chromosomes (figs. 1 to 5), and that it has no mate of like size is most clearly seen in figures 31 to 36. In all of the complete spermatogonial cells which he figures, Von Winiwarter has shown this small ball—like Y—element, although in the text of his paper he leaves the impression that he regarded it either as a chromosome seen in section or a long chromosome tipped on end. As I have pointed out, however, in my own preparations, careful focusing shows that this Y-chromosome is not a long element seen on end. The unpaired nature of the extra rod-like chromosome is not so apparent until one attempts to pair up individual elements of a spermatogonial cell, as has been done in figures 31 to 34. It is impossible to say just which one of the small rod-like or slightly bent chromosomes of the spermatogonial cells is the true odd chromosome, but it is obvious that no matter how one pairs up these smaller elements there is always one left over. These facts taken alone would be strong evidence for, or at least very suggestive of, the X-Y chromosome condition found in the opossum or in many insects, and would lead one to look for such a sex chromosome complex in the first spermatocytic division in man.


The evidence from the various thread stages will not be taken up here, since it is inconclusive one way or another. By diligent searching one can find cells which might be given as evidence to establish any particular type of sex chromosome towards which one has a leaning (p. 300). In the first maturation spindle there is a chromosome the two parts of which are very unequal in size. As a complete description of both the morphology and the behavior of this element has been given, little more need be said except to emphasize the fact that during the first maturation division the X- and the Y-components segregate to opposite poles of the cell. This behavior is, of course, in entire accord with what happens to similar sex elements in the opossum and in certain insects.

Secondary spermatocytes should be of two kinds, one carrying 23 autosomes and the X chromosome, and the other 23 autosomes plus the small Y chromosome. However, up to the present I have not found in my material cells where decisive counts could be made, so that no positive evidencecan be presented on this point.

In the absence of further positive evidence, from a study of the human testis, we may ask, Can the observed phenomena have any other interpretation than the one given? Can the peculiar element found in the first division be explained in any other way? That such an element exists in both the negro and white man is not to be questioned.

When the X-Y chromosome was first observed in the negro material it was immediately realized that it could be some sort of chromosomal irregularity (supernumerary or heteromorphic autosome) as well as the true sex chromosome, and judgment was withheld on the point until it was possible to ‘study the germ cells of another individual. When the first sperrnatocytes of a white man showed (figs. 17 and 18) the same kind of X-Y chromosome with just the same behavior as that element exhibited in the negro, the probability of a chromosome irregularity seemed extremely remote, and it was concluded that the true sex complex had been found. The evidence of the spermatogonia of both the white man and the negro fully substantiated this view.

From a study of the spermatogenesis alone one is not able to say which is the X or ‘female determiner’ and which the Ychromosome. To settle this point conclusively it would be necessary to study the chromosome complex of the female. The author has been unable to do this up to the present time. J udg— ing from the conditions found in another mammal, the opossum, the larger element is the X—chromosome. (In many other animals where the X-Y sex complex has been found, the Y has usually been the smaller element.)

The history of the sex chromosome of the opossum has a very direct bearing upon the present work, because, as far as they have been studied the X-Y chromosomes of man are strikingly like similar elements found in this marsupial, and the behavior is the same in both cases. In’ text figure d I have given a few figures which show the conditions which prevail in the opossum, so that the reader may readily compare these with what has been described for man. (The labeling on the drawings together with the legend of the figure will make the drawings clear. In the opossum we know the diploid chromosome number for both the male and female, the haploid or reduced number both for the egg and for primary spermatocytes, and complete history of the X-Y chromosomes as shown in the two maturation divisions of the male. Finally, the somatic chromosome complex of both male and female embryos have been worked out.)

There is a close similarity between the sex chromosomes of man and the opossum, so far as we have been able to follow the former in this study. In the spermatogonia of both forms there are two chromosomes without synaptic mates of the same size and shape. The X-Y chromosome of the first maturation division has a somewhat similar form, and it shows the same behavior, for the X— and Y-components segregate to opposite poles of the cell. Beyond this point we have no direct evidence to offer in the case of man, but it is confidently expected that a close study of the second spermatocyte division Wlll show that half of the spermatids receive an X- and half ‘a Y—chromosome. Sex determination in man then would be as follows:

Sperm contain Eggs contain 23 autosomes + X plus 23 autosomes + X = 46 autosomes 2X (female)

01'

23 autosomes + Y plus 23 autosomes + X = 46 autosomes XY (male)

Sex chromosomes have been reported for man by a number of investigators, as table 1 will show, but only three authors have given figures of the chromosome so interpreted (Guyer, Montgomery, and Wieman). Von Winiwarter did not identify, or at least did not signify in his figures which was the sex chromosome; for the most part he based his conclusions on second spermatocyte counts.

The sex chromosomes figured by Guyer and by Montgomery were in all probability either displaced tetrads, or the half of a tetrad which had divided, the other half remaining in the equatorial plane of the spindle (p. 301). This error in their interpretations was due, no doubt, to faulty preservation, as it will be recalled that these authors found 12 to be the haploid chromosome number. ,

Wieman (’17) described the presence of an X-Y chromosome in man. His evidence was drawn in part from the condition of the nucleoli in the growth period and in part from observations made on the chromosomes during the first spermatocytic division. In the latter stage he found a chromosome dividing early, and the two halves, as they lay at opposite poles of the cell, appeared to be composed of two arms which were unequal in length. He regarded the longer arm as the X and the shorter arm as a Ychromosome, and the first division was an equational one for these elements, as both the X and the Y were equally distributed to the poles of the cell. Wieman supposed that in the second maturation division the X- and the Y-components segregated apart, though he was unable to confirm this by direct observation on his material.

I have repeatedly observed cells in which Wieman’s figures are duplicated, both in the several spireme stages and in actual MAMMALIAN SPERMATOGENESIS 3 1 9

Text fig. (1 Showing essential facts in the sex determination of the opossum. Figures taken from author’s paper of 1922. A is a spermatogonial cell, showing the X and Y chromosomes. B, showing the segregation of the X and Y elements to opposite poles in the first maturation division. 0 and D are second spermatocyte divisions, showing that in part of these cells we have an X element and in part a Y component. In both cases the sex element divides equationally, so that half of the spermatozoa will carry an X‘ chromosome and half will carry a Y chromosome. E and G are somatic cells of two female embryos showing the 2X condition. F and H are somatic cells of two male embryos, showing the X-Y chromosome condition. Compare these last two cells with the spermatogonia. as seen in A. 320 THEOPHILUS s. PAINTER

mitosis. It is quite possible that the nucleoli which he regarded as the X- and Y—components (during the growth period) are homologous of the X and Y sex chromosomes which I have found during the first maturation division, but it is certain that the chromosomes which Wieman interpreted as the sex chromosomes in the spindles of primary spermatocytes have nothing in common with the sex elements which I have found. According tomy interpretation, what Wieman observed is simply a tetrad which has divided early, and the arms of the individual 'V’s at either pole of the cell are lying so that one is foreshortened, hence giving the impression of unequal length. In figure 21 I have shown a somewhat similar condition, only at one pole of the cell the arms of the V are lying so that their equal length is clearly shown.

Von Winiwarter reported an X chromosome for the male upon following evidence: first, the odd number of chromosomes in spermatogonia and, secondly, that in secondary spermatocytes he found cells, part with 23 and part with 24 chromosomes. He does not figure the X—chromosome during the first maturation division. In spermatogonia I find one more chromosome than von Winiwarter; in the first spermatocytic division our counts are the same (24), but I have found an X-Y element. On the basis of my observations, the second spermatocytes should all show 24 chromosomes, but the ‘Y’ component which should be in half of the cells is very small and hence could be overlooked easily. This probably accounts for the fact that in part of the second spermatocytes Von Winiwarter found only 23 chromosomes.

Summary

1. A study has been made of the spermatogenesis of man through the use of freshly preserved testes of two negroes and one white man. The testes were removed under local anaesthesia and the material preserved at once.

2. The spermatogonia of both the white man and the negro show 48 chromosomes. Two of these chromosomes are unpaired elements, that is, they have no synaptic mates of the same size and shape. These two elements are the X- and Y-components of the X-Y sex-chromosome complex. MAMMALIAN SPERMATOGE NESIS 32 1

3. The evidence from the several spireme stages (nucleoli), While in harmony with the X-Y chromosome theory, is not considered crucial.

4. In primary spermatocytes there are 24 chromosomes. This is the haploid or reduced number.

5. One chromosome in the first maturation division is made up of two very unequal parts. This is the X-Y sex chromosome.

6. The X-Y chromosome elements segregate apart in the first maturation division, the X going to one pole, and the Y to the other. This behavior has been observed in both the white man and the negro.

7. The author has been unable to make decisive chromosome counts in the second maturation division. It is inferred, however, that the X and the Y components divide equationally at this time (as similar elements do in the opossum), and as a result half of the sperm will carry an X and half will carry a Y chromosome.

Bibliography

See table 1 for references to Works on human chromosome numbers and the condition of the sex chromosomes. ALLEN, EZRA 1919 A technique which preserves the normal cytological conditions in both germinal and interstitial tissue in the testes of the albino rat. Anat. Rec., vol. 16. BRIDGES, C. B. 1921 Triploid intersexes in Drosophila melanogaster. Science, N. S., vol. 54, p. 252. BROMAN, IVAR 1902 Ueber atypische Spermien (speciell beim Menchen). Anat. Anz., Bd. 21. GUYER, M. F. 1914 Accessory chromosomes in man. Science, N. S., vol. 39, HANCE, R. T. 1917 The fixation of mammalian chromosomes. Anat. Rec., vol. 12. ‘ MORGAN, T. H. 1914 Chromosomes of the white man and the negro. Science, N. S., vol. 39. PAINTER, T. S. 1921 Studies in reptilian spermatogenesis. I. The spermatogenesis of lizards. Jour. Exp. Zool., vol. 34. 1922 Studies in mammalian spermatogenesis. 1. The sperma1:ogenesis of the opossum. Jour. Exp. Zool., vol. 35. EXPLANATION OF PLATES

All of the figures represent a magnification of about 3,300 diameters as they are reproduced. A camera lucida was used for drawing together with a T1; oi]_ immersion (B & L) and no. 15 ocular.

323

THE JOURNAL OF‘ EXPERIMENTAL 2051.001’, VOL. 37. NO. 3 PLATE 1

EXPLANATION OF FIGURES

1 Spermatogonium of a White man, showing 48 chromosomes including the small Y chromosome.

2 Same stage as figure 1.

3 to 6 Spermatogonia of negro. In all the cells illustrated 48 chromosomes are shown, but the Y chromosome could not be identified in figure 6.

324 325 PLATE 2

EXPLANATION OF FIGURES

7 Spermatogonium of negro, showing 47 chromosomes. The Y chromosome could not be found.

8 Spermatogonium of white man, showing apparently 49 chromosomes.

9 and 10 Early prophase stages of primary spermatocytes, showing in both cells 24 chromosomes.

11 and 12 Side Views of primary spermatocyt-e spindles.

326 MAMMALIAN SPERMATOGENESIS PLATE 2 T!-IEOPHILUS s. PAINTER PLATE 3

EXPLANATION OF FIGURES

13 to 16 Side views of primary spermatocyte spindles, showing the form of the various tetrads and also the X—Y sex chromosome components, in the negro.

17 and 18 Side views of primary spermatocytes of a white man, showing particularly the X—Y sex chromosomes.

328 MAMMALIAN SPERMATOGENESIS PLATE 3 'r:mo1>1-uws 5. PAINTER PLATE 4

EXPLANATION OF FIGURES

19 View of part of a primary spermatocyte, showing with especial clearness the morphology of the X-Y chromosome. Note the two small dots lying in the thread which connects the two components. (All negro germ cells.)

20 to 24 Cells which show the form of the X-Y chromosome as it lies in the spindle. Cells in figures 20, 21, and 24 are especially clear—cut. All cells taken from negro.

330 MAMMALIAN SPERMATOGENESIS H] s s. PAINTER

331 PLATE 5

EXPLANATION OF FIGURES

25 to 30 Primary spermatocytes in which the X—Y chromosomes are dividing early. The segregation of the X- and the Y—component to opposite poles of the cell is demonstrated. (Negro material‘) Note the small dots in the fiber joining the sex elements in figures 25, 26, and 30.

332 MAMMALIAN SPERMATOGENESIS PLATE 5 TI-IEOPHILUB s. PAINTER

333 PLATE 6

EXPLANATION OF FIGURES

The magnification of this plate as reproduced is about 3800 diameters.

31 to 34 Show the line-up of the spermatogonial chromosomes as they are seen in figures 3, 4, 7, and 1 of the present paper. (See foot-note 4 for details of preparation.)

35 and 36 Represent the spermatogonial chromosomes of Von Winiwarter’s figures 16 and 15, work of 1912.

37 to 41 Is an attempt to line up the chromosomes of the first maturation spindle, as explained on page 305. Fig. 37 taken from fig. 11; fig. 38 from fig. 12; fig. 39 from fig. 15; fig. 40 from fig. 14, and fig. 41 from fig. 13 (negro material). As far as possible the chromosomes have been matched. When, however, the position of a chromosome was not clear, it has been placed at the end of the row, thus in figure 41 While there are several blank files the chromosomes at the end of the row do not obviously fit into any of these places.



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