Difference between revisions of "American Journal of Anatomy 1 (1901-02)"

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intricately interwoven, the only indication of the segmentation being afforded by apparently free ends protruding on the side toward the idiozome.
 
intricately interwoven, the only indication of the segmentation being afforded by apparently free ends protruding on the side toward the idiozome.
  
Synapsis. — The beginning of the growth period of the spermatocyte is evidently a time of great importance in the process of spermatogenesis. The spermatocyte possesses chromosomes of one-half the number normally occurring in the mitoses of that species, and this pseudoreduction has generally been located as occurring in the growth of the spermatocyte. The view, too, that the reduction in number of the chromosomes is only an apparent or pseudo-reduction, is generally accepted. That the chromosomes of the spermatocyte are bivalent and are two joined together end to end, is of course well known and also seems quite generally accepted. The terms introduced by Moore to indicate this pseudo-reduction and the corresponding period, " synapsis " and "synaptic phase," have been used in rather a confusing way. As used by Moore, 95, in his work on the spermatogenesis of Elasmobranchs, synapsis is equivalent to Riickert's, 93, pseudo-reduction, though Moore apparently does not assume that the chromosomes of the spermatocyte necessarily represent two joined together and therefore bivalent, which pseudo-reduction does assume. At the time that this reduction was believed to take place, there occurred in the forms studied by Moore (Scyllium and Torpedo), a peculiar contracted condition of the chromatin, causing it to be massed upon one side of the nucleus, and giving the appearance of an artifact. This Moore belieyed characteristic of the synaptic period and to be of general occurrence, and to this phenomenon of chromatin contraction, by a species of m.etonymy, the term synapsis has been transferred by several investigators. For this Moore himself seems partly responsible, since in a later paper, in conjunction with Farmer, 95, he uses the term synapsis as equivalent to " the contraction figure." A distinction between these two uses of synapsis seems to be necessary. The first use is evidently the correct one and is followed in this article. The use of the term in my preliminary communication upon this subject is wrong. The contracted condition of the nucleus, Moore found in the elasmobranchs investigated by him and in Amphibia (the Triton). Brauor, 93, had figured and described the massing of the chromatin upon one side of the nucleus in Ascaris, and Toyama, 94, figures it in the spermatogenesis of the silk worm (Moore). Paulmier, 99, describes it in insects (see also Montgomery, 00, p. 354). By botanical workers similar figures occun'ing at comparable points in the process of sporogenesis (the maturation period of the spore or pollen mothercell) have been found in a large range of forms.
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Synapsis. — The beginning of the growth period of the spermatocyte is evidently a time of great importance in the process of spermatogenesis. The spermatocyte possesses chromosomes of one-half the number normally occurring in the mitoses of that species, and this pseudoreduction has generally been located as occurring in the growth of the spermatocyte. The view, too, that the reduction in number of the chromosomes is only an apparent or pseudo-reduction, is generally accepted. That the chromosomes of the spermatocyte are bivalent and are two joined together end to end, is of course well known and also seems quite generally accepted. The terms introduced by Moore to indicate this pseudo-reduction and the corresponding period, " synapsis " and "synaptic phase," have been used in rather a confusing way. As used by Moore, 95, in his work on the spermatogenesis of Elasmobranchs, synapsis is equivalent to Riickert's, 93, pseudo-reduction, though Moore apparently does not assume that the chromosomes of the spermatocyte necessarily represent two joined together and therefore bivalent, which pseudo-reduction does assume. At the time that this reduction was believed to take place, there occurred in the forms studied by Moore (Scyllium and Torpedo), a peculiar contracted condition of the chromatin, causing it to be massed upon one side of the nucleus, and giving the appearance of an artifact. This Moore belieyed characteristic of the synaptic period and to be of general occurrence, and to this phenomenon of chromatin contraction, by a species of m.etonymy, the term synapsis has been transferred by several investigators. For this Moore himself seems partly responsible, since in a later paper, in conjunction with Farmer, 95, he uses the term synapsis as equivalent to " the contraction figure." A distinction between these two uses of synapsis seems to be necessary. The first use is evidently the correct one and is followed in this article. The use of the term in my preliminary communication upon this subject is wrong. The contracted condition of the nucleus, Moore found in the elasmobranchs investigated by him and in Amphibia (the Triton). Brauor, 93, had figured and described the massing of the chromatin upon one side  
  
  
  
B. F. Kingsbury 111
 
  
The observations of the investigators enumerated would indicate that the massed condition of the chromatin at the beginning of the growtli period of the spore or sperm mother-cell is a natural phenomenon of general occurrence without affording any clue to its significance; on the other hand, the fact that careful investigations in spermatogenesis have been made without noting the occurrence of any such phenomenon, suggests that it is not of universal occurrence.
 
  
In Desmognathus, the contracted condition of the nucleus at the beginning of the growth period does occur, as was noted in my preliminary publication, and it was then assumed to be of constant occurrence in the spermatic cycle of this form. Subsequent and more careful study of the form makes this seem very doubtful. Spermatocytes are being formed during the fall, winter and spring, though growth in the winter is probably slow, and transformation ceases to take place at about the beginning of summer, as has been already stated. During this time contraction figures are found rarely, and it is not until late May or June, or in other words, in the last generations' ot spermatocytes, that contraction of the chromatin into a mass is general. The appearances so produced are quite similar to those found by Moore, Paulmier, Wiegand and others, and are, I think, the same phenomenon; the chromatin gathers at one side of the nucleus, leaving a space within the nuclear membrane, with which it remains connected by a few shreds of linin. At this time the chromatin is closely massed together and details of structure are diflieult to make out, Fig. 18. There is no indication that anything is cast out from the nucleus at this time, as Wiegand found. With other workers, I feel confident that it is not an artifact, though no examination of fresh tissue was made. The fact that it occurs only at the end of the season of transformation at a time when the process is almost ready to stop, dissociates it, I think, from the process of " synapsis " or reduction.
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[[Category:Journal]][[Category:Historic Embryology]][[Category:1910's]][[Category:USA]]
At about this time there begin to appear in the last generations of spermatogonia contraction-figures which are essentially similar to those described above, in which the contraction is excessive. In these, the nucleus gathers into an apparently perfectly homogeneous round mass, the chromatin often separating from it as though " squeezed out." Such contraction-figures become abundant during July and August and are clearly, I think, degeneration changes associated with the cessation of transformation into spermatocytes, whether as cause or effect need not be considered here. Further study of this interesting phase will be undertaken in connection with the spermatogonia.
 
 
 
It is suggested, therefore, that the contraction-figures, instead of
 
 
 
 
 
 
 
112 The Spermatogenesis of Desmoguatlius Fnsca
 
 
 
being constructive and a fundamental phenomenon in the formation of the spermatocyte, may be an expression of a " running out " in the spermatogonium stock, and represent a tendency toward degeneration. We know as yet too little of the occurrence of contraction-figures in different forms to drav/ any general conclusions; possibly quite different phenomena may be here included. The fact of their occurrence in DesmognatJius only at the end of the season of spermatocyte-formation is, I think, suggestive, and further knowledge of their presence in other forms from this point of view is desirable.
 
 
 
Groictli of the Spermatocyte. — The growth of the spermatocyte has already been well described by Meves, 96, Herman, 89, and McGregor, 99, so that a detailed discussion is unnecessary. The chromatin of the spermatogonium is irregularly distributed in the nucleus in the form of an apparent network, Fig. 1. The changes in the nucleus are the following: The chromatin in the form of small granules becomes more evenly distributed in the nucleus upon the linin frame-work, which still appears to be a close reticulum with the chromatin evenly distributed. Gradually, the chromatin is concentrated in the form of a thread (or threads) connected by the linin network. It is at first hard to say whether a single thread or several (twelve) threads are so formed. Soon, however. Fig. 2, the free ends of threads are discernible projecting from the tangled mass on the side toward the idiozome. From this time on, in the growth of the spermatocyte, the chromatin threads or cliromosomes shorten relatively and increase in thickness. Fig. 3; they may now be counted and are found to be twelve in number. Their free ends, typically at least, are toward the idiozome (see Montgomery, 00, p. 352), so that they form a more or less irregular horseshoe. They are made up of a succession of numerous and large chromatin granules connected together by a less chromatic substance, giving the characteristic beaded appearance shown by Hermann, and now so well known. They are not smooth but possess processes joining the linin network of the nucleus and giving to the thread (chromosome) a fanciful resemblance to a string of daisies, as has been said by others.
 
 
 
The establishment of the spirem from the resting nucleus of the spermatogonium, and the shortening and thickening of the chromatin threads, are but a part of one continuous process of growth, so I have considered them together rather than as belonging in part to the succeeding division. The process is one involving a considerable period of time, as judged from the large number of lobules containing growing spermatocytes in various stages of development. During this period, there is a steady increase in the size of the nucleus, as may be seen from
 
 
 
 
 
 
 
B. F. Kingsbury 1^'^
 
 
 
the figures 1-6 of Plate 1. The exact method of chromatin change during the establishment of the chromosomes is rather difficult to determine. The chromatin of the spermatogonium seems to migrate out on the linin network as small granules and accumulate in lines as the spirem threads (or thread). Just in how far there is an actual migration ot particles and how far it may represent a chemical change, as of the less chromatic particles to the more chromatic and the reverse (e. g., as the change of oxychromatin to basichromatm of Heidenham), giving the efEect of such a migTation, could not be detected.
 
 
 
Eisen, oo, in his richly illustrated contribution upon the spermatogenesis of Batrachoseps, devotes attention largely to the finer structure of the cells. My work upon the minute details has been insufficient to render a full criticism of Eisen's views justifiable. Chromioles, chromomeres and chromosomes (his terminology) are recognized, and it seems highly probable that the chromioles do unite to form the chromosomes, though not in the fantastic way he describes. Chromoplasts were not recognized, and in the "bouquet" stage, instead of twelve "leaders" there are twenty-four, as may be seen from the transection, Eig. 4. In other words, one "wreath" does not represent two chromosomes joined by a chromoplast, but a single chromosome bent m the form of a horseshoe. Although the term "Auxocyte" was introduced by Lee as a name for the spermatogonium which becomes the spermatocyte of the first order, he fails to use it in his main work on the spermatogenesis of Helix and its introduction does' not simplify nomenclature, especially when it is applied to the spei-matocyte of the first order, either mature or in any stage of its growth.
 
 
 
It seems quite certain that there is not in this period of growth any formation of twenty-four chromosomes which then actually unite to form twelve; nor does it seem probable that a single continuous spirem is formed, which subsequently segments into the twelve chromosomes, though, as already stated, this is possible. The process appears as one of a continuous change and growth by which the distributed chromatin is gathered together in the form of twelve loops. In this case synapsis, actual or potential, would not occur as an observable fact.
 
 
 
But little attention has been bestowed upon the achromatic structm-e of the nucleus or cell body at this stage. The increase in the size of the cell body is quite large, as may be seen by comparing the figures 1-b. The idiozome, with the two centrosomes contained, shows usually three zones, which are, however, ill defined. In tissue fixed in chrom-acetoosmic mixture there is a condensation of substance about the idiozome, as Eisen has figured it in BatracJioseps. This is not shown in tissue
 
 
 
 
 
 
 
114 The Spermatogenesis of Desmognathus Fusca
 
 
 
fixed in Platino-aceto-osmic mixture, and may be due to the precipitation of proteids dissolved out when the latter fluid is used, which suggests that the idiozome acts as the nutritive center of the cell.
 
 
 
The two points which seem to need emphasis in discussing this stage of the spermatogenesis of Desmognathus are: (1) the early establishment of definite chromosomes while growth is still taking place, and (2) their polar orientation during tbis period of growth in relation to the idiozome and centrosomes. The early formation of the chromosomes in other Amphibia does not seem to have been very definitely recognized or noted, save in the case of Batrachoseps by Eisen.
 
 
 
The First Division. — The first indication of the division of the spermatocyte occurs in the change of position of the chromosomes. They lose the polar arrangement which they have maintained throughout their period of growth and possess no recognizable arrangement in relation to the idiozome. They do, however, exhibit a tendency to take up a superficial position beneath the nuclear membrane. The splitting of the chromatin threads follows next in close succession. Fig. 6. The details of the splitting are difficult to determine. The chromatin segments are long and still possess a bead-like structure. In them the splitting appears as a succession of clefts originating (in some cases at least) in the less chromatic part and merging at last in one continuous space. In which case, it would seem that the division did not begin with the chromatin granules. Whether or not the splitting is complete and the daughter-threads are at first separated throughout their entire length and afterwards fuse at their ends, could not be satisfactorily determined. Moves, 96, believed this was the case in Salamandra, and it seemed to be the case in Desmognathus as well, though it was hard to determine whether or not the ends seen in a section were free or cut ends. The point, however, is a minor one on our present basis of 'knowledge, and need not be considered further. The splitting is followed by a thickening and shortening of the double chromosomes which now are seen to be fused at their ends. In this stage they are usually distorted and twisted, assuming a variety of shapes. Figs. 7, 10. They may be twisted once, presenting the figure of an 8; they may be twisted twice; and also twisted and bent. The typical ring, 0, is frequently found but by no means constantly, even in stages approaching the establishment of the equatorial plate stage.
 
 
 
The splitting of the chromosomes takes place before there is any perceptible change in the idiozome or the centrosomes. The latter become larger, Figs. 6, 8, 9, stain more intensely and are of vaguer outline, appearing surrounded by an umbra. They migrate apart within
 
 
 
 
 
 
 
B. F. Kingsbury 115
 
 
 
the idiozome which becomes the center of radiations extending throughout the cell-body and also penetrating the idiozome itself, Fig. 9. "When the centrosomes have moved apart, a delicate spindle may be seen extending between them. This spindle increases in size as the centrosomes move farther apart, and with the dissolution of the nuclear membrane, which occurs when the spindle is about half grown, penetrates the nucleus, some of the fibers becoming apparently attached to the chromosome rings as the mantle fibers. When the spindle is first formed its axis may make any angle with the nuclear membrane. Figs. 9, 10; but as it increases in size, it seems to rotate so that the axis is roughly tangential to the nuclear membrane. In some cases the spindle could be observed before the outline of the idiozome had disappeared, Fig. 10. This, together with the penetration of the idiozome by the radiations are features not observed in the other Amphibia, Moves finding in Salamandra that the centrosphere fragments took no part in the formation of the spindle. In Desmognathus, this does not seem to be the case. The chromosome loops do not seem to retreat to the opposite side of the nucleus, but rather to collect upon the side next the spindle.
 
 
 
The linin of the nucleus, I am sure, takes no part in the formation of the spindle, but has a different, irregailar appearance, as of degeneration, and stains somewhat differently.
 
 
 
The chromosomes, at the stage when they are drawn upon the spindle, are irregular rings, usually much distorted; occasionally presenting the form of a Y, by partial fusion of the sides. The typical stage of the spindle formation, such as is shown in Meves' figure, in which the chromatin rings bend in response to the attachment of the mantle fibers to the ends, are of rarer occurrence in Desmognathus, Fig. 13. The methods employed did not reveal any difference between mantle fibers .and central fibers, nor from the study of this division in Desmognathus did there appear to be sufficient evidence for the conclusion that the arrangement of the chromosomes on the spindle and their subsequent migration are due to the mechanical force of contraction in the mantle fibers. A careful study of the mechanics of mitosis in the spermatocyte has not been attempted, hoAvever.
 
 
 
The succeeding stages in the mitosis of the spermatocyte of the first order have been carefully described by Meves, Flemming and McGregor, and Desmognathus diff'ers from the forms investigated by them only in apparently unimportant details. The spindle varies in shape and size though the volume of the spindle appears to remain nearly constant, the shorter spindles being broader. In Desmognathus, as in the other forms, the chromosomes vary in shape and actual size. The amount of fusion 10
 
 
 
 
 
 
 
116 The Spermatogenesis of Desmognathiis Fiisca
 
 
 
of the ends of the chromosomes varies widely. In some cases the fused ends project prominently from the spindle in the equatorial plane, thus: [^ Such forms occur more often in the short and broad spindles, and it is believed represent an extreme expression of a tendency of the chromosomes to fuse, made possible by different " mechanical " conditions.
 
 
 
The disappearance of the polar radiations as the spindle develops has been commented upon hj McGregor and explained by Meves as due to the absorption of their ends as the spindle grows. In Desmognathus the polar radiations are possibly more marked in the metaphase, but rapidly disappear in the anaphase. In the anaphase the migration of the daughter-chromosomes and their secondary fission as they pass to the poles occur in much the same way as described in other forms, Fig. 16. The secondary longitudinal fission in Desmoganthus is shown in Fig. 15. As the chromosomes approach the poles, they become so closely massed. Fig. 17, that the individual outlines are completely lost. This massing serves to completely mask the secondary splitting that in the early anaphase is evident. The centrosomes pass to the extreme periphery of the cell and become no longer recognizable, so that I was unable to identify them and trace them continuously to the division of the spermatocyte of the second order. A well-defined astral shield was not recognized in Desmognathus, nor was there any indication of a migration of the centrosomes, such as both Meves and McGregor have found in the forms studied by them. The vacuole on the polar side of the chromatin mass, which in Salamandra occurs under the astral shield, is, however, present.
 
 
 
The mid-body and the remains of the central spindle present the same characteristic appearances already well known in other forms, as may be judged from Fig, 17. No attention has been devoted to their meaning and fate.
 
 
 
The Second Division. — The chromosomes in the telophase of the division of the spermatocyte of the first order are closely massed in an apparently structureless mass, much contracted. When the chromatin expands to form the nucleus of the spermatocyte of the second order, the chromosomes separate from each other, and it is then seen that they occilpy the same position as in the late anaphase of the previous division; their apices are turned toward the former pole of the spindle, and the branches of the Vs extend back toward the opposite pole. Fig. 19. This is, of course, of typical occurrence and needs no emphasis. The chromosomes, are, however, seen to be double, so that from the apex of each, where they are united, four chromatin threads radiate out. Fig. 20
 
 
 
 
 
 
 
B. F. Kingsbury 117
 
 
 
shows a stage slightly older than that illustrated iu Fig. 19. A careful count at this time reveals twelve of these groups of double chromosomes. Polar views of the nucleus of the spermatocyte of the second order are shown in Figs. 20 and 21, while Fig. 22 represents a deep (equatorial) section through a nucleus, showing the chromatin threads cut across. The chromatin threads are at first long, fine, and irregular in thickness, and rough in outline, though a regular succession of chromatin granules, such as made up the chromosomes of the growing spermatocyte of the first order, is not so evident. The linin is at first scanty in amount but increases as the nucleus gi'ows, though it never becomes relatively as abundant as in the spermatocyte of the first order. It forms a coarse mesh-work between the chromosome threads, attaching to them, and thus giving them their irregular outline. The question of the source of the linin of the spermatocyte of the second order is one to which no attention has been given in this investigation, though it is of considerable interest.
 
 
 
The chromosomes shorten and thicken corespondingly, thereby losing their rough outline. The groups of four chromatin threads are in this way converted into crosses or X's, which tend to lie superficially under the nuclear membrane, Figs. 23-27, as did the chromatin rings in the spermatocyte of the first order, and so lose their original polar orientation. At about the time of the dissolution of the nuclear membrane, the X's separate into their component V's which become involved in the spindle. Fig. 29. Their distribution at the time they are first formed is apparently without order or system, so that the equatorial plate stage is one of loose formation. Typically, when the cross is dissolved, the two component V's become applied to each other, and thus are drawn into the equatorial plate in pairs, Fig. 29. This, however, does not seem to be always the case, so that often the V's are separated from each other and promiscuously scattered.
 
 
 
The remaining stages in the mitosis of the spermatocyte of the second order have been well described hj other workers, Flemming, Meves, McGregor, and need not be repeated here. The equatorial plate, anaphase and telophase are shown in Figs. 30, 31 and 32, and from them one may see that the processes are essentially identical with those described long ago by Flemming in Salamandra. The chromosomes become closely massed as they pass to the pole, as was the case in the spermatocyte of the first order. Fig. 32. They become separated again in the dispirem stage, and are once more apparent as V's, twelve in number, with their apices toAvard the pole of the cell. Fig. 33. These soon lose their form and oive rise to a fine reticulum characteristic of
 
 
 
 
 
 
 
118 The Spermatogenesis of Desmognatlius Fusca
 
 
 
the spermatid. Fig. 34 is a transection of the expanding nucleus of the spermatid, showing the arms of the Vs cut across, while Fig. 35 shows the fully formed spermatid. That the nucleus of the spermatid undergoes a slight enlargement before the period of transformation into the spermatozoon is evident.
 
 
 
The determination of the exact method in which the spindle is established in the spermatocyte of the second order has been attended with considerable difficulty, and despite careful work with many specimens well fixed and stained, it has been im_possible to arrive at an absolute decision. The spindle is established from a stage similar to that shown in Fig. 27. One centrosome lies at an extreme side of the cell-body directly under the cell-membrane, and from it there extends a radiation of fibers. The other centrosome lies close to the nuclear membrane some distance from the first, and it likewise is the center of radiations. The spindle seems to be formed by the fusion of these two sets of radiations. The earlier history of the formation of the achromatic figure is not so clear. The identity of the centrosome at the end of the first mitosis is lost, as has already been stated, so that centrosomic continuity between the first and second divisions has not been established; nor has it been shown that the two centrosomes of the second division were derived from a single centrosome, which presumably would have been one of the daughter-centrosom_es of the previous division. Fig. 25 suggests that this is the case, and that my failure to trace them has been due to the fact that one of them moved close to the nuclear membrane and on that account and because of the absence of well-marked radiations became in most cases indistinguishable. The entire achromatic figure, in comparison with the one of the previous division, is weak. A centrosphere is lacking, radiations are not well marked, and the astral shield at the end of the division is not developed.
 
 
 
From an examination of the Amphibian literature, it appears that there are no figures showing the development of the spindle in the second division. McGregor failed to trace the continuity of the centrosome in the spermatocyte divisions of Antphiuma, though Meves, in his description, leaves no doubt as to his interpretation of centrosomic continuity. Eisen, in Batrachoseps, describes and figures the spindle in the second division as formed by a fusion of two sets of radiations (fibercones) resembling somewhat the method described above as the one believed to occur in Desmognatlius. In his work, new-formation of centrosomes (archosomes) is assumed to occur. His fiber-cone resembles closely one of the centrosomes in Desmognatlius, surrounded by its radiations, and undoubtedly the two are the same.
 
 
 
 
 
 
 
B. F. Kingsbury 119
 
 
 
The life of the spermatocyte of the second order must be short, very short as compared with the gi'owth period of the spermatocyte of the first order and the transformation period of the spermatid, since never at any time do more than a few lobules contain spermatocytes of the second order. In size these are markedly smaller than the spermatocytes of the first order, and during their existence do not show an appreciable increase in size. The chromosomes remain distinct throughout, never losing their individuality, though a nuclear membrane is formed with a development of linin.
 
 
 
General Considerations.
 
 
 
In the brief discussion just given of the divisions of the spermatocyte, questions of their interpretation as mitoses have been entirely ignored. While sutScient attention has not been devoted to the study of the " mechanics " of the divisions, a word may be vouchsafed on certain points. Any contribution offering correct or suggestive interpretations in this most difficult field must, at the present time, be the result of comparative work, and that I have not done in this case. My aim has been, therefore, to keep the analysis of cell and nuclear structure as simple as possible, ensuring only its being sufficient for the purposes of this paper. Eisen's more elaborate analyses of granosphere, plasmosphere, hyalosphere and cone-fibers, were not found applicable in Desmognathus; they are felt to be premature. The purely mechanical interpretation of the processes of division given by Eisen (and others), I cannot consider at all satisfactory, nor can his statement that " the mitosis of the cells of the testis of Batrachoseps is the result of two independent parallel processes cooperating only at certain points," receive my confirmation from the study of the same divisions in Desmognathus. In the maturing spermatocyte of the first order, the arrangement of the developing chromosomes with their free ends toward the idiozome, suggests an interaction of the two during this period, and the idiozome as the metabolic center of the cell-body. As has been said, if Flemming's fluid is employed instead of Hermann's fluid, there is revealed a massing of substance about the idiozome not indicated by the other fixer — quite possibly soluble proteids. The loss of orientation when growth is attained, the splitting of the chromosomes followed by the changes in the idiozome point, I believe, to an intimate interrelation of the two sides of the phenomena. Meves, McGregor and Eisen have assumed that the arrangement of the chromosomes on the spindle and their subsequent migration axe due to a force of contractility in the
 
 
 
 
 
 
 
120 The Spermatogenesis of Desmognathus Fusca
 
 
 
mantle fibers, which seems to me quite inadequate as an explanation from the conditions in Desmognathus.
 
 
 
The chromatin changes in Desmognathus, considered by themselves and briefly stated, are as follows: The chromosomes, twelve in number, (presumably) one-half that of the somatic mitoses, develop as horseshoe-shaped threads, the free ends pointing toward the idiozome. These split longitudinally and incompletely, the ends being fused and open out to form rings in the manner typical in Amphibia. In the anaphase, however, the fused ends separate and daughter-V's are formed. As these pass to the poles, a secondary splitting takes place, which (presumably) masked in the late anaphase, reappears in the expansion of the nucleus of the spermatocyte of the second order, when the chromosomes are found to be united at their apices. These united V's shorten and thicken to fotm crosses and X's, which in the metaphase separate into the two component V's. In my preliminary paper was set forth a discussion of the chromatin changes in view of the possibility of a " reducing " division in Desmognathus, and a portion of what was then said may be repeated here. It was there pointed out that the formation of the crosses in the spermatocyte of the second order and their subsequent solution into V's introduced the possibility of a reducing division, since it was not possible to determine in what plane the separation into V's took place. Granted the V's represent
 
 
 
bivalent chromosomes, the result of the splitting and cross formation
 
 
 
a h ^
 
 
 
gives ^ If the separation into V's simply completes the longitudinal
 
 
 
c d • ha
 
 
 
splitting, we have ^ and no reducing division; if, however, it takes
 
 
 
 
 
 
 
V
 
 
 
 
 
 
 
place at right angles the resulting V's are j^ and the division is a
 
 
 
Z) a "reducing" divisions. To quote from that article, 99: "If the second division in Desmognathus is to be looked upon as a reducing division, it may be considered in two ways. The original union of the chromosomes, after two longitudinal splittings of the united chromosomes, is now dissolved and a new union between the daughter-chromosomes established; or, from the standpoint of the more typical mode of reduction by tetrad formation with longitudinal and transverse divisions, there would occur in Desmognathus, a reduction in number to one-half, a longitudinal (equation) division, which, however, is not completed, and is prevented from being completed, by the second division, which is transverse. Shorten the interval elapsing between the first and the second divisions, and (possibly thereby) eliminate the second longitu
 
 
 
 
 
B. F. Kingsbury 121
 
 
 
dinal splitting, and the process is reduced to the typical form." Such a transverse division is not believed to occur in Desmognathus, however, and the above is written simply to present all the possibilities of interpretation. If we compare other Amphibia we find that the occurrence of a fusion between the apices of the daughter- Vs with X-formation does not exist as far as reported, save perhaps in the oogenesis of the Triton, as investigated by Camoy and Le Brun, 98. Flemming, 87, in his work, indeed, did not recognize that the longitudinal splitting of the chromosomes of the secondary spermatocyte took place early (in the previous cell generation), nor that the splitting of the daughterchromosomes in the anaphase of his heterotypic mitosis was the precocious splitting for a second, following division; though he recognized its importance and normal occurrence, he confessed ignorance as to its significance. Meves, 96, leaves no doubt that this second precocious splitting becomes completed in the spermatocyte of the second order as the longitudinal division of the chromosomes of that cell division; his words are: "Die zweite homootypisch verlaufende Eeifungstheilung schliesst sich an die erste heterotypische an, ohne dass ein eigentliches Euhestadium des Kerns durchlaufen wurde, sondern dieser tritt aus dem Dispiremstadium von neuem in Mitose. Indem sich die chromatischen Faden aufiockem, wird zunachst die im Dyaster der heterotypen Form aufgetretene Langsspaltung welche wahrend des folgenden Dispiremstadiums undeutlich geworden war, von neuem sichtbar" (p. 61). McGregor,* too, inclines to the same result in Amphiuma, Avhile Eisen does not refer to the steps in sufficient detail, stating simply that both divisions in Batraclioseps are equation divisions. Carnoy and Le Brun, 98, in their work on the oogenesis of the Tritons, agree with the other workers on Amphibia in that they find both divisions are longitudinal. Their results are unique in several particulars. Their figures show the occurrence in the oogenesis of Triton, of X's entirely similar in appearance to the structures in Desmognathus, though it does not appear that they are daughter-V's united at their apices and formed by an incompleted longitudinal splitting. Their figures illustrating the second longitudinal splitting do not appeal to me as satisfactory, and perhaps permit of a different interpretation. The two divisions in the oogenesis of the Tritons follow each other more rapidly, so that a species of tetrad •* " The chromatin emerges from the spirem in the form of twelve Vs longitudinally split, which are probably identical with those of the anaphase of the preceding division, though this cannot be stated with absolute certainty, for it is impossible to discover exactly how the new double Vs arise from the spirem." McGregor, 99, p. 80.
 
 
 
 
 
 
 
122 The Spermatogenesis of Desmognathus Fusca
 
 
 
formation occurs. The second (axial) splitting may be postponed and occur in the anaphase as ihe chromosomes are passing to the poles. The figures strongly suggest that there is a close resemblance to the divisions in the spermatogenesis of Desmognathus, modified by the more rapid succession of the division in the polar-body formation.
 
 
 
From these comparisons there seems little doubt as to the interpretation of the second division in Desmognathus as a longitudinal splitting, nor as to its being the persistent longitudinal splitting which occurred in the anaphase of the first division. The second splitting in Flemming's heterotypic mitosis is clearly, then, the precocious division of the chromosomes for the succeeding division, and should not be considered an essential character of heterotypic division, since it would not necessarily occur, I believe that Amphiuma agrees with Desmognathus in this respect. The interpretations of Carnoy and Le Brun are unique, and cannot be reconciled with my own findings.
 
 
 
It is not necessary here to refer in detail to the influence Weismann's theory of the germ-plasm has had upon spermatogenesis work. Practically the only detailed work that had appeared prior to his first publication, in 1887, touching on the question of a reduction was Flemming's classical paper upon the divisions of the spermatocyte in Salamandra maculosa. Under the stimulating influence of Weismann's essay, paper after paper appeared — by Henking, vom Eath, Eiickert, Hacker and others — some of which seemed to bring wonderful proof of the correctness of his prophecy, while in other cases, as those of Brauer, Boveri, Moore, Moves, etc., the results were contradictory.
 
 
 
Owing largely to Weismann's theory and its apparent confirmation, there has been a powerful impetus given to the work in oogenesis, spermatogenesis, fertilization and cleavage, and from the standpoint of his brilliant theory new possibilities of interpretation of the phenomena of development have been brought out in testing its accuracy. In so far as it has led to these results, much could be said of the beneficial influence " Weismannism " has had in biology. On the other hand, in the investigation of oogenesis and spermatogenesis, the study of the phenomena has been made too largely a search for the occurrence of tetrad-formation and reducing divisions. An unproved theory, a speculation, highly suggestive and stimulating, but altogether hypothetical and not admitting of even partial proof, has been made the basis of the work, and it has diverted attention from other points of view that would have given a more normal, though perhaps not so rapid, development of this field of work.
 
 
 
A truer basis upon which an interpretation of the phenomena of
 
 
 
 
 
 
 
B. F. Kingsbury 123
 
 
 
spermatogenesis should be attempted is that of mitosis. The two " reducing " divisions are mitoses with certain peculiarities and should be considered simply as such and investigated from that standpoint. Any explanation of oogenesis or spermatogenesis must be first of all an explanation of cell-divisions. I do not mean that this has not been done by many workers on spermatogenesis; and full appreciation is felt of the excellence of the work of those employing spermatogenesis divisions for the investigation of mitosis. Flemming's classical paper in 1887, with its recognition of the divergent types of mitosis, uninfluenced as it was by theoretical interpretations, seems to me to represent a much more healthy attitude than do many of the later contributions. Occasionally the influence of theory has been responsible for evident eri'ors of interpretation, as, in Amphibia, vom Rath's work on the spermatogenesis of Salamandra.
 
 
 
As is well known, in several groups, by repeated and confirmatory investigations, the absence of " reducing ". divisions has been shown, and this is especially evident in Amphibia, Ascaris, and Lilium. In Amphibia, Flemming, Meves, McGregor, Cai-noy et Le Brun, Eisen and myself have furnished strong demonstration, Ascaris megalocephala has been tested by Boveri, Brauer and Hertwig. Among plants, small doubt may be felt about the divisions in the Liliacece from the work of Strassburger, Guignard, Mottier, Sargent and Dixon. A single wellauthenticated case of the absence of transverse divisions seems to me to be fatal to the theory of a qualitative reduction, and warrants its rejection as a working hypothesis. In its abstract form, it is a theory that cannot be disproved, although as reconstructed it cannot offer a more suitable basis for interpretation. While in certain forms both divisions of the spermatocyte have been shown to be longitudinal," in other groups I think it may be considered fairly well proved, that one of the divisions is as certainly transverse. Carnoy and Le Bruri, it seems to me, go too far in doubting correctness of observation in the finding of transverse divisions. In Insects and Copepods, certainly, the concordance of results permits but one interpretation — that one of the divisions is transverse. Both conditions must be harmonized, then, in any theory of spermatogenesis, and this the Weismann theory does not do.
 
 
 
If we view the divisions of the spermatocyte from the standpoint of
 
 
 
5 In a recent paper by King on the oogenesis of Bufo, tlie conclusion reached is that there both divisions are equation divisions, the "splitting" in the first maturation division taking place very early.
 
 
 
 
 
 
 
124 The Spermatogenesis of Desmognathus Fusca
 
 
 
mitosis, three features are to be noted; the first of these, is the rapidity with which the divisions follow each other, witliout an intervening interval of rest and growth. The effect of this is, theoretically, a reduction in the size of the nucleus of the grand-daughter cells one-half. In ordinary mitoses, the nucleus, n, increases by growth to 2n, divides so that the daughter-nuclei represent n: by growth each of these increases to 3n, to be reduced in the ensuing division to n, and so on. Omit one of the periods of growth so that the second division follows immediately after the first_, and the nuclei in the daughter-cells of the second division are reduced to -Jn. A quantitative reduction of the nuclear matter to one-half is accomplished as an inevitable result of the two rapidly following divisions. The difference in relative size of the cells of the generations of the spermatocyte divisions may be easily seen by comparing the figures, after reducing those of Plates III and IV, as directed. The spermatogonium has a nucleus Avith a diameter of, say, 25n, the nucleus of the mature spermatocyte measures 32n, that of the secondary spermatocyte has a diameter of 25n, while the spermatid has, as the corresponding measurement, 18n. This, of course, gives but the grossest idea of the size differences of the cells and nuclei. The size of the nucleus depends in part upon the growth period it has enjoyed, and this, in turn, must depend upon numerous factors, among them the metabolic interrelation of nucleus and cellbody, so that, considered quantitatively, the size of the nucleus is largely relative and variable. In the divisions of the spermatogonia there is such a variation in the size of the nuclei that it would be very difficult to estimate the size relative to the original embryonic nuclei. The primary spermatogonia possess large nuclei; these undergo rapid division and there is a decrease in the size accordingly. The period of growth of the spermatocyte again increases the size of the nucleus, restoring it — may we assume? — to the original size before a division. Only in case we assume that the quantity of nuclear matter in embryonic cells remains approximately constant, and that the mature spermatocyte has a nucleus as large as that of an embryonic cell before a division, is it safe to state that the divisions of the spermatocyte accomplish a quantitative reduction to (approximately) one-half.
 
 
 
The second point that seems well established is that in the spermatocyte mitoses the chromosomes appear in one-half the number that has been found in the ordinary tissue (and embryonic) mitoses in the respective forms. The significance that this seems to possess is the prevention of the doubling of the chromosomes and the maintenance of their numerical constancy in the species. It is, therefore, prophetic,
 
 
 
 
 
 
 
B. F. Kingsbury 125
 
 
 
anticipatory. It lias theoretical bearings on the meaning of the constancy of the number of the chromosomes and their individuality. Great as the evidence is, my inclination is to regard the generalizations as to the imjDortance of a constancy in number of the chromosomes and the adherence to that number in the mitoses of different cells in the organism, as yet unsafe.
 
 
 
The same doubt may be applied to the question of the individuality of the chromosomes, for which the evidence is not as strong. On the basis of the individuality of the chromosomes rests the interpretation of the reduction in number of the chromosomes as to a synapsis or a joining together in pairs, so that each is bivalent. This view may be purely hypothetical and unobserved, as in Moore's work, or based on actual observation, as in Montgomery's. In Desmognathus there is no evidence that the chromosomes of the spermatocyte are bivalent; nor in other Amphibia do we find evidence reported, save perhaps by Eisen in Batrachoseps where the chromoplasts may be interpreted as joining or separating single chromosomes. In that case, however, the number of chromosomes is one-half what it should be in that form.
 
 
 
The third general feature that attracts attention is the existence of the peculiar chromosomxC-forms that characterize the spermatocyte divisions, among which may be included, tetrads, ring-forms, X-forms, Y-f orms, and V-forms different from the V-shaped chromosomes of ordinary mitoses. It seems likely that the differences which exist between the spermatocyte divisions in different forms is due to minor modifications of the procedure, and that they are not intrinsic, so that if the modifying causes could be recognized, the variations could be more easily understood.
 
 
 
As a peculiarity of the spermatocyte mitoses has been mentioned the lack of a period of growth between them. The rapidity with which the second division follows the first in spermatogenesis seems to vary. Perhaps Scyllium, according to Moore's account, presents, among observed forms, the most complete resting stage between the first and second divisions. Here a complete resting period intervenes, with new formation of chromosomes in the second division. In Mammals likewise there is apparently a new-formation of the chromosomes after a resting period, Lenhossek, 96.
 
 
 
In Amphibia there is encountered a step toward the shortening of the interval. In Salamandra, according to Meves' account, a true dispirem, not to say a reticulum, does not seem to exist, and in Desmognathus the chromosomes remain distinct, though they become irregular and threadlike. The second splitting, furthermore (if we may accept this inter
 
 
 
 
 
126 The Spermatogenesis of Desmognathus Fusca
 
 
 
pretation), has moved forward from the second spermatocyte into the anaphase of the primary spermatocyte. In the oogenesis of the Tritons, Carnoy et Le Brun, as already stated, tind that the second splitting follows the first so rapidly that by the two splittings, a chromatin ring (tetrad) is formed, though sometimes the second splitting appears in the anaphase of the first division. In all these cases the chromatin division is by longitudinal splittings not (in this respect) markedly different from those of ordinary mitoses, but becoming less typical as the resting period is shortened and the chromatin fission is shifted toward the first mitosis. In all, however, the intervening period of growth is inadequate to restore the chromosomes to their former size, and the chromosomes of the second division are markedly smaller than those of the first, presumably approximately one-half their size, as commented on by Moore, Lenhossek, Meves, Carnoy et Le Brun.
 
 
 
From the Amphibia, it is but a step to the condition described by Boveri in Ascaris, where the second chromosome division, occurring as a longitudinal fission before the first mitosis, forms tetrads by a double longitudinal splitting, which Boveri himself interpreted as a precocious splitting due to or associated with the lack of nuclear reconstruction between the two divisions.
 
 
 
Why it is that, in what is generally regarded as the more usual method of tetrad formation, the chromatin preparation for the two divisions is accomplished by the first longitudinal splittings being (in the typical case) followed by a transverse splitting as the second division instead of another longitudinal one, is, of course, on the basis of bur present knoAvledge, entirely inexplicable. The nature of the changes that go on in the cell and induce spirem and chromosome formation and longitudinal fission is equally unknown, and the explanation of ,why in certain forms and certain mitoses the daughter-chromosomes are formed by a transverse instead of a longitudinal separation, must be wrapped up with the explanation of the former; in other words, the exception must be explained with the rule. Any discussion of the side of mitosis, to which this leads, is beyond the scope and ambition of this article, but the interpretation of the transverse division is to be sought in that portion of mitosis phenomena in general. The fact, however, that in the same divisions, in different forms, the separation occurs in different ways, indicates that the plane of fission is not the determining factor, or intrinsically important, but is itself determined by other factors. Forms in which tetrad formation is accomplished by means of at least one transverse division are, I believe, all forms in which
 
 
 
 
 
 
 
B. F. Kingsbury 1^'^
 
 
 
the second division of the cell follows the first without any resting
 
 
 
period.
 
 
 
A second point of view from which the chromosome forms may be considered is that of their manifest tendency to fuse. Thus, ringformation typically occurs by the fusion or incomplete fission— which is in effect the same-of the ends of the chromosomes. Almost every conceivable variety and modification of the typical ring is to be met with depending on the region and extent of the fusion; Y-forms Tforms, solid rods, crosses and V's, have all been found. The ring-form is the type and from it all others of these varieties may be derived by the increase of the fused area. Rings and their modifications are found almost constantly in the large majority of forms, with or without tetrad formation, in the first division of the spermatocyte. The more exceptional forms derived from it. while rarer, yet occur also in the first division of the spermatocyte. Thus, Griffin found in Thalassema, Y-forms, rod-forms, and X-forms; van der Stricht found in Tkijsanozoon rod-forms with gradations of fusion modified from the ring-form; v. Klinckowstrom, ring-forms, cross-forms, and rod-forms in the Planarian, Prostliecermis. Many o,ther instances of excessive fusion m the chromosomes of the spermatocyte of the first order might be given in both animals and plants; e. g., Belajeff, 92, Atkinson, 99.
 
 
 
Fusion of the daughter-chromosomes in their middle instead of at the end produces the characteristic X- and +-forms, when they are more or less V-shaped, and as the fusion extends out on the legs of the V- Y- and T-forms as well. These have also been found in the divisions of 'the spermatocyte in a number of forms, and usually in the division of the spermatocyte of the second order rather than in the previous mitosis. Thus, by van der Stricht in Thijsanozoon; by v. Klinckowstrom in Prosthecemus; in Cyclops by Haecker; by myself in Desmognathus. Among plants crosses were found in Larix by Belajeff, in Hemerocalhs by Juel. In Allium, Ishikaua reports the X-formation by fusion of the daughter-chromosomes at their apices in the first division; while in the second division, the chromosomes unite by their ends to form rings, there being thus a reversal of the condition found by me in Desmognathus, which is perhaps more typical. Crosses may, therefore, be formed as a modification of the end fusion (ring-formation), or by the center (apex) fusion of the daughter-chromosomes. These two formsand X— seem to be the types from which other varieties of chromosome form in the spermatocyte are derived.
 
 
 
In tetrad-formation, based on the ring-formation, the tendency toward
 
 
 
 
 
 
 
128 The Spermatogenesis of Desmognathus Fusca
 
 
 
fusion of the daughter-chromosomes inter se is not so marked, though in certain cases it seems to be exhibited; e. g., in Anasa, Paulmier, gg.
 
 
 
The significance of this tendency toward fusion and its cause are, of course, obscure. Possibly it is due in part to a more labile condition of the chromatin in the spermatocyte, which would cause the chromosomes to run together and round off whenever other forces permitted and in a corresponding manner and degree. This marked lability is indicated by a number of facts that appeal to one in studying the divisions in a particular form, as well as in examining the published figures of the conditions in other forms: the strong tendency of the chromosomes to mass together as they approach the poles of the spindle in the anaphase; the great irregularity of the forms of chromosomes presented; and the readiness with which they change shape. Typical tetrad-formation itself may be in part the expression of this same tendency of the chromatin to round off, due to a more fluid consistency.
 
 
 
Whatever the factors upon which the ring and cross formations depend, it is my belief that they cannot be interpreted apart from the entire problem of cell division. I fully appreciate that no real explanation is offered in what has been written above; nor is it meant as a criticism of the excellent work done upon spermatogenesis and oogenesis. Those investigations in which the spermatocyte divisions have been studied as mitoses are also recognized. I simply present my view as to the standpoint from which the phenomena of spermatogenesis should be considered in order that a firmer basis be given for interpretation; that view is, — that the divisions be studied as such, and the case and effect of the omission of the second growth period be sought; that the chromatin changes be considered from the standpoint of chromatin changes elsewhere and as a part of the entire phenomenon of cell division.
 
 
 
If we return again to a consideration of Desmognathus, we find in the first mitosis, ring-formation, usually quite irregular and variable; in the second division, cross-formation. In the first division it is the ends of the chromosomes that show the fusion more strongly, and rings result; in the second division their middle points (the apices of the V's) fuse and X's result, the behavior of the daughter-chromosomes of the two divisions in this respect being antithetic, a condition that may have significance and which occurs also in other forms (v. Klinckowstrom, van der Stricht, Ishikaua). If we consider the chromosomes in the two spermatocytes in their relation to the pole of the cell (centrosome), in the growing spermatocyte, the chromosomes form loops with their ends toward the centrosomes; in the spermatocyte of the second order, the
 
 
 
 
 
 
 
B. F. Kingsbury 129
 
 
 
apices of the Vs (their middle points) are toward the centrosome (?), and in each case the portion toward the pole of the cell becomes fused. This is offered without comment as worth consideration, in the firm conviction that the chromatin changes cannot be explained in themselves.
 
 
 
Assuming that the chromosomes of the spermatocyte are bivalent (of which, as said, there is not evidence in Desmognathus), then it is that in the first mitosis one end of the united chromosomes fuses; in the second, the opposite end, suggesting a polarity in the chromosomes themselves, which, indeed, their bivalence itself might be interpreted as indicating.
 
 
 
Summary.
 
 
 
1. The " contraction figures " in the nucleus of the growing spermatocyte do not occur constantly in Desmognathus.
 
 
 
2. The chromosomes of the spermatocyte are twelve in number, and in their growth are horseshoe-shaped with- the ends toward the idiozome and centrosomes, suggesting a polarity of the cell at this stage.
 
 
 
3. Synapsis was not observed in the formation of the chromosomes of the spermatocyte.
 
 
 
4. The first division of the spermatocyte is heterotypic, with ringformation by incomplete splitting.
 
 
 
5. The second splitting of the chromosomes in the first division is believed to be the precocious fission of the second division.
 
 
 
6. The daughter-chromosomes of the second spermatocyte remain fused together at their apices to form X's.
 
 
 
7. Both divisions of the spermatocyte are believed to be equation divisions, and no qualitative reduction takes place.
 
 
 
8. The spindle in the second division is believed to be formed by the fusion of two sets of radiations.
 
 
 
9. The first and second divisions of the spermatocyte have certain similarities and differences that are suggestive. A comparison with other forms is given.
 
 
 
10. The structure of the testis, spermatogenetic cycle, and the life
 
 
 
cycle of the lobules are discussed.
 
 
 
Histological Laboratory, Cornell University, August 30, 1901.
 
 
 
LITEEATUEE CITED.
 
 
 
Atkinson, G. F., gg. — Studies on reduction in plants. I. Reducing division in Arisaema triphyllum by ring and tetrad-formation during sporogenesis. 11. Reducing division of the chromosomes in Trillium grandiflorum during sporogenesis. Bot. Gazette, Vol. XXVIII, pp. 1-26.
 
 
 
 
 
 
 
130 The Spermatogenesis of Desmognathus Fusca
 
 
 
Belajeff, W., 92. — Ueber die Karyokinesis in den Pollenmutterzellen bei Larix und Fritillaria. Sitzb. Warsch. Naturf. Ges.
 
 
 
Bidder, F. H., 46. — Verg-leicliend Anatomische und Histologishe Untersuchungen iiber die miinnliche Gesehlechts und Harnsorgane der nakten Amphibien. Dori)at., 1846.
 
 
 
BovERi, Th., 90. — Zell Studien, Heft. 3. Ueber das Verhalten den chromatischen Ivernsubstanz bei der Bildung der Richtnngskorper und bei der Befruchtung. Jen. Zeitschr. f. Naturw., Bd. XXIV, 1890.
 
 
 
Brauer, a., 93. — Zur Kenntniss der Spermatogenese von Ascaris megalocephala. Arch. f. mikr. Anat., Vol. XLII, pp. 153-213.
 
 
 
CARJsroY, J. B. et Le Brun, H, 98. — La vesicule germinative et les globules polaires chez les batraciens. La Cellule, Vol. XVI.
 
 
 
Dixon, H. H., 94.— Annals of Botany, Vol. VIII.
 
 
 
EisEN, GuSTAV, 00- — The spermatogenesis of Batrachoseps. Polymorphous Spermatogonia, Auxocytes and Spermatocytes, Journ. of Morph., Vol. XVII, pp. 3-117.
 
 
 
Flemming, W., 87, 91. — Neue Beitrage zur Kenntniss der Zelle. Arch. f.
 
 
 
mikr. Anat. Theil I, Vol. XXIX; Theil II, Vol. XXXVII. McGregor, J. H., 99. — The Spermatogenesis of Amphiuma. Journ. of
 
 
 
Morph., Vol. XV, Supplement, pp. 57-104.
 
 
 
Griffin, B, B., 99. — Studies on the maturation, fertilization and cleavage of Thalassema and Zirj)haea. Journ. of Morph., Vol. XV, pp. 583-634.
 
 
 
GuiGNARD, L., 91. — Nouvelles Etudes sur la Fecondation. Ann. d. sci. nat.
 
 
 
Bot., T. XIV, 1891. Haecker, v., 99. — Praxis und Theorie der Zellen und Befruchtungslehre.
 
 
 
Jena, 1899. Henking, H., 91. — Untersuchungen iiber die ersten Entwicklungsvorgiinge
 
 
 
in den Eiern der Insekten. Ill Specielles und Allgemeines.
 
 
 
Zeitschr. f. wiss. Zool., Vol. LIV; also Vol. XLIX; LI. Hermann, F., 89. — Beitrage zur Histologic des Hodens. Arch. f. mikr.
 
 
 
Anat., Vol. XXXIV. Hertwig, O., 90. — Arch. f. mikr. Anat., Vol. XXXVI. Hoffmann, C. K., 78- — Bronns Klassen und Ordnungen des Thierreiches.
 
 
 
Bd. VI, 2te Abth. Amphibien.
 
 
 
Hoffmann, C. K., 86. — Zur Entwicklungsgeschichte der Urogenitalorgane
 
 
 
bei den Anamnia. Zeitschr, f. wiss. Zool., Vol. XLIV, 1886, pp. 570 643. ISHIKAUA, M., 97. — Studies of Reproductive Elements. III. Die Entwick lung der Pollenkorper von Allium fistulosum L., ein Beitrag zur
 
 
 
chromosomenreduction im Pflanzenreiche. Journ. Coll. Sci.,
 
 
 
Tokyo, X, 2. Jordan, E. O., 91. — The Spermatophores of Diemyctylus. Journ. of
 
 
 
Morph., Vol. V, pp. 263-270. JuEL, H. O., 97. — Die Kerntheilungen in den Pollenmutterzellen. Jahrb.
 
 
 
d. wiss. Bot., Vol. XXX.
 
 
 
 
 
 
 
B. F. Kingsbury 131
 
 
 
Kingsbury, B. F., 95. — The Spermatlieca and Methods of Fertilization in some American Newts and Salamanders. Proc. Am. Micr. Soc, Vol. XVII, 1895, pp. 261-304.
 
 
 
Kingsbury, B. F., gg. — The Keducing Divisions in the Spermatogenesis of Desxnognathus fusca. Zool. Bulletin, Vol. II.
 
 
 
Klinckowstrom, a. v., 97. — Beitrage zur Kenntniss der Eireife und Befruchtnng bei Prostheceraeus. Arch. f. mikr. Anat., Vol. XLVIII.
 
 
 
Lee, a. B., gy. — Les Cineses Spermatogenetiques chez I'ilelix pomatia. La Cellule, Vol. XIII, pp. 199-278, 1897.
 
 
 
Lenhossek, M. von, g6. — Untersuchungen iiber Spermatogenese. Arch. f. mikr. Anat., Vol. LI.
 
 
 
Leydig. Untersuchungen iiber Fischen und Reptilien.
 
 
 
Meves, F., g6. — Ueber die Entwicklung der mannlichen Geschlechtszellen
 
 
 
von Salamandra maculosa. Arch. f. mikr. Anat., Vol. XLVIII. Moore, J. E. S., 95. — On the Structural Changes in the Eeproductive Cells
 
 
 
during the Spermatogenesis of Elasmobranchs. Quart. Journ.
 
 
 
Mikr. Sci., Vol. XXXVIII, 1895.
 
 
 
MooRE, J. E. S. and Farmeb, J. B., g5. — Essential Similarity of the chromosome reduction in Animals and Plants. — Annals of Botany, Vol. IX, p. 435, 1895.
 
 
 
MoTTiER, D. M., ^'j. — ^Beitrage zur Kenntniss der Kerntheilungen in den Pollenmutterzellen. Jahrb. d. wiss. Bot., Vol. XXX, 1897.
 
 
 
Montgomery, Th. H., 98.— The Spermatogenesis of Pentatoma up to the formation of the Spermatid. Zool. Jahrb., Vol. XII; Abt. f. Anat. u. ontogenie.
 
 
 
Montgomery, Th. H., 00. — The Spermatogenesis of Peripatus (Peripatopsis) balfouri up to the Formation of the Spermatid. Zoolog. Jahrbiichern: Abth. f. Anat. \md Ontogenie der Thiere, Vol. XIV, pp. 277-368, 1900.
 
 
 
Paulmier, gg. — The Spermatogenesis of Anasa tristis. Anat. Anz., Vol. XIV; Journ. of Morph., Vol. XV, suppl., pp. 223-272.
 
 
 
VoM Eath, 0., g3. — Zur Kenntniss der Spermatogenese von Salamandra maculosa. Zeitschr. f. v^ass. Zool., Vol. LVII.
 
 
 
VoM Rath, O., g2. — Zur Kenntniss der Spermatogenese von Gryllotalpa vulgaris Latr. Mit besonderer Berlicksichtigung der Frage der Eeductionstheilung. Arch. f. mikr. Anat., Vol. XL, p. 102.
 
 
 
EiTTER, W. E., and Miller, Loye, gg.— A contribution to the life-history of Autodax lugubris, Hallow, a Californian Salamander. Am. Nat., Vol. XXXIII, 1899, pp. 691-704.
 
 
 
EucKERT, J., g3. — Die Chromatinreduction der Chromosomenzahl im Entvs^icklungsgang der Organismen. Merkel & Bonnet, Ergebnisse, Vol. III.
 
 
 
Sargent, Ethel, g6, 97.— The formation of the Sexual Nuclei in Lilium Martagon. Annals of Bot., Vol. X; Vol. XL
 
 
 
Sherwood, W. L., 95. — The Salamanders found in the Vicinity of New York City, with notes on extra-limital or allied Species. Proc. Linnean Soc. of New York, No. 7, pp. 21-37. 11
 
 
 
 
 
 
 
132 The Spermatogenesis of Desmognathus Fusca
 
 
 
Spengel, 76. — Das Urogenitalsystem der Amphibien. Arbeiten des zoolog'. zootom. Instituts in Wlirzburg, Vol. Ill, Ileft. 1, 1876. La Valette, St. Geoege, 76- — Ueber die Genese der Samenkorper. Die
 
 
 
Spermatogenese bei den Amphibien. Arch. f. milir. Anat., Vol. XII. Steassburgek, E., u. Mottier, 97. — Ueber den zweiten Theilungsschritt in
 
 
 
Pollenniutterzellen. Ber. d. Deutsch. Bot. Ges., Vol. XV, nr. 6. VAN der Stricht, 98. — Contribution a I'etude, de la forme, de la structure,
 
 
 
et de la division du noyau. Bull, de I'acad. de Belgique, Ser. 3,
 
 
 
T. XXIX, 1898. ToYAMA, 94. — On the Spermatogenesis of the Silk-worm. Bull. Agr. Coll.
 
 
 
Imp. Univ., Tokio, Vol. II, No. 3. Wbismann, a., 87. — Ueber die Zahl der Eichtungskorper und iiber ihre
 
 
 
Bedeutung fiir die Vererbung. Jena, 1887. Weismann, a., 93. — The Germ-plasm. New York. WiiiDEK, H. H., 99. — Desmognathus fusca (Eafinesque) and Spelerpes bili neatus (Green). Am. Nat., Vol. XXXIII. Wilson, E. B., 00. — The Cell in Development and Inheritance. The Mac Millan Co., New York, 1900. VON WiTTiCH, 53. — Beitrage zur Morphologische und Histologische Ent wicklung der Ham und Geschlechtswerkzeuge der nakten Amphibien. Zeitschr. f. wiss. Zool., Bd. IV, 1853. Zeller, Ebnest, 90. — Ueber die Befruchtung bei den Urodelen. Zeitschr.
 
 
 
f. wdss. Zool., Vol. XLIX, pp. 583-602, 1890.
 
 
 
Explanation of Plates.
 
 
 
All the figures of the tour plates are drawn at the same magnification with a Leitz microscope, draw-tube in, Leitz 1/16 in. immersion objective, and Leitz No. 4 Ocular. All figures on plates III and IV are reduced only Yz the extent of plates I and 11. % was taken off any diameter of figures on plates I and II. Only i/g taken off any diameter of figures on plates III and IV. Hence take 14 more off any diameter of a figure on last two plates, to compare size with a figure on first two plates. All were drawn from longitudinal sections of Desmoffnathxis testes, fixed in Hermann's fluid (except Fig. 7, where the fixer was Flemming's chrom-aceto-osnaic mixture, strong formula) and stained with Heidenhain's iron hematoxylin. The sections from which they were drawn, were paraffin sections, 10 and 7 fi thick.
 
 
 
PLATEIS I AND II.
 
 
 
Division of Spermatocyte I.
 
 
 
Fig. 1. Secondary spermatogonium of the last generation, showing as yet no indication of the beginning of the growth period.
 
 
 
Fig. 2. Spermatocyte soon after the beginning of the growth has become evident. The chromatin is becoming arranged in the form of threads. A nucleolus is shown.
 
 
 
Fig. 3. Spermatocyte, later stage of growth; spirems larger and better defined. Indication of the chromosome threads vdth their free ends towards the idiozome.
 
 
 
 
 
 
 
SPERMATOGENESIS OF DESMOGNATHUS FUSCA. B. F. KINGSBURY.
 
 
 
 
 
 
 
 
 
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AMERICAN JOURNAL OF ANATOM Y--VOL. I.
 
 
 
 
 
 
 
 
 
 
 
 
 
SPERMATOGENESIS OF DESMOGNATHUS FUSCA. B. F. KINGSBURY.
 
 
 
 
 
 
 
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14
 
 
 
 
 
 
 
AMERICAN JOURNAL OF AN ATOMY--VOL.
 
 
 
 
 
 
 
17
 
 
 
 
 
 
 
V
 
 
 
 
 
 
 
18
 
 
 
 
 
 
 
B. F. Kingsbury 133
 
 
 
Fig. 4. Spermatocyte. Transection of the nucleus when growth is nearly completed. Most of the chromosome-loops are cut twice.
 
 
 
Fig. 5. Spermatocytes. Growth nearly completed. Three cells are cut nearly "longitudinally "; one cell shows the idiozome but not the nucleus. The chromosomes are in the form of looi^s with their free ends toward the idiozome. This is about the same stage as Fig. 4.
 
 
 
Fig. 6. Splitting of the chromosome threads in two adjacent cells. Chromosomes have lost their orientation in relation to the idiozome. Splitting in some of the threads is not continuous.
 
 
 
Fig. 7. Three Spermatocytes, showing ring-formation; some of the chromosomes are twisted as 8's; while fragments only of others are seen. But one cell is cut in the right plane to show the idiozome. The two centrosomes are yet close together.
 
 
 
Fig. 8. Spermatocyte. Section at the beginning of spindle-formation. The centrosomes within the idiozome have moved slightly apart and faint radiations have appeared. The chromosome rings are situated superficially under the nuclear membrane and are cut irregularly.
 
 
 
Fig. 9. Spermatocyte. Stage slightly older than that of Fig. 8. Kadiations are more distinct and penetrate the' idiozome. Centrosomes naore distinct. Chromosomes near the nuclear membrane.
 
 
 
Fig. 10. Stage older than Fig. 9. Two cells are shown with young spindles forming. The outline of the idiozome is still preserved. Note the angle of the spindle axis. The nucleus of one of the cells is cut superficially, and the chromosome forms are well shown. In the other cell, the chromosonQes are on the side of the nucleus toward the idiozome.
 
 
 
Fig. 11. Spermatocyte. Later stage; spindle well formed, with but two chromosomes shown at the level.
 
 
 
Fig. 12. Spermatocyte. Deeper section of the same cell shown in Fig. 11. Ring chromosomes are w^ell shown. Note their shape and bending. Two Y-shaped chromosomes are shown.
 
 
 
Fig. 13. Spermatocyte. Spindle at a later stage. The chromosomes are being arranged on the spindle. The rings are bent in a typical manner.
 
 
 
Fig. 14. Spermatocyte. " Equatorial plate " stage, showing a typical spindle. The chromosomes are breaking apart in the equatorial plate. Fused ends of some of the chromatin loops are seen projecting.
 
 
 
Fig. 15. Spermatocyte. An oblique polar view of the daughter-chromosomes as they pass to the pole, showing the second splitting.
 
 
 
Fig. td. Spermatocyte. Late anaphase. The chromosomes approaching the poles.
 
 
 
Fig. 17. Spermatocyte. Telophase. The chromosomes have become closely massed; a vacuole caps the mass. Mid-body and remains of the spindle are shown.
 
 
 
Fig. 18. Spermatocyte. At the beginning of the growth period, with the nucleus in a " contracted " condition. The chromatin is in a dense mass, still connected with the nuclear membrane by strands. Detail of structure in tlie nucleus cannot be made out.
 
 
 
 
 
 
 
134 The Spermatogenesis of Desmognathus Fiisca
 
 
 
PLATES III AND IV.
 
 
 
The figures of tlhcse two plates must he reduced y^ more {i.e., y^ off any diameter) to permit of correctly comparing size with figures on the first two plates.
 
 
 
Division of Spermatocyte II.
 
 
 
The following- figures may be compared with the corresponding stages of the Spermatocyte I:
 
 
 
Fig. 19. Secondary Spermatocyte. Two danghter-cells of the first division at the " dispirem " stage. The chromosomes have expanded somewhat; the apices of the V's are still toward the pole of the cells.
 
 
 
Fig. 20. Secondary Spermatocj^te. Later stage. Slightly oblique view showing five of the chromosomes, which are seen to be double. They still maintain their polar arrangement.
 
 
 
Fig. 21. Secondarj^ Spermatocyte. Polar view of a similar stage. Eight chromosomes more or less complete are shown. The polar arrangement is still maintained.
 
 
 
Fig. 22. Secondary Spermatocyte. Ti'ansection (equatorial section) of the nucleus at a deeper level. The cut chromosome threads are shown; and their arrangement in fours is indicated. The apex of one group of four threads is seen.
 
 
 
Fig. 23. Secondary Spermatocyte. Later stage. Three cells are shown; in one the section is near the middle and the chromosomes, now shortened to X's, are seen to lie superficially. In the other tw^o cells, the section cuts the nucleus nearer the s\:rface.
 
 
 
Fig. 24. Secondary Spermatocyte. Stage similar to that shown in Fig. 23. Crosses and X's are shown.
 
 
 
Fig. 25. Secondary Spermatocyte. Longisection at the beginning of spindle formation. The chromosomes are in the form of crosses and are next the nuclear membrane.
 
 
 
Fig. 26. Secondary Spermatocyte. A later (?) stage. The nucleus is cut superficially. One centrosome only can be seen next the cell membrane.
 
 
 
Fig. 27. Secondary Spermatocyte. Spindle formation. The one centrosome is located near the cell-membrane, the other at a distance, near the nucleus, and a spindle is not yet formed between them.
 
 
 
Fig. 28. Secondary Spermatocyte. Later stage; spindle almost completed. Nuclear membrane has been dissolved.
 
 
 
Fig. 29. Group of five secondary spermatocytes. The equatorial plate stage just being established. The crosses in most cases have been dissolved, and the daughter-Vs have become applied to each other. Tw^o of the cells are cut somewhat obliquelj' so that only one pole of the spindle is shown.
 
 
 
Fig. 30. Secondary Spermatocyte Typical equatorial plate stage. The spindle is quite spherical; mantle fibers are shown.
 
 
 
Fig. 31. Secondary Spermatocyte. Anaphase. Daughter-Vs passing to the poles of the spindle.
 
 
 
 
 
 
 
SPERMATOGENESIS OF DESMOGNATHUS FUSCA. B. F. KINGSBURY.
 
 
 
 
 
 
 
 
 
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AMERICAN JOURNAL OF ANATOMY--VOL.
 
 
 
 
 
 
 
SPERMATOGENESIS OF DESMOGNATHUS FUSCA. B. F. KINGSBURY.
 
 
 
 
 
 
 
PLATE IV.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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AMERICAN JOURNAL OF ANATOMY-VOL. I.
 
 
 
 
 
 
 
34
 
 
 
 
 
 
 
35
 
 
 
 
 
 
 
B. F. Kingsbury \3o
 
 
 
Fig. 32. Secondary Spermatocyte. Telophase. Chromosomes closely massed at the poles. Division of the cell-body completed. Mid-body and spindle-relic seen.
 
 
 
Fig. 33. Spermatid. Oblique polar view of a daughter-nucleus of the division of Spermatocyte II, at the " dispirem " stage, when the chromosomes again expand.
 
 
 
Fig. 34. Spermatid. Transection of the nucleus at the stage of Fig. 33, showing the legs of the V's cut across.
 
 
 
Fig. 35. Spermatid, fully formed. The chromatin is irregularly distributed in small masses. The two centrosomes at the edge of the cell-body, near the " Sphere."
 
 
 
 
 
 
 
ON THE ORIGIN OF THE PULMONAEY AETERIES IN
 
 
 
MAMMALS.
 
 
 
BY
 
 
 
JOHN LEWIS BREMER, M. D. From the Embryological Laboratory of Harvard Medical School.
 
 
 
With 9 Text Figukes.
 
 
 
The material used in prepai'ing this paper is from the collection of the Laboratory of Embryology at the Harvard Medical School; the original numbers of the series and sections have been preserved. The drawings are from reconstructions, and represent, as it were, casts of the lumina of the arteries without reference to the thickness of their walls. They are all of the same magnification (X 80 diameters); the arteries are seen from behind, and the pulmonary arches can be followed until they unite to form the truncus pulmonalis, or until, as in Fig. 1, they enter the heart itself.
 
 
 
In 1857, H. Rathke published his monograph, "Die Aortenwiirzeln und die von ilinen ausgehenden Arterieii der Saurier," in which appear the diagrams of the aortic arches now made more familiar by their reproduction by Kolliker, Hertwig, Quain, and many others, Avith or without slight modifications. In these diagrams the right and left pulmonary arteries are represented as arising, in lizards and birds, from their respective fifth, or pulmonary, arches, while in snakes and in mammals one fifth arch alone gives rise to both pulmonary arteries, the other arch becoming obliterated; in snakes the right pulmonary arch remains, in mammals the left. Since this monograph there has been, so far as I know, no special investigation into the origin of the pulmonary arteries.
 
 
 
The earliest buds of the pulmonary arteries, in the rabbit, appear in embryos of about 4.0 mm., one bud from each of the puhuonary arches, on the mesial aspect of each. The growth of these buds is at first backward, then downward and inward, giving a small twist, Figs. 1, 2, 3, x, near the proximal end of the pulmonary artery, which seems peculiar to the rabbit. From this twist, the course is straight downward, on each side of the trachea and slightly anterior to it, to the lungs, where the usual branches are given off. During this downward course no branches
 
 
 
 
 
 
 
138 On the Origin of the Puhnonary Arteries in Mammals
 
 
 
are seen. As the arteries increase in length their proximal ends, where they arise from the aortic arches, seem to approach each other actually, as can be seen by comparing Figs. 1, 2 and 3. The mechanism of this change is probably as follows: the truncus pulmonalis is at first short, soon dividing into its two branches, the right and left fifth aortic arches; as it becomes twisted around the aorta, following the turn of the heart, the truncus pulmonalis pulls on the two fifth arches, which are thus crowded together, forming a double tube, and at the same time the two pulmonary arteries, arising from the mesial aspect of the two arches, are
 
 
 
 
 
 
 
 
 
Fig. 1.
 
 
 
 
 
 
 
Fig.
 
 
 
 
 
 
 
Fig. 1. Kabbit of 5.0 mm. Frontal series No. 14S, sections 250-261. X 80 diameters. A, B, left and right pulmonary arches, opening directly into heart, H. C, D, pulmonary arteries. F, G, fourth aortic af-ches, opening into heart.
 
 
 
.Fig. 2. Eabbit of 8.0 mm. Frontal series No. 154, sections 291-311. X 80 diameters. E, junction of A and B. H, valve of heart.
 
 
 
 
 
 
 
brought nearer together. By fusion of the two parallel arches the truncus pulmonalis is increased in length, and its two branches shortened; this fusion may extend until the origins of the pulmonary arteries are very near the bifurcation, or until the left artery springs actually from the bifurcation.
 
 
 
The diameter of the pulmonary arteries remains small in comparison to their increasing length, as one might expect from the slight necessity
 
 
 
 
 
 
 
John Lewis Bremer
 
 
 
 
 
 
 
139
 
 
 
 
 
 
 
of blood in the unused lungs. The left pulmonary arch grows rapidly in diameter as well as in lengtli, while the right becomes entirely obliterated beyond the point where the pulmonary artery arises, leaving finally no trace of its existence; from this point to the junction with the left arch to form the truncus pulmonalis, the right arch remains of the same calibre as the pulmonary artery. The small twist marking the origin of the pulmonary artery gradually straightens out, and the whole right side, i. e. the anterior portion of the fifth arch and the
 
 
 
 
 
 
 
 
 
 
 
Fig. 3.
 
 
 
 
 
 
 
Fig. 4.
 
 
 
 
 
 
 
Fig. 3. Habbit of 10.0 mm. Frontal series No. 157. sections 347-367. X 80 diameters. ,
 
 
 
Fig. 4. Pig- of 7.8 mm. Frontal series No. 430, sections 270-297. X 80 diameters.
 
 
 
 
 
 
 
pulmonary artery, being now unattached to the right dorsal aorta, is drawn to the left by the larger left aortic arch, which is constantly tending to become straight. As a result of these changes, the left pulmonary arch seems to give rise, at about its mid-point, to two arteries, with their origins close together (or there may be a very short common stem); the right one, the longer of the two, arising anteriorly, and taking its course
 
 
 
 
 
 
 
140 On the Origin of the Pulmonary Arteries in Mammals
 
 
 
at first almost horizontally across to the right side of the trachea, then bending down toward the right lung, the left pursuing a straight course to the left lung. The portion of the left fifth arch posterior to the pulmonary arteries becomes later the Ductus Botalli, and is closed at birth.
 
 
 
It will be seen from this description that, in the actual origin of the pulmonary arteries, the rabbit is identical with birds and reptiles, as drawn by Rathke and verified by many other writers. In the rabbit, as well as in birds and reptiles, one pulmonary artery arises from each pulmonary arch, but in birds and reptiles the growth of these arches is equal until birth, so that the picture is symmetrical, a fifth arch, a pulmonary artery, and a Ductus Botalli on each side; while in the rabbit the left pulmonary arch alone remains until birth, and the picture is distorted. It was this distortion, this early disappearance of that portion of the right pulmonary arch posterior to the pulmonary artery, which made possible the diagram of Eathke, and his statement that '^in mammals the left fifth aortic arch at a very early period of embryonic life sends out from about its mid-point a small branch which is intended for both lungs, and posterior to its place of origin divides into two twigs."
 
 
 
Eathke examined, of mammals, the pig, sheep, and hare, with special reference to the pulmonary arteries.^ In the rabbit, cat, and in the few human embryos within my reach, I have found the pulmonary arteries to arise as I have stated, that is, in the beginning, symmetrically, one from each pulmonary arch. In Eathke's original diagrams the arteries of lizards and of birds arise symmetrically, as do they also in the frog, as described by Gaupp.^ Of snakes, according to Stannius and others, while most species have only the right lung, and therefore only the right pulmonary artery, in adult life, some species have the left lung and left artery alone, and others even both lungs and both arteries, more or less fully developed. In two cases, recently cited by F. Hochstetter,^ of Trojudonotus tessellatus (a species with only the right lung normally developed), a slender artery was found, which, although finally ramifying in the oesophageal wall, resembled in origin and course a left pulmonary artery. From these facts it seems probable that in the younger snake embryos, of all species, both pulmonary arteries will be found present. If this is the case, the proof will be strong that in all
 
 
 
iMiiller's Archiv, 1848, p. 276.
 
 
 
2 Anatomic des Frosches, diagram, p. 285.
 
 
 
3 Morpliologisches Jahrbucb, 1901, p. 419.
 
 
 
 
 
 
 
John Lewis Bremer
 
 
 
 
 
 
 
141
 
 
 
 
 
 
 
vertebrates with lungs the laulmonary arteries originate one from each pulmonary arch, and "that Rathke's diagrams, though describing perfectly the adult and late embryonic conditions, are, as regards this origin, incorrect.
 
 
 
In the pig, one of the animals examined by Rathke, although the symmetrical origin is preserved, one pulmonary artery arising from each pulmonary arch, and although the ultimate appearance, that of both
 
 
 
 
 
 
 
 
 
Fig. 5.
 
 
 
 
 
 
 
 
 
Fig. 6.
 
 
 
 
 
 
 
Fig. 5. Pig of 9.0 mm. Frontal series No. 54, sections 462-502. X 80 diameters.
 
 
 
Fig. 6. Pig of 11.0 mm. Sagittal series No. 8, sections 96-113. X 80 diameters.
 
 
 
 
 
 
 
pulmonary arteries arising from the mid-point of the left pulmonary arch, is the same as in the other mammals I have examined, the intermediate steps are different, as is shown in Figs. 4 to 9. Instead of remaining comparatively parallel, as in the rabbit, the pulmonary arteries, after attaining considerable length (pig of 7.8 mm.), bend toward
 
 
 
 
 
 
 
142 On the Origin of the Puhnonary Arteries in Mammals
 
 
 
each other, and instead of remaining without branches (except those developed later in the lungs) send out buds, each toward the other artery. Fig. 4, x, y. This bending toward the median line of these two
 
 
 
 
 
 
 
 
 
Fig. 7. Pig of 12.0 mm. Transverse series No. 5, sections 366-404, X 80 diameters.
 
 
 
 
 
 
 
 
 
Fig. S. Pig- of 12.0 mm. diameters.
 
 
 
 
 
 
 
Frontal series No. 6, sections 429-464. X SO
 
 
 
 
 
 
 
pulmonary arteries is perhaps caused by the great growth of the auricles of the heart in the pig. Both processes continue until in a pig of 9 mm. there is at least one connection between the right and left pul
 
 
 
 
 
John Lewis Bremer
 
 
 
 
 
 
 
143
 
 
 
 
 
 
 
nionary arteries, often two, as is suggested in Fig. 5, x, y, while in a pig of 11.0 mm. the two arteries, along a considerable part of their length, have merged into one channel. Fig. 6. Meanwhile the upper or proximal part of the right pulmonary artery, which often shows signs of
 
 
 
 
 
 
 
 
 
Fig. 9. Pig of 20.0 mm. diameters.
 
 
 
 
 
 
 
Frontal series No. 61, sections 270-279. X 80
 
 
 
 
 
 
 
irregularity, such as a double origin. Figs. 4 and 7, D, ceases to increase in size, then grows smaller, and soon becomes obliterated, so that all the blood to both lungs flows through the left pulmonary ai-tery. This gradual change is shown at D, Figs. 5, 6 and 7, and in Fig. 8, where only the remains of the right artery are seen. For a little while after the obliteration of the lumen, a cord of connective tissue marks the
 
 
 
 
 
 
 
144 On the Origin of the Pulmonary Arteries in Mammals
 
 
 
former course of the right pulmonary artery, but soon even this disappears.
 
 
 
Along with this change, another, common to all mammals, has taken place, namely, the obliteration of the right pulmonary arch; but this is not the cause of the obliteration of the right pulmonary artery, since the lumen of the latter is the first to close. Fig. 8. Still another change is seen, as in the rabbit, in the lengthening of the truncus pulmonalis at the expense of the two pulmonary arches, and the consequent apparent movement of the left pulmonary artery toward the right pulmonary arch. In the pig, considerable variation seems to occur in regard to the stage of growth at which this last mentioned change takes place, as may be seen by comparing Figs. 6 and 7, where the distance between the points of origin of the pulmonary arteries is about the same in two pigs of 11.0 and 12.0 mm., respectively, and Fig. 8, where the distance is much greater, although the length of the embryo is again 12.0 mm. It will be seen that of the two 12.0 mm. pigs, one still has, and one has already lost, the connection of the right pulmonary artery.
 
 
 
Whether all ungulates, or only pigs, have this odd method of arriving at the adult relations of the pulmonary arteries, I do not as yet know; certainly there is nothing like it in the rabbit, the cat, the dog, or in the human embryos within my reach.
 
 
 
 
 
 
 
THE DEVELOPMENT OF THE AKM IN MAN.
 
 
 
BY
 
 
 
WARREN HARMON LEWIS, M. D.
 
 
 
Instructor in Anatomy, The Anatomical Laboratory, Johns Hopkins University.
 
 
 
With 3 Plates and 14 Text-Figures.
 
 
 
The wandering of the trapezius and the latissimus dorsi and also of muscles in the ahdominal wall was noted by Dr. Mall ' several years ago. At his suggestion I undertook, in the spring of 1897, a more careful study of these and other changes in the development of the arm region in man. Similar studies were undertaken later by Dr. Bardeen on the leg and body wall. We have embodied many of the important points obtained from our studies in a joint article ^ which appeared in the first number of this journal, and of which this present article may be considered a continuation.
 
 
 
In the present paper I purpose to consider the origin of the tissue which fills the arm bud, the entrance of nerves into this tissue and its differentiation into skeleton, ligaments, muscle and tendon, and finally the growth and wandering of these structures until practically the adult conditions are present.
 
 
 
I wish here to express my most sincere thanks to Dr. Mall for his constant interest and many suggestions, and also for the use of the valuable embryological material in this laboratory.
 
 
 
The embryos studied, with the exception of the one belonging to Dr. Buxton, of Cornell University, are in the collection belonging to Dr. Mall. Most of those considered in this paper are tabulated on page 2, Vol. I, of this journal.
 
 
 
From the serial sections of embryos CLXIII, CIX, XLIII and XXII I have made reconstructions of the arm region after the Born method. The arm region in Plates III to IX in the paper by Bardeen and Lewis
 
 
 
' Mall, Development of the ventral abdominal walls in man. Jour, of Morph., Vol. XIV, 1898.
 
 
 
2 Bardeen and Lewis, Development of the limbs and body wall in man, Am. Jour, of Anat., Vol. I, p. 1.
 
 
 
 
 
 
 
14:0 The Development of the Arm in Man
 
 
 
are drawn from these models, and are to be consulted in connection with the descriptions given in this article. Dr. Mall's embryos are stained in alum caraiine or alum cochineal.
 
 
 
 
 
 
 
PART I.
 
 
 
Relation of the Myotomes to the Arm Bud.
 
 
 
literature.
 
 
 
Considerable study and perhaps even more theorizing has been done on the relation of the myotomes to the musculature of the limbs. The present state of our knowledge upon this subject is far from satisfactory, especially in the higher vertebrates. The great difficulty or impossibility in many cases of distinguishing between the cells at the ventral edge of the myotomes and those in the neighboring portion of the limb bud renders the problem very difficult. Experimental work, such as has been done by Byrnes,^ may lead to a clear understanding of the relations in the lower vertebrates. The majority of workers appear to have been able to trace myotomic processes into the limbs. Mollier ■* has shown in Selachians that myotome buds enter into the fin anlage or pectoral plate. From these buds are developed the muscles. From the mesoderm between the buds are developed the fin rays. Braus " also shows myotome buds going into the pelvic fins of Selachians. Dohrn * finds two buds from each myotome, an anterior and a posterior, entering the fin anlage, these he believes form the fin muscles. Balfour^ holds that the limb muscles in Elasmobranchii come from muscle plate buds. Harrison * has shown that in teleosts the pectoral fins are derived wholly from the somatopleure and that the myotomes take no part in the
 
 
 
3 Byrnes, Experimental studies on the development of the limb muscles in Amphibia, Jour, of Morph., Vol. XIV, 1898.
 
 
 
Mollier, Zur Entwickelune: der Selachierextremitaten, Anat. Anz., Vol. VII, 1893, p. .3.51. Die paarigen Extremitaten der Wirbelthiere, Anal Hefte, Bd. Ill, 1893.
 
 
 
5 Braus, Beitrage zur Entwi-ckeluno; der Muskulatur und des peripheren Nervensystem der Selachier, Morph. Jahr., Bd. XXVII, 1899, p. .501.
 
 
 
6 Dohrn, Studien zur Urgeschichte des Wirbelthier Korpers, VI, etc., Mittheil. aus der Zool. Station zu Neapel, Bd. V, 1884.
 
 
 
1 Balfour, Comp. Emb., 3nd. Ed., 1885.
 
 
 
8 Harrison, Die Entwickelung d. unpaaren und paaren Flosseu der Teleostier, Archiv f. Mikr. Anat., Bd. XLVI, Ileft 3, 1895.
 
 
 
 
 
 
 
Warren Harmon Lewis 147
 
 
 
formation of these fins. Boyer^ believes elements from the peripheral layer of certain myotomes are contributed to the pectoral plate which comes from the somatopleure. Corning" believed in 1894 that the pectoral fins in teleosts received muscle-plate buds, but he has since come to the conclusion " that these fins in teleosts do not receive such buds, and agrees with Harrison that the myotomes take no part in the formation of the pectoral fims. Kaestner " was not able to show in Anura that the myotomes take any part in the formation of the limbs, though he believes they do at a very early period. Field " believes that the elements which form the muscle of the extremities in Amblystoma are separated at a very early age from the ventral part of the myotome. Byrnes " work, both her embryological and experimental studies, shows that " the myotome processes, as such, take no part in the formation of the limbs . . . The limbs are of somatopleuric origin, i. e., the muscle, cartilage, and connective tissue." Goette " believes that the limb muscles in Bombinator develop from the outer layer of the muscle plate. Van Bemmelen" believes that in the lizard the limb muscles are derived from the myotome buds. Mollier" finds cells from myotome buds go into the arm anlage in Lacerta niuralis. According to Patterson," the limbs of the chick are derived wholly from the somatopleure. He does not find muscle buds or homologous structures entering into the limbs. In a recent paper by Maj," the conclusion is reached that the myotomes
 
 
 
9Boyer, The mesoderm in Teleosts, etc., Bull. Museum Comp. Zool., Harvard Univ., Vol. XXIII, 1893.
 
 
 
lo Corning, Ueber die Ventralen Urwirbelknospen in der Brustflosse der Teleostier, Morph. Jahr., Bd. XXII, Heft 1, 1894.,
 
 
 
"Corning, Ueber die Entwickelung der Zungen Musculatur bei Reptilien, Anat. Anz. (Gesellschaft) 1895.
 
 
 
i^l^aestner, Extremitaten- und Bauehmusculatur bei Auuren, Arcbiv f. Anat. und Phys. (Anat. Abtheil.), Hefte .5 und 6, 1893.
 
 
 
13 Field, Die Vornieren Kapsel, ventrale Musculatur und Extremitiitenanlagen bei den Amphibien, Anat. Anz., Bd. IX, No. 33, 1894.
 
 
 
14 Byrnes, Op. cit.
 
 
 
13 Goette, Die Entwickelungsgeschichte der Unke., 187.5.
 
 
 
16 Van Bemmelen, Ueber die Herkunf t der Zungen- und Extremitiitenmusculatur bei Eidechsen, Anat. Anz., Bd. IV, 1889.
 
 
 
1' Mollier, Die paarigen Extremitaten der Wirbelthiere. Anat. Hefte, Bd. Ill, Heft VIII; Bd. V, Heft XVI.
 
 
 
18 Patterson, On the fate of the muscle plate and the development of the spinal nerves in birds and mammals. Quart. Jour. Micr. Sci., Vol. XXVIII, 1887.
 
 
 
"Maj, Contribute alio studio dello sviluppo della musculatura negli arti. Osservazioni sur polio (Gallus domesticus), Dal Bollettino della Soc. Med.-Chir. di Pavia, 1901.
 
 
 
12
 
 
 
 
 
 
 
148 The Development of the Arm in Man
 
 
 
do enter the limbs in the chick. He pictures the ventral end of the myotome entering the limb in company with the nerve and splitting into dorsal and ventral lamellae. Fischel '" believes that in birds and mammals myotome cells mix in the limb bud and give rise to the muscles. There is a diffuse entrance of cells from the myotomes but not of myotome buds. In a section of a human embryo of the fourth week he pictures these myotome cells as forming a peripheral layer around the arm bud and even extending into the somatopleure. The rest of the bud comes from the somatopleure. Kollmann '^ pictures in a very diagrammatic manner the downgrowth of the outer lamella of the muscle plate into the arm bud where it lies between the ectoderm and the mesenchymal core.
 
 
 
In the lower vertebrates it would appear therefore that the limb muscles may arise either from distinct buds of the myotome or they may arise independently of the myotomes from the somatopleure. In the higher vertebrates no distinct myotome buds have been traced into the limbs. Myotome cells are supposed by most observers to enter the limbs and take part in the formation of the muscles.
 
 
 
The question as to whether in man the muscles of the arm are derived from cells of the myotomes which have migrated into the arm bud at a very early period, I have not been able to determine satisfactorily. Neither am I convinced by the work of Pischel or Kollmann that the myotomes take such a part in the formation of the arm. Their pictures are quite unlike any of the conditions found in the human embryos which I have studied.
 
 
 
EAELT STAGES OF THE AEM BUD.
 
 
 
In Embryo CLXIV, 3.5 mm. in length, there are thirteen myotomes. No signs of an arm bud are present. The myotomes are sharply limited and do not give off any cells into the region where the arm bud is soon to sprout. Cells appear to be migrating from the myotome towards the chorda. The somatopleure, hov/ever, shows a proliferation of the cells lining the ccelom. This is very close to the place where the arm bud is soon to appear and lateral to the Wolffian duct and tubules. (See Fig. 1.)
 
 
 
5io Fischel, Zur Entwickelung der ventralen Rump- und Extremitiiteumusculatur der Vogel und Saugethiere, Morph. Jahr., Bd. XXIII, 1895.
 
 
 
21 Kollmann, Die Rumpfsegmente menschl. Embryonen von 13 bis 35 Urwirbel, Archiv f. Anat. und Pliys. (Anat. Abtbiel.), 1891.
 
 
 
 
 
 
 
Warren Harmon Lewis
 
 
 
 
 
 
 
149
 
 
 
 
 
 
 
Embryo XII," 2.1 nun. in length, has fourteen myotomes and is slightly older than CLXIV. The first definite signs of an arm bud are here noticed by a slight swelling ventrolateral to the myotomes in the lower cervical region. Its position is seen in Fig. 2. The origin of the cells which cause this swelling I am not able to determine, though there are suspicious looking processes from the myotomes. No spinal nerves are present.
 
 
 
Embryo LXXYI is 4.5 mm. in length and about three weeks old. Between embryos XII and LXXVI there is quite a gap. There are
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
' IVolffian duct.'^ '(
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
^yi-^^s-!^
 
 
 
 
 
 
 
Fig. 1. Cross section through the eighth mj'otome of embryo CLXIV. X 100 diameters.
 
 
 
35 myotomes. The arm bud is quite large and filled with uniformly and closely packed cells whose nuclei take a deep stain with the alum carmine. A few thin-walled blood-vessels are scattered here and there. The base of the arm bud lies opposite the fifth cervical to the
 
 
 
 
 
 
 
^- Dr. Mall considers embryo CLXIV slightly older than embryo XII. The greater length of CLXIV he accounts for by a straightening of the body of the embryo through mechanical injury to the ovum. See Mall, On the development of the human diaphragm, The Johns Hop. Hosp. Bui., Vol. XII, 1901, p. 160.
 
 
 
 
 
 
 
150
 
 
 
 
 
 
 
The Development of the Arm in Man
 
 
 
 
 
 
 
first thoracic intervertebral disks. The cells of the median lamella of the myotomes have been converted into muscle fibers. The myotomes are fairly well defined and do not show buds, or, so far as I can determine, migration of their cells into the arm bud. The general trend of the growing ventral end of the myotome is not out towards the arm bud, biit ventrally towards the ccelom. It will be seen in Fig. 3 that a considerable portion of the root of the arm lies close to the dorsal end of the ccelom and that a proliferation of cells from its lining might easily contribute to the arm tissue. The spinal nerves are not formed though a few anterior root fibers appear to pass directly lateral from the anterior horn. Most of them are lost in the surrounding mesen
 
 
 
 
 
Sth cervical =«j'S^(S%^,ob jS^ muscle plate .•^WiJfel^tS .felST»=
 
 
 
 
 
 
 
 
 
 
 
 
 
somaiopleure
 
 
 
Wolffian duct.
 
 
 
 
 
 
 
V#*%.
 
 
 
 
 
 
 
muxcle plate AMii'tif-^-^;,-^
 
 
 
■^'■ISk '■' ■"■■'\ ' '^'
 
 
 
ai-m bud j/}^J}'^^M,>'X'%-('-% '""^ A
 
 
 
■'-^~r'.'X%J-i-;y j'
 
 
 
 
 
 
 
border vein
 
 
 
 
 
 
 
Fig. 3.
 
 
 
 
 
 
 
 
 
•I
 
 
 
 
 
 
 
Fig. 3.
 
 
 
Fig. 2. Cross section through the eighth cervical myotome of embryo ,XII. X 100 diameters.
 
 
 
Fig. 3. Cross section through the eighth cervical myotome of embryo LXXVI. X 50 diameters.
 
 
 
 
 
 
 
chyma, a few, however, appear to reach the group of muscle fibers on the median surface of the myotomes. This is an exceedingly important stage. The arm bud is filled with a peculiar closely packed mesenchyma which, so far as I am able to judge, is the same sort of tissue from which at a later stage the skeletal and muscular tissues differentiate. This tissue fills the arm before the nerves are developed, and if there are cells from the myotomes present they have migrated there without the nerve supply.
 
 
 
In Embryo LXXX, 5 mm. in length, the arm bud has increased con
 
 
 
 
 
Warren Harmon Lewis
 
 
 
 
 
 
 
151
 
 
 
 
 
 
 
siderably in size. The cells which fill it resemble those in LXXVI, but are more closely packed together and stain deeper. There is no differentiation of this mesenchyma. Thin-walled blood-vessels are numerous, the border vein (Eandvene of Hochstetter) is present. Numerous mitotic figures indicate that there is a rapid proliferation of the mesenchymal cells. The myotomes are fairly well defined, though in places the ventral end is not always sharp and the possibility of wandering of cells
 
 
 
 
 
 
 
 
 
tj^ '" ^ I \^ ,intei\ertebral disc ^-Ur- i^\i'i^^; %\ ^tlnen-ical
 
 
 
 
 
 
 
 
 
bolder vein
 
 
 
 
 
 
 
Fig. 4. Cross section through the eighth cervical myotome and nerve of embryo LXXX. X 50 diameters.
 
 
 
 
 
 
 
from it into the arm bud cannot be denied. The almost constant presence of several blood-vessels at the ventral end of the myotome would interfere somewhat with that process. The spinal nerves have grown out some distance from the cord, they however pass by the median side of the myotomes without sending branches into them. The distal ends of the nerves reach beyoud the myotomes. The lower four cervical and first dorsal end at the root of the arm. As will be seen in Fig. 4, this end of the nerve spreads out somewhat and is surrounded here as well
 
 
 
 
 
 
 
152
 
 
 
 
 
 
 
The Development of the Arm in Man
 
 
 
 
 
 
 
as along its course by loose mesenchyma which is quite different from that in the arm bud, but like that which lies between the myotome and the aorta, and through which the nerve has pushed, probably carrying some of this tissue with it and before it. The first beginnings of the cervical and brachial plexuses are present in the form of anastomoses of the brush-like ends of the first cervical to the third thoracic. It is an interesting fact that at this stage the upper thoracic myotomes extend to a considerable distance ventral of the ventral union of the arm bud
 
 
 
 
 
 
 
sp. gang_ y/yjjTrk:^^
 
 
 
 
 
 
 
'neural process
 
 
 
 
 
 
 
condensed m^s
 
 
 
 
 
 
 
Sth muscle Plateilpf^.^^V/'h/ •
 
 
 
Sth nerve #i^FH^>^^■^■ ' ' ' ^ " / Ttli nerve ^,,jt^.i,Sl-im^li\l:^>i:^i^
 
 
 
 
 
 
 
 
 
 
 
 
 
border ^eln
 
 
 
 
 
 
 
intervertebral disc
 
 
 
 
 
 
 
^ -jom iiopleure
 
 
 
 
 
 
 
Fig. 5. Cross section through the eighth cervical myotome of embrj^o II. X 50 diameters.
 
 
 
 
 
 
 
with the body wall. The tip of one of these thoracic myotomes is seen in Fig. 4. In the cervical region, on the contrary, the myotomes are much shorter and do not extend so far ventrally.
 
 
 
In the Buxton embryo about the same conditions exist as in LXXX. Embryo Buxton is 5 mm. in length and about 25 days old. It was stained in haematoxylin and eosin, thus bringing out the muscle plates even better than with the alum carmine in which Dr. Mall's embryos were stained.
 
 
 
Eiribryo II is 7 mm. in length, and about four weeks old. It shows
 
 
 
 
 
 
 
Warren Harmon Lewis 153
 
 
 
some advances over LXXX. The arm bud is filled with the closely packed mesenchyma similar to that seen in LXXVI and LXXX, with this important difference however that in the center of the mass the cells are somewhat more closely packed than at the periphery, and represent the first beginnings of differentiation in the arm. This probably represents the hnmerns. It will be seen from Fig. 5 that the peculiar tissue in the arm bud has spread some distance into the membrana reuniens. Here, as in the previous stages it is impossible to determine whether cells may not go from the myotomes into the arm bud. The nerves, as in LXXX, pass along the median side of the myotome without sending branches into the myotome or, so far as I can determine, taking a portion of the myotome along into the arm bud. They extend farther into the arm than in the preceding stage. The beginning of the cervical and brachial plexus is even more marked, and is formed by anastomoses of the brush-like ends of the first cervical to the second thoracic. The root of the arm lies at the level of sixth cervical to the second thoracic intervertebral discs.
 
 
 
SUMMAET.
 
 
 
The tissue from which the muscles, ligaments, tendons, and cartilages of the arm develop is present at a very early stage in the arm bud, probably by the beginning of the third week. No distinct myotome buds take part in the formation of this tissue. That cells from either or both myotomes and somatopleure enter into this early arm mesenchyma cannot be determined from the material at my disposal. If cells do migrate from the myotomes, they apparently do so independently of the nerves which are not present until the tissue is formed and fills the arm bud. The first beginnings of differentiation of this peculiar tissue which fills the arm bud occur durine; the fourth week.
 
 
 
 
 
 
 
PART 11.
 
 
 
The Diffeeentiation of the Mesenchyma of the Aem Bud into muscrlae and skeletal elements, and the
 
 
 
GeOVYTH of THE NeEVES.
 
 
 
Embryo CLXIII.
 
 
 
The first indication of a differentiation of the mesenchyma of the arm bud occurs as we have seen in an embryo of about four weeks. In
 
 
 
 
 
 
 
154
 
 
 
 
 
 
 
The Development of the Arm in Man
 
 
 
 
 
 
 
our next stage, embryo CLXIII, quite marked changes have taken place.
 
 
 
Embryo CLXIII is 9 mm, in length, and about four and one-half weeks old.
 
 
 
In order to gain a clear conception of the form and various relations of the structures in this embryo, I found it necessary to construct a model, and in order to do so it was necessary to draw sharp lines about the various structures when in reality there were no sharp limits. One mass often shading off into another, while the central portion of each was very distinct. Thus, in most places the skeletal core of the arm
 
 
 
 
 
 
 
"eural process
 
 
 
 
 
 
 
 
 
Fig. 6. Skeleton and nerves of the arm region in embryo CLXIII. X 40 diameters.
 
 
 
 
 
 
 
shades off into the surrounding premuscle tissue, or as in the region of the hand plate into the primitive condensed mesenchyma, filling the distal end of the arm bud. The same was often true of the various premuscle masses. The main portion of these are quite distinct, but they often shade off into each other and into the surrounding mesenchyma. We find in this embryo in the arm region that the premuscle masses most closely associated with the trunk are the farthest advanced, those connecting the arm and trunk next, and the least devel
 
 
 
 
 
Warren Harmon Lewis 155
 
 
 
oped is the general arm jjremuscle slieatli, especially its distal portion.
 
 
 
I have given the name premuscle to various Jiiasses of condensed mesenchyma from which at a later period I believe muscle develops by histogenetic changes of the cells.
 
 
 
The Skeletal System. — There is no cartilage at this stage. The skeleton is composed of condensed or closely packed mesenchyma which takes a deeper stain than the surrounding tissue.
 
 
 
The vertebral column consists in the arm region of the intervertebral discs, and their nenral processes which lie in the posterior third of each segment. Between the discs is a loose mesenchyma, the cells of which, as well as those in the disc, have a concentric an-angement about the chorda.
 
 
 
The ribs spring from the adjacent portions of the disc and neural process. A line of separation is visible. They take a ventrolateral direction into the body wall. The sixth and seventh cervical intervertebral discs have short rib-like processes.,
 
 
 
In the arm the exact limits of the skeletal strnctures cannot be determined as this central core which is easily recognized, shades off into the surrounding mesenchyma, which develops into muscle. The scapula is a quadrilateral mass at the level of the fourth and fifth cervical discs. There are no indications of coracoid, acromion or spinous processes. The scapula is continuous with the humerus, which is a cylindrical mass occupying the center of the proximal portion of the arm bud. Practically all of it lies at a level anterior to the first rib. At the level of the first rib the humerus is continuous with tlie ulna and radius. There is a slight flexion of the forearm. They are short and thick. The ulna is the larger and is more directly a continuation of the humerus. Partially surrormding the ulna and radius is a plexus of blood-vessels which helps to outline them. The continuation of this plexus is seen in Fig. 8. Both ulna and radius are continuous, with the very ill-defined mass of condensed tissue which lies in the center of the distal end of the arm bud. This rather thin plate composed of cells more closely packed together than those of the surrounding tissue, shows no signs of division into the various elements of the hand. I name it the hand plate.
 
 
 
The Muscular System. — The muscle plates are fused into a continuous column. Indications of segmentation remain. This column lies close and lateral to the neural processes. In the cervical region it ends abruptly at the brachial plexus. In the costal region, however, it extends ventrally into the body wall, between, and partially surrounding the ribs. It ends ventrally beyond the tips of the ribs. The muscle
 
 
 
 
 
156
 
 
 
 
 
 
 
The Development of the Arm in Man
 
 
 
 
 
 
 
plate system is easily distingnished from the surrounding tissues by its fibrillation.
 
 
 
Lateral to the muscle-plate system are ill-defined masses of condensed tissue without fibrillation, but from which muscles differentiate.
 
 
 
Lateral to the anterior six ribs lies the lateral premuscle mass. It occupies most of the space between the costal portion of the muscle-plate system and the integument. It shades off into the surrounding loose mesenchyma everywhere, but at the anterior end, at about the level of the first intercostal space, it splits into four divisions which pass anteriorlv.
 
 
 
 
 
 
 
 
 
Fig. 7. Outline of the arm region of embryo CLXIII from Plate III. Bardeen and Lewis, Vol. I, No. 1, this Journal. X 15 diameters.
 
 
 
 
 
 
 
The first or dorsal division lies lateral to the muscle-plate column, and extends to the level of the fifth cervical disc.
 
 
 
The second, third and fourth divisions correspond so closely with the position in which I find certain muscles in the next stage, and as they also have the same nerve supply as the muscles into which I believe they develop, that I have called them in order: The (2) levator scapulae and serratus anterior, the (3) latissimus dorsi and teres major, and the (4) pectoral premuscle masses.
 
 
 
The second division, the levator scapulce and serratus anterior premuscle mass, ventral to the first and opposite the ventral portion of the muscle-plate column is fairly well defined. It extends into the upper cervical region. It lies in a more median plane than the scapula, and at this stage is in no way attached to it.
 
 
 
 
 
 
 
Warren Harmon LeAvis
 
 
 
 
 
 
 
157
 
 
 
 
 
 
 
The third division, the laUssimus dorsi and teres major premuscle mass, passes anteriorly along the dorsal side of the brachial plexus and becomes continuous with the arm premuscle sheath at the proximal
 
 
 
 
 
 
 
trapezius intia. hjoid apular
 
 
 
 
 
 
 
lev. scap. premuscle mass muscle plate column / neural process
 
 
 
spinal gang.
 
 
 
 
 
 
 
pectoral
 
 
 
 
 
 
 
premuscle sheatl
 
 
 
 
 
 
 
 
 
,5th nerve
 
 
 
phrenic nerve
 
 
 
brachial plexus sympathetic
 
 
 
diaphragm disc
 
 
 
 
 
 
 
border vein
 
 
 
hand plate
 
 
 
 
 
 
 
tip 4th rib
 
 
 
 
 
 
 
Fig. 8. Ventral view of the arm region of embryo CLXIII. X 40 diameters.
 
 
 
 
 
 
 
portion of the humerus. The humeral end is thicker and broader, and continuous also with the arm premuscle sheath about the scapula.
 
 
 
The fourth division, the pectoral premuscle mass, passes ventral to the brachial plexus and joins the arm premuscle sheath near the proximal end of the humerus. This pectoral premuscle mass is continuous medially with an irregular mass of condensed tissue which extends to the
 
 
 
 
 
 
 
158 The Development of the Arm in Man
 
 
 
base of the tongue. The cephalic portion of it is supplied by two nerves, one a branch of the first and second cervical, and the other a branch of the third cervical nerve. These two branches form a loop on the siu'face of the mass. They correspond to the ramus descendens n. hypoglossus and ramus communicans hypoglossus uniting to form the ansa hypoglossus. Hence I have called this the infra-hjoid premuscle mass. I have not been able to determine the fate of the condensed tissue on the median side of the pectoral premuscle mass, and caudal to the infrahyoid mass. The phrenic nerve ends very close to it, and very likely this is diaphragm premuscle mass.
 
 
 
The rJwmhoid premuscle mass lies lateral to the second division of the lateral premuscle mass and is an ill-defined plate of condensed tissue. It lies at the level of the fifth cervical vertebra and receives a branch from the fifth cervical nerve, arising in connection with a nerve to the levator scapulae mass.
 
 
 
The caudal end of the trapezius premuscle mass is seen in Fig. 7, lateral to the levator scapulse mass. The main portion of the trapezius premuscle mass lies opposite the cephalic four cervical vertebrae. It is supplied by the spinal accessory and communicating branches from the first four cervical nerves.
 
 
 
Arm premuscle slteath. — The skeletal core of the arm is surrounded by a mass of tissue which shows no signs as yet of splitting into separate masses. Along the median side of the humerus this sheath is interrupted by the entrance of the brachial plexus and nerves. In places the sheath is separated from the skeletal core by blood-vessels, but in most places no sharp line of separation can be seen. Toward the distal end of the arm the sheath merges into the more primitive mesenchymal tissue which fills most of the distal end of the arm. In Fig. 8 the ,distal limit of the premuscle sheath is indicated, a portion of the primitive arm mesenchyma having been removed to show the limit of the sheath, the hand plate, the border vein and the venous plexus between the hand plate and the mesenchyma.
 
 
 
The Nerves. — The muscle plate column is supplied by branches of the dorsal rami from all the nerves in this region. They enter the median side of the muscle-plates branch Avithin them, one branch passing through to the subcutaneous tissue.
 
 
 
Branches from the anterior rami of the III, IV, V, VI and VII cervical nerves supply the levator scapulfe and serratus anterior premuscle mass. The rhomboid premuscle mass is supplied by a branch which comes off with the one from the V cervical.
 
 
 
 
 
 
 
Warren Harmon Lewis 159
 
 
 
The phrenic nerve arises from the median side of the trunk formed by the IV and V cervical nerves. It does not reach quite to the level of the first rib. See Fig. 6.
 
 
 
The brachial plexus is formed from the ventral divisions of the IV, V, VI, VII, VIII cervical and I thoracic nerves. The main portion of the plexus forms a continuous sheet of nerve tissue in which only indications of the three cords can be distinguished. The plexus passes laterally into the arm without any caudal inclination. On reaching the arm it splits into a dorsal and a ventral division. The dorsal division corresponds to the continuation of the posterior cord. It passes around the dorsal side of the humerus, decreasing rapidly in size and ends in the premuscle sheath near the distal end of the humerus. Most of it represents the musculo-spiral nerve. A small branch, which is probably the circumflex, is given off near its beginning. Fibers from all the spinal nerves forming the plexus can be traced into this dorsal division. The ventral division is partially divided into two parts, which probably represent the outer and inner cords. From the outer arises the suprascapular nerve, having fibers from the IV, V, and VI cervical. It passes ventral to the scapulo-humeral junction into the arm premuscle sheath. The rest of this outer cord splits into the musculo-cutaneous and the outer head of the median. The musculo-cutaneous passes into the premuscle sheath on the ventral side of the humerus, and the median into the sheath distal to this, reaching as far as the distal end of the humerus. The inner cord terminates in the ulnar nerve, which runs into the premuscle sheath along the median side of the humerus as far as the beginning of the ulna. Branches going into the pectoral premuscle mass leave the median side of the plexus, one mostly from the outer and the other two from the inner cord. They correspond to the external and internal anterior thoracics. In Fig. 6 the lengths of the various nerves are indicated.
 
 
 
Embryo CIX.
 
 
 
Embryo CIX measures V. B. 10.5 mm. and X. B. 11 mm. in lengtli and is about five weeks old. There is a marked advance over the preceding stage. Cartilage has made its appearance both in the vertebrae and in portions of the arm skeleton. There is considerable difference in the character of the cartilage of the vertebra from that in the arm. The latter seems more advanced and lias more the appearance of true hyaline cartilage. It is possible that the cartilage appears first in the arm, though I have not been able to examine intervening stages to
 
 
 
 
 
 
 
160 The Development of the Arm in Man
 
 
 
determine this with certainty. Other portions of the arm skeleton are in the precartilage and condensed tissue stages. Both cartilage and precartilage are surrounded in most places by a distinct perichondrium. This takes a very deep stain with the alum carmine. This perichondrium shades off into the condensed tissue of the carpus, which is like that composing the skeletal core in the preceding stage. This again shades into the even less differentiated tissue of the digits, which is at a.bont the same stage of development as the hand plate of the preceding stage, and it in turn shades off into the surrounding mesenchyma.
 
 
 
The muscles in the arm region show very different degrees of development. Those derived from the muscle plate system are in advance of most of the others. The trapezius, levator scapulae and serratus magnus are about as far advanced as those from the muscle plate system, they show distinct muscle fibers and are for the most part quite sharply limited from the surrounding loose mesenchyma. In position they correspond with their premuscle masses of the preceding stage. The pectoral muscle is next in advance and the latissimus dorsi next. These two muscles grow from the humeral region towards their future attachments on the body wall. It is this portion which lies farthest from the humerus which seems to show the most advance in fibrillation and the sharpest limitation from the surrounding mesenchyma. At the humeral end these muscles gradually shade into a condensed mesenchyma, which fuses with neighboring muscle and skeletal elements. Both muscles correspond in position to their premuscle masses of the preceding stage. As in the preceding stage, embryo CLXIII, the trapezius and serratus premuscle masses were in advance of the pectoral and latissimus; in embryo CIX we find the same relation still continues.
 
 
 
The remaining muscles of the arm apparently develop in situ from the premuscle sheath and undergo practically no migrations. They do not appear to be as far advanced as any of the above mentioned muscles. Of these muscles developing from, the arm premuscle sheath, the more proximal ones are more developed than the ones more distal. In the scapulo-humeral region most of the muscles show partial fibrillation, while those in the palm of the hand are in about the same condition as the proximal portion of the premuscle sheath in the preceding stage.
 
 
 
The fibrillation, position and nerve supply have made it possible to determine the presence of most of the muscles of the arm.
 
 
 
The Skeletal, System. — The Vertebral Column. The inteiTertebral discs are composed of condensed mesenchyma, the cells having a concentric arrangement about the chorda. The vertebral bodies between the discs are each composed of two masses of cartilage, one on either
 
 
 
 
 
 
 
Warren Harmon Lewis 161
 
 
 
side of the chorda. Tliey are surrounded by a perichondrium. Along the ventral surface of the vertebral coluuni is a layer of dense mesenchyma, which probably represents both perichondrium and the anterior common ligament. The neural processes, composed of condensed mesenchyma, are clearly defined. They are continuous with the discs and form a wide, shallow groove for the spinal cord. The transverse processes arise by two roots, one from the base of the neural process and the other from the disc. They are of condensed mesenchyma.
 
 
 
The Ribs. — The ribs are more sharply defined than in CLXIII. They are of condensed tissue except for a small area near the head, which is of precartilage. They extend farther into the body Avail than in the preceding stage.
 
 
 
 
 
 
 
 
 
Fig. 9. Skeleton of the arm region of embryo CIX, lateral view. X 12 diameters.
 
 
 
The Arm Skeleton. — The Scapula is composed of precartilage and has greatly altered in shape. It lies in the region of the lower four cervical and first one or two thoracic vertebrae. From the anterior border, which corresponds to the spine, springs the large curved acromion process. On the median surface at the junction of the humerus with the scapula arises the large hooked coracoid process. Eunning across the median surface of the scapula to the vertebral border is a slight ridge which separates the supraspinatus from the subscapularis muscles and corresponds to the future anterior border. The condensed tissue is thickened on the medial surface into a perichondrium, while on the lateral surface the precartilage shades off into the surrounding mesenchyma.
 
 
 
The Clavicle. — A rather poorly defined mass of condensed tissue continues from the tip of the acromion toward the tip of the first rib, extending for about one-third this distance. This mass represents the clavicle. From it a mass of ill-def]ned tissue extends to the coracoid process and represents the coraco-clavicular ligament.
 
 
 
 
 
 
 
162
 
 
 
 
 
 
 
The Development of the Arm in Man
 
 
 
 
 
 
 
The Humerus is directly continuous with the scapula and root of the coracoid process. No signs of joint surfaces or ca-vity arc present. Both ends of the shaft are enlarged and Ihe distal end shows both external and internal condyles. The core of the shaft is of hyaline cartilage; this is surrounded by very thick perichondrium, which shades ofE into the condensed tissue of each end in' which is enclosed an area of precartilage. The distal end seems more advanced than the proximal.
 
 
 
The Radius and Ulna are continuous with the distal end of the humerus, no indications of joint surfaces or cavities being present.
 
 
 
 
 
 
 
 
 
Fig. 10. Outline of the arm region of embryo CIX, lateral view from Plate IV. Bardeen and Lewis, Vol. I, No. 1, this Journal. X 12 Diameters.
 
 
 
 
 
 
 
There is more flexion at the elbow than in CLXIII. The forearm occupies a position about half way between pronation and supination. The core of each shaft is composed of hyaline cartilage. This is surrounded by a very thick perichondrium, which continues into the condensed tissue at either end of the bone, in which precartilage is enclosed. The Hand-plate is continuous with the distal ends of the radius and ulna. It is composed of condensed mesenchyma. There are several centers of increased condensation which I believe must correspond to the carpal bones, namely, the scaphoid, lunar, pyramidal, trapezium, trapezoid, OS magnum and unciform. The scaphoid is in line with the radius and the lunar with the ulna, while the pyramidal is at the ulnar side of
 
 
 
 
 
 
 
Warren Harmon Lewis 16B
 
 
 
■the carpus, and as the metacarpal V continues from it more tlian the unciform tlie whole hand has a peculiar bend toward the ulnar side. From the carpus five masses of condensed tissue project. They shade off into the surrounding mcsenchyma which fills the distal end of the arm. The condition of these finger masses corresponds to the condition of the hand-plate in CLXIII. There is not the sliglitest indication of segmentation into metacarpals and phalanges. The radial of the five projections probably consists of both trapezium and metacarpal I, which have not yet shown signs of separate centers of condensation.
 
 
 
The Musculae System. — The muscle plate system has become differentiated into several muscles, namely, the deep dorsal muscles, the intercostals, the abdominal muscles and the deep ventral neck muscles.
 
 
 
The infrahyoid muscles correspond in position and nerve supply with the infrahyoid premuscle mass of the preceding stage. They extend nearly to the region where the median end of the clavicle will eventually extend.
 
 
 
The trapezius muscle has extended posteriorly to the level of the fifth cervical vertebra. Its posterior end lies near to the lateral surface of the body and is connected to the tips of the neural processes as far posteriorly as the second thoracic vertebra by a considerable interval of fascia. As the muscle passes anteriorly it lies deeper and deeper from the surface, being separated from it by the platysma and facial muscles. Its ventral border is free from attachment to the scapula and clavicle. At the level of the second cervical vertebra it is joined by the stemomastoid muscle, which has ascended from the more ventral neck region. The nerve supply is as in the adult.
 
 
 
The rhomboid mass lies in the region of the Y and A^I cervical vertebrae. It connects with the fascia passing to the dorsal tips of the neural processes but has no scapular attachment. A branch of the fifth cervical nerve supplies it.
 
 
 
The levator scapula; and serratus anterior muscles form a continuous fibrillated mass, extending from the first cervical vertebra to the ninth rib. It occupies much the same position that its premuscle mass did in embryo CLXIII except that the posterior end now extends to the ninth rib. Digitations go to all the cervical transverse processes and to each of the anterior nine ribs. The anterior and posterior digitations are very slender and contain but few fibers. The thickest part of the- muscle lies in the scapular region. There is no scapular attachment. The ventral edge of the muscle lies at about the same level as the dorsal edge of the scapula but in a more median plane. Branches from the second to the seventh cervical nerves supply the muscle. The first three 13
 
 
 
 
 
 
 
164: The Development of the Arm in Man
 
 
 
penetrate directly into the muscle. The last three form a trunk which runs along the lateral surface of the muscle as far as the fourth rib.
 
 
 
The pedoralis major and minor "^ are united into a common muscle mass, which is well differentiated from the surrounding tissue. It forms a thick oval mass, which extends from the level of the second rib to the proximal portion of the humerus. The greater part of the muscle thus lies anterior to the first rib. As the mass bends towards the humerus it is attached also to the clavicle. So probably both sterno-costal and clavicular portions are present. The median side of the mass bulges towards the coracoid process and represents the minor. Most of the mass shows distinct fibrillation, but toward the humerus this passes into the condensed tissue which is not sharply outlined from the surrounding structures. The position of the pectoral muscle corresponds to the position of the pectoral premuscle mass in embryo CLXIII. Branches from the median side of the brachial plexus supply the pectoral. Two from the external cord contain fibers from the fifth, sixth and seventh cervical nerves. Two come from the inner cord. Within the muscle complicated anastomoses occur from which fibers spread out in all directions.
 
 
 
The muscles thus far considered were fairly definite, and, as we have seen, come from quite definite premuscle masses. The remaining muscles of the arm are in process of differentiation from the arm premuscle sheath. The exact limits of the individual muscles are almost impossible to determine.
 
 
 
The deltoid muscle extends from the acromion and clavicle and fascia over the infraspinatus to the humerus. It is very closely connected with the infraspinatus and only by the difference in the nerve supply can the two be separated. The position of the teres minor is also only indicated by its nerve and not by any line of separation between it and the infraspinatus or deltoid. The origin of part of the deltoid from the acromion and clavicle helps to distinguish some of its fibers, but a short distance from this origin no line of separation can be made between it and the infra- and supraspinatus muscles. Condensed tissue connects it with the triceps and pectoral muscles. The circumflex nerve supplies this muscle and also sends a branch to fibers which are closely associated with the infraspinatus and probably constitute the teres minor muscle.
 
 
 
That portion of the infraspinatus which lies on the lateral surface of the scapula is fairly distinct except where the deltoid and teres minor
 
 
 
23 Lewis, Observations on the pectoralis major muscle in man, Johns Hopkins TTosp. Bui., Vol. XII, 1901.
 
 
 
 
 
 
 
Warren Harmon Lewis 165
 
 
 
muscles join it. The portion of tlie supraspinatus on the anterior onefourth of the median surface of the scapula is distinct, but after it passes the acromion it is inseparably' connected with the infraspinatus and deltoid and pectoral muscles. These muscles shade off into the 23roximal end of the humerus. The main portion of each of these muscles contains muscle fibers. The suprascapular nerve supplies the supra- and infraspinatus muscles.
 
 
 
The subscapularis muscle arises from the posterior one-half of the median surface of the scapula and passes beneath the coracoid process to the humerus. The circumflex nerve separates a portion of it from the teres major muscle, but the scapular portions of the two are closely imited, as is also the long head of the triceps. A branch from the circumflex and another from the posterior cord of the brachial plexus supply the subscapularis.
 
 
 
The teres major and latissimus dorsi muscles are closely associated at "their humeral end. The latissimus dorsi' lies in the lateral thoracic region, extending posteriorly as far as the fourth rib. It has no attachments to the ribs or vertebral column. The two muscles are inserted together into the proximal portion of the humerus. The teres major arises from the axillary border of the scapula near its posterior angle. The common portion of the latissimus and teres passes close to the posterior cord of the brachial plexus, from which a large branch is given off that runs into the latissimus and has a brush-like ending near the posterior limit of the muscle. A smaller branch of the posterior cord is given off to the teres major.
 
 
 
The triceps muscle extends along the posterior and lateral surfaces of the humerus, extending from the scapula to the ulna. Indications of the three heads are present. The portion of the muscle lying near the insertion of the latissimus dorsi and the infraspinatus muscles is not sharply defined from them. The musculo-spiral nerve passes through the muscle and gives branches to it.
 
 
 
The biceps and coracohracJnaUs muscles lie along the median side of the humerus, extending from the coracoid process to the radius. The two heads of the biceps are quite closely united nearly to their origins, Avhich are but a short distance apart. The portion of the coracoid process from which the long head arises must ultimately become a portion of the head of scapula. The attachment of the coracobrachialis to the humerus is by condensed tissue, as is the distal end of the biceps to the radius. The distal end of the biceps blends with the brachialis and the flexor mass. The musculo-cutaneous pierces this group and gives off branches to it.
 
 
 
 
 
 
 
166 The Development of the Arm in Man
 
 
 
The hracJiialis muscle is closely attached to the distal one-half of the hmnerns over the anterior and median surfaces. It is also closely attached to the overlying biceps muscle and it is impossible to determine just the line between the two or between it and the brachioradialis muscle. It is also impossible to determine the exact line between the muscle and the underlying perichondrium. It is closely associated with the triceps on one side and the deltoid on the other. The main portion of the muscle is fibrillated and is inserted into the ulna by condensed tissue, which is closely associated with the, flexor mass of the forearm. The musculo-cutaneous nerve gives off a large branch which has a brush-like endins; within the muscle.
 
 
 
 
 
 
 
Spinal accessoiy
 
 
 
 
 
 
 
 
 
Fig. 11. Outline of the arm region of embryo CIX, median view, from Plate V. Bardeen and Lewis, Vol. I, No. 1, this Journal. X 15 diameters.
 
 
 
The flexor muscle mass of the forearm forms a thick layer over the median surface of the ulna, radius, carpus and proximal end of the metacarpus. It is with considerable difficulty that I have separated this mass into two layers. The superficial layer is smaller in extent and lies in the proximal region of the forearm. It is connected with the radial portion of the forearm by a condensed tissue mass and distally fuses with the deep layer to become continuous with the condensed tissue of the digits. The median nerve passes through the proximal portion and then comes to lie between the two layers. From its position and relation to the median nerve I believe this to be the layer from which the flexor carpi radialis, flexor sublimis digitorum, pronator teres and palniaris long-us muscles differentiate. Branches from the median nerve supply this layer. Both layers arise partly from the inner condyle of the humerus, and are continuous more or less with the muscles of the upper arm. The deep layer is closely attached to the perichondrium of
 
 
 
 
 
 
 
Warren Harmon Lewis 167
 
 
 
the forearm and hand. It is wider in extent than the superficial and shows indications of separations into muscles. Tlie portion for the flexor carpi nlnaris shows most advance. The extension into the hand probably constitutes the portion from which the interossei and lumbrical muscles and flexor tendons develop. It is continuous with the condensed tissue of the digits. The portion on the forearm forms the flexor profundus digitorum, flexor pollicis longus, flexor carpi ulnaris and pronator quadratus muscles. Both the ulnar and median nerves supply the deep layer.
 
 
 
The extensor mass of the forearm is farther advanced than the flexor. It can be differentiated into three groups of muscles which accord well with the adult groups. The first group, the largest and most superficial, extends from the lateral condyle to the proximal ends of the digits, where it blends with the condensed mesenchyma. It is a thin layer and spreads out over the ulnar two-thirds of the forearm and is quite closely applied to the perichondrium and cpndensed mesenchyma of the skeletal structures beneath. A portion of it overlaps the second and third groups. It is the still undifferentiated extensor communis digitorum, extensor carpi ulnaris, and extensor minimi digiti. It is supplied by branches of the posterior interosseus nerve.
 
 
 
The second group occupies the proximal portion of the radial side of the forearm. It arises in connection with the first group from the external condyle and adjoining portion of the humerus. The muscle mass passes distally along the radius and soon divides into two parts between which the radial nerve passes. The radial part fuses with the condensed tissue of the distal end of the radius. It is the brachioradialis muscle. The second part passes beneath the third group and fuses with the condensed mesenchyma at the proximal ends of the second and third digits. It is the extensor carpi radialis longior et hrevior muscle. Branches of the musculospiral nerve supply this second group.
 
 
 
The third group arises beneath the first from the ulna and radius. Its fibers pass toward the radial side of the forearm, passing from beneath the first group and over the second group, and finally end in the condensed tissue of the first and second digits. The portion to the second digit is closely fused with the portion of the first group which goes to this digit. This group is quite closely applied to the underlying skeletal condensed tissue. The third group represents the abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus and extensor indicis proprius. Branches of the musculospiral nerve supply this group.
 
 
 
The supinator I believe must arise in connection with the third group, judging from its position and the direction 'of its fibers.
 
 
 
 
 
 
 
168 The Development of the Arm in Man
 
 
 
The muscle fibers of the extensor groups do not extend as far distally as do those of the flexor mass.
 
 
 
The Nerves. — The enormous size of the lower cervical nerves attracts the attention at once. In the plates and figures they are given in their true proportion to the other structures. The main portion of the brachial plexus has but a very slight posterior inclination.
 
 
 
A branch from the V cervical supplies the rhomboid muscle mass.
 
 
 
The V, VI, VII and VIII cervical and I thoraeic nerves unite to form the brachial plexus. The IV cervical does not connect with the plexus. The main portion of the plexus forms a continuous sheet in which indications of the three cords can be seen. The V and VI unite before joining the others and from this union is given off the -suprascapular. It leaves the trunk at right angles and has the appearance of having its proximal end dragged distally to\\'ard the arm by the main portion of the plexus. The VIII and I thoracic unite before joining the plexus. The continuous sheet formed by these five nerves soon splits into a lateral (dorsal) and median (ventral) division. The lateral corresponds to the posterior cord and from it arise the circumflex, subscapular and musculospiral nerves. These nerves take the normal course found in adult and supply the same muscles as in adult. Cutaneous branches are also given ofl^. The median sheet of the plexus quickly divides into several bundles. The anterior one corresponds to the distal end of the external cord. From it are given off the musculo-cutaneous, two branches to the pectoral mass, and one head of the median nerve. The posterior division corresponds to the distal end of the inner cord. From it arise branches to the pectoral mass, the inner head of the median, the ulnar and internal cutaneous nerves. The distal end of the median splits into a peculiar fan-like arrangement of its branches. Both median and ulnar give branches to the deep flexor mass and anastomose within the mass.
 
 
 
I have attempted to trace the origin of the fibers in the main nerves of the arm. The results are given in the following table:
 
 
 
Cervical. Thoracic.
 
 
 
Suprascapular V, VI ?
 
 
 
Subscapularis V, VI, VII
 
 
 
Long thoracic VII, VIII I
 
 
 
Anterior thoracics V, VI, VII, VIII I
 
 
 
Musculo-cutaneous V, VI, VII ?
 
 
 
Median V, VI, VII, VIII I
 
 
 
Circumflex V, VI, VII
 
 
 
Musculospiral V, VI, VII, VIII I
 
 
 
Ulnar VI?, VII, VIII I
 
 
 
 
 
 
 
Warren Harmon Lewis 169
 
 
 
Embryo XLIII.
 
 
 
Embryo XLIII measures 16 mm. V. B. and 11 mm. jST. B. It is about six weeks old. Many changes have talcen place during the sixth Aveck. The entire arm has migrated posteriorly, dragging muscles and nerves with it. The brachial plexus has a decided posterior inclination. The skeletal system is much farther advanced and consists for the most part of cartilage; its individual elements are assuming more the adult form. The clavicle now unites the arm and thoracic skeletons.
 
 
 
The muscular tissues have become more clearly differentiated and except in the hand are easily distinguished. Muscles, such as the trapezius, serratus, and pectoral, have spread out into sheets and acquired more their permanent attachments, in the case of the trapezius, latissimus and pectorals by migTation or extension of their fibers.
 
 
 
In the hand, however, we find the interossei still in an undifferentiated condition like that of the deep flexor layer in embryo CIX or the serratus and infrahyoids in embryo CLXIIL'
 
 
 
The Skeletal System. — The vertebral column. The intervertebral discs are of still more compact tissue than in embryo CIX, but they occupy only about one-fourth height of the segment, while in CIX they occupied nearly one-half the anterior-posterior length of the segment. The body of each vertebra contains a large mass of cartilage, which is continuous with the cartilage in the transverse and neural processes. Indications of the hypochordal brace of Froriep are present in connection with the ventral side of the first three discs. The anterior one is the largest, the others decreasing rapidly in size.
 
 
 
The ribs are composed of long, slender cartilages, surrounded by a thick perichondrium. This is continuous with the condensed tissue of the tips of the ribs. The tips of the first seven ribs are connected by a narrow strip of condensed tissue which appears to be formed by the turning anteriorly of their tips until they touch the rib above and fuse with it. Thus is formed the' anlage of one-half the sternum on either side some little distance from the median line. There is at present no sign of union of the two halves of the sternal anlagen. The first rib is fused with the median end of the clavicle. The ribs show a marked increase in their lateral convexity, as in embryo CIX there was scarcely any. There are no joint cavities between the ribs and vertebrte.
 
 
 
The scapula is composed largely of cartilage. It has migrated posteriorly so that less than one-half of it lies above the level of the first rib. The whole scapula is larger than in embryo CIX. There is a thick layer of perichondriuui around the cartilage and a considerable
 
 
 
 
 
 
 
170
 
 
 
 
 
 
 
The Development of the Arm in Man
 
 
 
 
 
 
 
mass of condensed tissue along the vertebral border, and at the posterior angle, the cartilage reaches to the level of the third rib and the condensed tissue nearly to the fifth. The anterior border is somewhat irregular and thickened and gives origin in part to the supraspinatus muscle. The lateral lip of this border probably represents the spine and the median lip the anterior border. Projecting from the lateral side of the head and continuous with the lateral lip of the anterior border is the acromion process. It is large, curved and mostly of condensed tissue and contains a slender core of cartilage continuous with the cartilage
 
 
 
 
 
 
 
trapezoid metacarpal
 
 
 
trapezium ( scaphoid
 
 
 
 
 
 
 
OS magnum .
 
 
 
 
 
 
 
ant. border acromion , spine coracoid ^ft^^.
 
 
 
 
 
 
 
 
 
unciform
 
 
 
 
 
 
 
,FiG. 12. Cartilaginous slceleton of tlie arm of embryo XLIII, lateral view. X 20 diameters.
 
 
 
 
 
 
 
of the body. The coracoid process arises from the median side of the head, is larger than the acromion, and contains a much larger cartilao-inous core, which is continuous with the cartilage of the body. The acromio-clavicular ligament is strongly developed.
 
 
 
The clavicle consists of a thick mass of condensed tissue, extending from the acromion to the tip of the first rib, where it continues with the half sternal anlage. There is no line of separation at either end. There is a small core of a peculiar precartilaginous tissue.
 
 
 
The Immerus is larger, longer and more slender in proportion to its
 
 
 
 
 
 
 
Warren Harmon Lewis 171
 
 
 
length than in the preceding stage. The two ends are enlarged. The main portion is of cartilage snrronnded by a thick perichondrihm which is continuous with that of the head of the scapnla, forming the Ijeginning of the capsular ligament. There is also a strip of perichondrium between scapula and humerus in which there are no signs of a joint cavity. At the proximal end the perichondrium shows thickenings for the tuberosities, while at the distal end the condyles are for the most part of cartilage continuous with that of the main portion. Considerable masses of condensed tissue, however, help to increase the size of the condyles. A portion of the head of the humerus rests against the base of the eoracoid process, indicating that a portion of this is to be incorporated with the head of the scapula.
 
 
 
The ulna and radius are of cartilage surrounded by a thick perichondrium. This is continuous with that of the distal end of the humerus, forming the beginning of the capsule. The perichondrium of the proximal end of the radius is continuous with that of tlie adjoining surface of the ulna. The cartilages of the humerus, radius and ulna are separated from each other by condensed tissue in which no signs of cavities are present. The olecranon is quite well developed and consists mostly of cartilage. The coronoid process is mostly of condensed tissue. The great sigmoid fossa is rather shallow. The bicipital tuberosity is of condensed tissue. The distal ends of these bones are enlarged and separated from each other by condensed tissue continuous with the perichondrium of each.
 
 
 
The carpus consists of a condensed tissue matrix in which lie imbedded the various cartilages. The distal row is complete, the trapezium, trapezoid, os magnum and unciform. The latter has spread in between the fifth metacarpal and the cuneiform (pyramidal). In the proximal row the cuneiform and scaphoid are of cartilage and the hmar and pisiform of condensed tissue.
 
 
 
The metacarpus shows five slender cartilages surrounded by very thick condensed tissue layer or perichondrium. The first metacarpal cartilage is only about one-half the length of the others.
 
 
 
The ulnar four plialanges of the first row are present as short slender cartilages deeply imbedded in condensed tissue. In the first digit condensed tissue takes the place of the cartilage. At the tip of each digit is a mass of condensed tissue.
 
 
 
There are no joint cavities between the cartilages of the hand, each one is separated from its neighbor by an area of condensed tissue.
 
 
 
 
 
 
 
172 The Development of the Arm in Man
 
 
 
Ilagen '* has reconstnicted the cartilaginous skeletal system of a hnman en'ibryo of abont this age. A comparison of the drawings from the reconstructions shows that there is considerable variation in the carpal region. In none of my stages does the metacarpal come irt contact with the radius, either before or after the cartilages of thecarpus and metacarpus appear, and there is a considerable area of dense mesenchyma between metacarpus and radius. I am inclined to believe what he calls metacarpal I, may be trapezium and his so-called firstphalanx the metacarpal.
 
 
 
The Muscular System. — Plates I and II, Figs. A and B. The trapezius muscle has both clavicular and acromial attachments. The muscle has extended posteriorly so that the muscle fibers run from theocciput to the level of the fifth rib. They are connected by a considerable interval of fascia with the dorsal ends of the cervical and all the thoracic neural processes.
 
 
 
The levator scapulce and serratus anterior muscles are greatly altered iit shape. The latter forms a broad, thin sheet between the dorsal border of the scapula and the first nine ribs, being attached by a digitation tO' each rib. The scapular attachment is into the condensed tissue along its dorsal border.
 
 
 
The pectoral mass is now spread out into a large, thin sheet, which has split into the major and minor muscles. The clavicular and sternocostal portions of the pedoralis major are separated by a considerable interval. The clavicular fibers arise from the median one-third of the clavicle and pass to the humerus. They overlap the humeral ends of the sterno-costal fibers which arise from the first six ribs and the sternal anlage.
 
 
 
The pedoralis minor is a distinct muscle arising from the second, third and fourth ribs and passing to the coracoid process.
 
 
 
The subelavius muscle is quite Avell developed and runs from the first ril) to the clavicle, having a course nearly at right angles to the latter.
 
 
 
The latissimus dorsi has spread out into a broad, thin sheet of muscle fibers, which are connected by fascia with the lower thoracic and lumbar neirral processes. Its humeral end is closely miited with the teres major.
 
 
 
The teres major muscle has about the relations found in the adult. It and the latissimus dorsi are inserted together into the humerus.
 
 
 
Tlie deltoid muscle is very much like the adult in its attachments and shape.
 
 
 
-'•* Hagen, Die Bildung des Knorpelskeletes beim mensclalicben Embryo, Arch, fiir Anat. u. Pbys., 1900.
 
 
 
 
 
 
 
Warren Harmon Lewis 173
 
 
 
Tlic infraspinatus nniscle arises from the anterior portion of the lateral sin-face of the scapula and can bo easily traced to its insertion into the great tuberosity of the humerus. The teres minor cannot be separated from it.
 
 
 
The supraspinal us muscle arises from the anterior thickened border of the scapula and passes to the great tuberosity of the humerus.
 
 
 
The suhscapularis muscle occupies the central portion of the median surface of the scapula. It is separated from the teres major. It passes beneath coracoid process to the lesser tuberosity of the humerus.
 
 
 
The triceps muscle is easily traced from its origin by the three heads to its insertion into the olecranon process. The three heads are quite easily distinguished. The long head is smaller in proportion than in the adult.
 
 
 
The biceps muscle is more elongated and shows more of a separation of its two heads than in embryo CIX. The long head still arises from the base of the coracoid process. The two heads join about the middle of the humerus and pass to a thickening of condensed tissue on the radius. The short head arises in common with the coracobrachialis muscle from the tip of the coracoid process. This latter muscle is inserted into the middle of the median surface of the humerus. It is closely connected with the biceps for most of its length.
 
 
 
The hracJiiaUs muscle is spread out more over the distal portion of the humerus and its muscle fibers extend farther toward the insertion into the coronoid process of the ulna than in the preceding stage.
 
 
 
The fte.ror mass of the forearm and hand show a most marked advance over the preceding stage. The various muscles of the superficial layer which arise from the internal condyle are easily recognized. They are more or less fused at their origin and for some little distance from it.
 
 
 
The pahnaris longus muscle, the most superficial one, is thin and wide, ends in the condensed tissue of the palmar fascia.
 
 
 
The pronator teres muscle passes to the middle of the shaft of the radius.
 
 
 
The flexor carpi radialis muscle lies mostly on the radial side of the forearm, towards the distal end of which it bends under the deep flexor and ends in a condensed tissue tendon which fuses with the condensed tissue near the proximal end of the second metacarpal. This portion of the muscle is not yet clearly differentiated from the condensed tissue on the palmar surface of the caqms.
 
 
 
The flexor digitonim suhlimis muscle arises beneath the palmaris longus in connection with it from the internal condyle, and also from the shaft of tlie ulna, for a little distance distal of the coronoid ]~)rocess.
 
 
 
 
 
 
 
l74 The Development of the Arm in Man
 
 
 
It is very broad and spreads out over the middle of the forearm and carpus, where it divides into fonr broad, thin tendons which fnse with the condensed tissue surrounding the distal end of the four ulnar metacarpals and first row of phalanges. The muscle fibers continue distal as far as the middle of the carpus, where the muscle becomes wider and thicker. The tendons do not show the split which is later to appear and enclose the deep flexor tendon. The strongest part of the tendons lie on the ulnar side of digits.
 
 
 
The deep layer of the preceding stage has undergone marked changes.
 
 
 
The flexor carpi ulnaris muscle is quite distinct. It arises partly from the internal condyle superficial to the sublimis and closely connected with it and the palmaris longus and partly from the ulna. The muscle at its origin is broad and thin but narrows into a condensed tissue tendon which is inserted into the os pisiform.
 
 
 
The flexor digitorum profundus and the flexor polUcis longus muscles arise from the surfaces of the radius and ulna and the internal condyle. They are closely imited and pass to the carpal region where division takes place into five well-formed oval tendons, which pass beneath the tendons of the sublimis, and fuse with the condensed tissue about the ends of the digits.
 
 
 
The pronator quadraius muscle is a small, oval mass connecting the distal ends of the ulna and radius.
 
 
 
The lumhride muscles are fonned. They arise from the profundus near the angles formed by the iive tendons. They are short and contain distinct muscle fibers which end in tendons that fuse with the condensed tissue on the radial side of the ulnar four digits.
 
 
 
The intrinsic muscles of the hand, the interossei, and muscles of the thumb and little finger, are represented by a late premuscle tissue in which a few muscle fibers are beginning to appear. These masses are more or less continuous with each other and lie on the palmar surface of the carpus and metacarpus and partially in between the latter. The distal ends of these masses fuse with the less differentiated condensed tissue about the digits.
 
 
 
The extensor muscles of the forearm show considerable advance over the preceding stage, but the development does not seem to have been as rapid as in the case of the flexor muscles.
 
 
 
Of the first group, the extensor communis digitorum and the extensor minimi digiti are united into a broad, thin sheet which divides in the metacarpal region into four broad, thin tendons that end in the condensed tissue of the four ulnar digits. The extensor carpi ulnaris closely associated with this muscle at its origin from the external condyle arises
 
 
 
 
 
 
 
Warren Harmon Lewis 173
 
 
 
also partly from the ulna and is inserted into the condensed tissue at the proximal end of the fifth metacarpal. It is quite separate from the common extensor for the greater part of its length.
 
 
 
Of the second group, the hracMoradinlis is quite distinct from the extensor- carpi radialis longior et hrevior for most of its length, but at their origin, however, the two are closely connected. Both muscles are broader and larger than in the preceding stage. The extensor passes beneath the third gTOup and ends in the condensed tissue near the proximal ends of the second and third metacarpals.
 
 
 
The tJiird group, which arises beneath the first from both radius and ulna, has split more or less into four parts. The proximal one, which is the most completely separated, is the supinator and passes from the ulna and external condyle to the radius. It is united with rest of this group along their ulnar origins, forming thus a continuous sheet for a short distance. The next two pass over the extensor carpi radii tendon, and fuse with the condensed tissue of the first digit. They are the abductor pollicis longits, extensor poUicis brevis and the extensor poUicis longus muscles. The fourth division is broad and thin and soon joins the deep surface of the tendon of the extensor communis and goes with it to be inserted into the condensed tissue of the second digit.
 
 
 
The Kerves. — By the migration of the arm posteriorly the brachial plexus has been pulled caudally and given a decided posterior inclination. It has also divided into the various cords more than in the preceding stage.
 
 
 
The distribution of the muscle and cutaneous nerves is much as in. the adult and as in the next stage.
 
 
 
Embryo XXII.
 
 
 
Embryo XXII measures 20 mm. V. B. and 18 mm. X. B. It is about seven weeks old. The entire arm has a more posterior position. The lower angle of the scapula is at the level of the sixth rib, its anterior limit is about at the seventh cervical vertebra. The entire arm as well as its various parts have increased in size. The muscles are sharper and better developed than the preceding stage. Every muscle that the adult arm presents can now be recognized and each one now contains muscle fibers. The tendons are better formed and can be traced farther towards their final insertions. The ligaments and fasciae are also more distinct. The process of ligament and tendon formation from the condensed mesenchyma is still in progress at the distal ends of the digits. The skeletal elements are for the most part fairly well formed in cartilage except the distal row of phalanges.
 
 
 
 
 
 
 
176 The DeveloiDinent of tlie Arm in Man
 
 
 
The Skeletal System. — The vertebral column. The intervertebral discs are reduced in thickness, while the bodies of the vertebra have increased and occnpy about four-fifths of each segment. The neural and transverse processes are larger and for the most part of cartilage. At the tip of the neural processes, which reach about one-half way around the spinal cord, is a small mass of condensed tissue at what juay be considered the growing point. These processes arise entirely from the body and not from the disc. So the body has probably grown at the expense of the disc. The perichondrium, wliich surrounds the body and its processes, is thickened along the ventral side of the bodies into the anterior common ligament.
 
 
 
The rihs are of cartilage surrounded by thick perichondrium, wliicli is continuous with the condensed tissue anlage of the one-half the sternum. The distance between the two halves of the sternum is not as great as in the preceding stage and at the anterior end they ai'e just beginning to come in contact with each other. There are no joint cavities between the ribs and vertebrae.
 
 
 
The clavicle is composed of cartilage somewhat different in appear ance from that in the other bones. It is continuous with the acromion and sternum by an area of condensed tissue. It is surrounded by a typical perichondrium. There are distinct coraco-clavicular, costoclavicular, and interclavicular ligaments.
 
 
 
The cartilaginous scapula is very much larger than in the precedingstage and contains no large areas of condensed tissue. It has moved posteriorly and lies in the region from the last cervical to the fifth thoracic vertebrae. Its dorsal border also extends farther dorsal than in any of the preceding stages. The acromion and coracoid processes are large and of cartilage with only the ordinary thickness of perichondrium which is continuous with that surrounding the rest of the scapula. The spine has not yet appeared but the thickened anterior border from which the supraspinatus muscle arises probably represents by its lateral lip the spine and by its median lip the future anterior border. The acromion arise partially from the lateral side of the anterior border. The head seems to have enlarged at the expense of part of the base of the coracoid process as the long head of the biceps now arises from the junction of the coracoid and the head, and the head of the humerus does not rest against such a large proportional area of the coracoid. There is a distinct suprascapular and a coraco-acromial ligament. At the posterior angle of the scapula there is small mass of condensed tissue which gives attachment to a portion of the serratus, latissimus, and teres major muscles.
 
 
 
 
 
 
 
Warren Harmon Lewis ITv
 
 
 
The humerus is much hirger than in embryo XLIII, and has much the adult shape, though of course it is thicker in proportion to its length. It is composed of cartilage. There is a capsular and a coraco-humeral ligament. No joint cavity exists between the scapula and humerus. The tuberosities and condyles are fairly well formed in cartilage and •condensed tissue. The bicipital groove is present.
 
 
 
The ulna and radius are larger and longer than in the preceding stage 3ind are well formed in cartilage. The olecranon, coranoid and styloid processes are partially formed in cartilage and condensed tissue. The perichondrium about the ulna and radius is quite thick. The capsular .and orbicular ligaments are present. No joint cavities exist and the cartilages are separated by condensed tissue continuous with the peri•ciiondrium.
 
 
 
 
 
 
 
 
 
Fig. 13. Cartilaginous skeleton of the arm of embrj-o XXII, lateral view. X 12 diameters.
 
 
 
 
 
 
 
All the bones of the carpus are represented by cartilage, and in about their relative positions. The amount of condensed tissue matrix is much less than in the preceding stage. The condensed tissue matrix is continuous with the ulna and radius and the five metacarpals without joint cavities. Indications of ligaments of the wrist are present.
 
 
 
The five metacarpals are present in cartilage surrounded by thick perichondrium. The first is the shortest.
 
 
 
The first tAvo rows of phalanges are present in all the digits. They are of cartilage surrounded by a very thick perichondrium, which is continuous with the condensed tissue between them and the metacarpals and between the phalanges them.selves. It is also continuous with the ■enlarged condensed tissue tip of each digit. There are thickenings for the various ligaments connecting the metacarpals and phalanges and the phalanges with each other.
 
 
 
The Musculak System. — (Plate II, Fig. C.) The trapezius muscle fibers extend from the occiput to the level of the sixth rib. There is a
 
 
 
 
 
 
 
178 The Development of the Arm in Man
 
 
 
considerable interval of fascia connecting them to the neural processes of the lower cervical and the thoracic vertebrae. There is a tendonous attachment to the clavicle and acromion and into fascia or condensed tissne on the surface of the infraspinatus between the trapezius and the deltoid.
 
 
 
The rliomhoid muscle lies in the region of the seventh cervical to the fourth thoracic vertebrae. It is inserted into the condensed tissue along the dorsal border of the scapula.
 
 
 
The latissimus dorsi muscle fibers extend from the humerus to the level of the ninth rib. There is a considerable interval of fascia between them and the neural processes of the lower thoracic and the first two or three lumbar vertebrae. This dorsal fascia is not very well marked. The latissimus also has fibers attached to the condensed tissue at the inferior angle of the scapula.
 
 
 
The serratus anterior muscle is separate from the levator scapulae except near its attachment to the scapula. It is a broad, thin sheet, having digitations to the first eight ribs.
 
 
 
The pedoralis major muscle is well developed. The separation between the clavicular and the sterno-costal portions is less marked than in the preceding stage.' The muscle is attached as low as the sixth rib.
 
 
 
The pedoralis minor muscle is quite distinct from the major, as a considerable layer of loose mesenchymal tissue lies between them. It arises from the second, third and fourth ribs and passes to the coracoid process.
 
 
 
The siibclavius muscle is inserted into the clavicle at an angle of 45°. As the scapula and clavicle sink down towards the level of the first rib the angle at which this muscle is inserted into the clavicle decreases.
 
 
 
The teres major muscle arises from the lower angle of the scapula and passes to the humerus. It is interesting to note that at this stage tendon of the latissimus dorsi twists around the lower l^order of the teres to be inserted with it into the humerus.
 
 
 
The deltoid muscle is large and well developed.
 
 
 
The suprasfinatus muscle arises from the thickened anterior border of the scapula. It cannot be said to take origin more from the lateral surface than from the median surface of the scapula.
 
 
 
The infraspinatus muscle occupies the middle of the lateral surface of the scapula and passes beneath the deltoid to the great tuberosity of the humerus.
 
 
 
The suhscapularis muscle arises from most of the median surface of
 
 
 
 
 
 
 
Warren Harmon Lewis 179
 
 
 
the scapula. Its tendon of insertion is broad and thin and closely applied to the capsular ligament.
 
 
 
The three heads of the triceps muscle are easily distinguished. The long and external heads are of about the same size. The anconeus muscle is continuous with the triceps but arises from the external condyle and passes to the side of the olecranon and adjoining surface of the shaft of the ulna.
 
 
 
The long 'head of the biceps muscle arises from the junction of the eoracoid process and the head of the scapula and passes through the bicipital groove. The two heads are inserted together into the condensed tissue swelling on the radius.
 
 
 
The corncobracJtviUs muscle and short head of the biceps are intimately connected for most of the length of the former.
 
 
 
The brachialis muscle has spread out over more of the distal surface of the humerus than in the preceding stage.
 
 
 
The flexor muscles of the forearm are easier to distinguish than in the preceding stage.
 
 
 
The tendon of the palinaris longus is narrower in proportion than in embryo XLIII.
 
 
 
The tendon of the flexor carpi raclialis muscle can be traced farther towards its insertion into the base of the second metacarpal than in embryo XLIII.
 
 
 
The muscle fibers of the flexor sublimis digitorum still run to the carpus before the ^\ide tendon begins. This tendon soon splits into four tendons which go to the four ulnar digits. These tendons are better formed than in the preceding stage and split to surround the tendons of the deep flexor. Their ends fuse with the thick perichondrium about the phalanges.
 
 
 
The flexor carpi uhiaris muscle shows distinctly its two heads of origin. It has a well-formed tendon of insertion.
 
 
 
The deep flexor muscles can be separated into the flexor polUcis longus and the flexor profundus digitormn muscles. The muscle fibers of the profundus continue into the carpal region and end in a broad tendon which divides at the base of the metacarpus into four well-formed tendons. These fuse with the condensed tissue at the tips of the digits. There is a slight split in each of these tendons near its end. The tendon of the flexor longus pollieis behaves similarly.
 
 
 
The pronator quadratns muscle is oval in cross section, and connects the distal ends of the radius and ulna.
 
 
 
The lumbricales are quite well developed and their fairly well-formed tendons end in the perichondrium on the radial side of the digit. 14
 
 
 
 
 
 
 
180
 
 
 
 
 
 
 
The Development of the Arm in Man
 
 
 
 
 
 
 
The interossei mnscles and the small mnscles of the thumb and little finger are now fairly well developed. Muscle fibers are present.
 
 
 
The extensor muscles of the forearm show considerable advance over the preceding stage. The tendons of the extensor communis digitoriim are longer and narrower. The muscle fibers continue to the base of the metacarpus, where the splitting into the four tendons takes place. The tendons are inserted into the condensed tissue tips of the digits. The edge of the tendons near their insertions are more or less continuous with the perichondrium about the digit.
 
 
 
The tendon of the extensor carpi ulnaris is beginning to. form. One branch of it seems to join the communis tendon. This may be the tendon of the extensor minimi digiti.
 
 
 
 
 
 
 
briich,ora,lml,s /and .■xl! indf
 
 
 
 
 
 
 
 
 
Fig. 1-t. Lateral view of the arm of embrj'o XXII. from Plate VIII. Bardeen and Lewis, Vol. I, this Journal. X 12 dia.
 
 
 
 
 
 
 
The extensor carpi radialis longior et hrevior are not to be separated.
 
 
 
The supinator muscle is well developed and has the posterior interosseus nerve passing through it.
 
 
 
The abductor pollicis longus and extensor pollicis brevis muscles are only to be separated where the muscle fibres pass into tendons, which fuse with the perichondrium of the first digit. The separation occurs at the lower end of the radius. These two muscles are fairly distinct from the supinator and the extensor pollicis longus and extensor indicis projiriiis muscles. The last two muscles are inseparable for part of their course and shortly after dividing each forms a round tendon. The extensor pollicis longus then spreads out into the sheath about the first
 
 
 
 
 
 
 
Warren Harmon Lewis ISl
 
 
 
digit. The extensor indicis muscle joins the nlnar side of the tendon of the commimis to the second digit.
 
 
 
The Nehves.— The drachial plexus has a decided posterior inclination and seems to have been pulled down against the first rib. The three cords are so close together that it was impossible to separate them satisfactorily though indications of the cords are present. There is nothing especially peculiar about the distribution of the nerves from the plexus^ either motor or sensory, which is not present in the adult.
 
 
 
Summary.
 
 
 
The first indications of the arm bud appear during the third week as a slight swelling in the lower cervical region on the anterior portion of the Wolffian ridge. This gradually enlarges and by the time the embryo is 4.5 mm. in length and three weeks old the arm is of considerable size. The base now lies opposite the lower four cervical and first thoracic vertebrae. The arm bud is at first filled with a homogeneous and closely packed mesenchyma. No nerves or myotome buds have entered the arm, yet it contains the tissue from which the muscular and skeletal elements develop.
 
 
 
During the fourth week before the nerves enter differentiation begins by an increased condensation for the skeletal core. The nerves, however, have reached the base of the arm and have united by their expanded ends into the first beginnings of the cervico-brachial plexus. During the fifth week the nerves from this plexus push into the pre-, muscle sheath which surrounds the skeletal core.
 
 
 
By the end of the fifth week the skeletal core can be differtotiated into many of the skeletal elements, three of Avhich contain cartilage, namely, the humerus, ulna and radius. The premuscle sheath also has become more or less differentiated into muscles or groups of muscles, between which, however, no sharp lines can, as a rule, be drawn. Toward the distal end the differentiation is less complete, and in the hand premuscle tissue still represents the intrinsic muscles. The nerves have grown into the hand and spread out in a veiy peculiar manner. Most of the branches of the brachial plexus found in the adult are now present.
 
 
 
By the end of the sixth week most of the muscles of the arm are easily recognized. The intrinsic muscles of the hand are just beginning to show fibrillation and are still mostly of premuscle tissue. The tendons and ligaments are also becoming more sharply differentiated. Most of the skeletal elements consist of cartilage and the surroundino
 
 
 
 
 
183 The Development of tlie Arm in Man
 
 
 
thick perichondrium. The clavicle, some of the carpals and the second row of phalanges are of condensed tissue, while the distal row of phalanges are not differentiated as yet. The nerves, both sensory and motor, are distributed much as in the adult.
 
 
 
By the end of the seventh week all the skeletal elements are of cartilage except the distal row of phalanges from the second to fifth digits, which are of condensed tissue. All the muscles are to be recognized and are composed of muscle fibers. The tendons and ligaments, except in the distal part of the digits, are well formed. The digits present a very interesting picture of the differentiation of the cartilage, perichondrium, ligaments, and tendons from the condensed tissue tip of each.
 
 
 
During the process of differentiation other important changes are taking place, namely, the migration caudally of the whole arm, the migTation or extension of certain muscles from the arm caudally along the body wall and the migration of other muscles from more anterior regions to the arm, shoulder girdle and thorax.
 
 
 
We may consider the position of the scapula and the inclination of the brachial plexus as indicators of the migration of the arm. We find, in an embryo of four and one-half weeks, that the scapula lies in the region of the fourth and fifth cervical vertebrae. The brachial plexus and the nerves forming it run to the arm without any caudal inclination. The nerves which leave the plexus do bend posteriorly in the arm. At five weeks the scapula has greatly enlarged and extends from the fourth cervical to the first dorsal vertebrae. Its center has evidently shifted posteriorly. The brachial plexus and the anterior nerves which go to it have a slight caudal inclination. By the end of the sixth week the greater portion of the scapula lies below the level of the first rib, its posterior angle, including the condensed tissue, having extended to the level of the fifth rib. The brachial plexus has been pulled along with the shifting of the arm and has a decided posterior inclination. By the end of the seventh week very little of the scapula lies above the level of the first rib, and its lower angle reaches to the fifth intercostal space. The brachial plexus has a very marked caudal inclination and appears to be bent over the first rib. Before the adult conditions are attained the sca]nila must migrate some distance posteriorly. Part of this movement will take place with the sinking posteriorly of the ventral portion of the thoracic wall, for in these stages the ventral ends of the ribs are as far anterior as the vertebra? from Avhicli they arise.
 
 
 
The migration of the pectoralis major and minor and the latissimus dorsi muscles from the arm posteriorly to the thoracic wall is very
 
 
 
 
 
 
 
Warren Harmon Lewis 183
 
 
 
evident from the stages we have studied. At a very early stage these masses receive their nerves and later drag them posteriorly. By the seventh week the pectoral mnscles have reached their adnlt positions so far as the thoracic attachments are concerned. The latissimns dorsi, even Ly the end of this week, only extends to the ninth rib.
 
 
 
Another A'ery important group of muscles migrate from the head and anterior cervical region to the arm and thorax. In an embryo, four and one-half weeks old, the posterior end of the trapezius })remuscle mass lies at the level of the fourth cervical vertebra, at five weeks the muscle fibers extend to the level of the fifth cervical vertebra, and at six weeks to the fifth thoracic vertebra. At this age also the muscle has acquired its attachment to the scapula and clavicle. At seven weeks the muscle extends to the level of the sixth thoracic vertebra. The sp'inal accessory nerve is connected to the premuscle mass as early as the middle of the fifth week, and as the muscle extends posteriorly the nerve is carried along with it.
 
 
 
The sterno-mastoid muscle originates high up in the neck with the trapezius. It extends posteriorly and ventrally, reaching the clavicle and sternum during the sixth week.
 
 
 
The infrahyoid muscles also migrate from the anterior neck region, carrying their nerves down with them.
 
 
 
The rhomboid premuscle mass at 4.5 weeks lies at the level of the fifth cervical vertebra and gets its nerve supply at this time from the fifth cervical nerve. At five weeks it has extended to the sixth cervical vertebra, and at seven weeks it is for the most part in the thoracic region and has acquired its scapidar attachment.
 
 
 
The serratus anterior premuscle mass at four and one-half weeks already extends into the upper thoracic region and has its cervical nerve supply. It has probably already migrated from the cervical region. At five weeks it has reached its posterior attachment on the thorax, but is not as yet attached to the scapula. This occurs during the sixth week. The serratus anterior muscle is thus one of the first of these migrating muscles to attain its permanent attachments. It is also evident that the various serrations of this muscle are of secondary origin.
 
 
 
 
 
 
 
 
 
conilensed. mfs. tip
 
 
 
 
 
 
 
Plate I, Fig. A. Lateral view of the arm region of embryo XLIII. X 20 diameters.
 
 
 
 
 
 
 
DEVELOPMENT OF THE ARM IN MAN. WARREN HARMON LEWIS.
 
 
 
 
 
 
 
PLATE I.
 
 
 
 
 
 
 
 
 
AMERICAN JOURNAL OF ANATOMY—VOL. I.
 
 
 
 
 
 
 
onder.seti. mes. tip.
 
 
 
 
 
 
 
 
 
Plate II, Fig. B. Median riew of left arm of embryo XLIII. X 20 diameters.
 
 
 
 
 
 
 
 
 
Plate TI, Fig. C. Median view of right arm of embryo XXII. X 20 diameters.
 
 
 
 
 
 
 
DEVELOPMENT OF THE ARM IN MAN. WARREN HARMON LEWIS.
 
 
 
 
 
 
 
PLATE II.
 
 
 
 
 
 
 
 
 
 
 
FIG C.
 
 
 
 
 
 
 
AMERICAN JOURNAL OF ANATOMY — VOL. I.
 
 
 
 
 
 
 
THE DEVELOPMENT OF THE EYE MUSCLES IN ACANTHIAS/
 
 
 
BY
 
 
 
ARTHUR B. LAMB.
 
 
 
From the Biological Laboratories of Tufts College.
 
 
 
With 9 Test Figukes.
 
 
 
These studies were imdertaken at the suggestion of Dr. J. S. Kingsley and were carried on during the year 1899-1900 under his direction, at the Biological Laboratory of Tufts College.
 
 
 
My specimens were killed either in aqueous corrosive sublimate or in Davidolf's corrosive-acetic mixture. Dektield^s hematoxylin was principally used as a staining agent. Wax reconstructions were made according to the method of Born. My results are largely but confirmatory of those of YdiW Wijhe, Miss Julia Piatt, Hoffmann and Neal. It is hoped, however, that a presentation of the subject freed from externals may, together with the series of reconstructions submitted, assist in the comprehension of the process of development.
 
 
 
The discovery that m selachii the eye muscles are developed from the epithelial walls of the Ist, 2nd and 3rd head somites was made by Marshall. His results have been repeatedly confirmed, and there can be no reasonable doubt of their validity. I use the term " somite " advisedly, being convinced that in Acanthias the head cavities are comparable with trunk somites. Neal, 98, p. 187, presents an excellent summary of the evidence on this point. A history of the development of the eye muscles is therefore a history of the origin and differentiation of these head somites. At this point I wish to call attention to the very detailed accoimt of the early history of these somites, presented by Hoffmann, 96, in his " Embryology of the Selachii."
 
 
 
AxTERiOR Somite. — Before passing to a consideration of the eye muscle somites proper. I propose, for the sake of completeness, to consider the anterior somite. Van Wijhe saw (82, p. 13), in the single specimeu of Galeiis at his disposal, on either side of the head, anterior to the 1st or premandibular somite, a slender cavity with distinct and
 
 
 
1 Studies from the Biological Laboratories of Tufts College, under the direction of J. S. Kingsley, No. XXIX.
 
 
 
 
 
 
 
186 The Development of the Eye Muscles in Aeanthias
 
 
 
thickened walls. He homologised this with the anterior prolongation of the first somite observed by him in Pristiurus and Scyllium. He therefore considered it as merely a secondary subdivision of that cavity. Miss Piatt, 91, observed similar cavities in Aeanthias, and to her is due the name of " anterior head cavities " ; it seeming unwise to alter the numbering of the remaining head somites for the sake of a pair of somites which are only known to occur in two forms. It also de
 
 
 
 
 
A. V.
 
 
 
 
 
 
 
OS. V.
 
 
 
 
 
 
 
S. A.
 
 
 
 
 
 
 
 
 
Op. Ves.
 
 
 
 
 
 
 
Fig. 1. — Reconstruction of auditory and optic vesicles, ganglia of fifth and seventh nerves and the anterior, first, second and third somites of the right side of an Aeanthias embryo, 12}^ mm. total length. Lateral view.
 
 
 
 
 
 
 
serves mention that Zimmermann, 91, p. llo, recognized, in the same year and quite independently of Miss Piatt, the presence in Aeanthias, of this somite. According to Miss Piatt, the archenteron, which extended forward to the anterior neuropore as a solid mass of cells, was divided into an anterior and a posterior part by the down-growing infundibnlum. Both parts .grew laterally; the anterior forming the anterior head somite, the posterior the 1st or premandibular somite. Hoffmann was able to add to this the observation, that the downgrowing anlage of the infundibnlum not only divides this process of the archenteron into an anterior and a posterior part, but that it also sub
 
 
 
 
 
Arthur B. Lamb
 
 
 
 
 
 
 
is:
 
 
 
 
 
 
 
divides the anterior part into three smaller portions: one axial in position, two lateral and paired. From these lateral portions develops, on either side, the anterior head cavity of Miss Piatt. Of the axial portion as much as lies beneath the infundibulum is aborted, while its anterior part persists and forms a connecting stalk by which the somite of one side is joined with that of the other. This connecting stalk is similar to that (C. sf. in figures) which joins the first (premandibular) somite of one side with that of the other. The only difference is that there we have a canal, while here we have a loose and solid strand of
 
 
 
 
 
 
 
0.5101
 
 
 
 
 
 
 
 
 
Fig. 2. — Parasagittal section of somites A, I and II of Acantliias embryo of 16 mm. total length showing the early ditlerenliation of muscles inferior and superior obliquus and inferior rectus.
 
 
 
 
 
 
 
connective cells. It wdl be seen that Hoffmann differs from Miss Piatt, simply in the recognition of an axial portion which later gives rise to a connecting strand. Stages in which 29-33 somites were differentiated showed very clearly the presence of this axial portion as described and figured by Hoffmann. See also Minot's figures (oi, pp. 82, 83).
 
 
 
At a 32 segment stage the somite is elongate in form, its main axis extending obliquely downward beneath the eyeball. It is smaller at its dorsal end, where the cells have assumed a somewhat epithelial arrangement, and a slight lumen is present. Ventrally. the somite consists of a
 
 
 
 
 
 
 
188 The Development of the Eye Muscles in Acanthias
 
 
 
large mass of cells rather loosely packed, and occupying the space between eye-ball and epidermis. The somite is pressed closely against the. anterior walls of both the 1st and the overlapping 2nd somite.
 
 
 
As the embryo grows older the somite assumes a much more elongate form (Fig. 1). The lumen increases in size and extends far into the ventral process, while the walls become considerably thinner and butone cell in thickness; still, they never become so thin as those of succeeding head somites. At a fourteen mm. stage the walls have again become thickened. This is especially true of the median and posterior walls. At a later stage (IG mm.) the cells are being freely proliferated into^ the lumen of the somite, and the median thickening is very marked. This proliferation continues, probably externally as well as internally, for the outline of the somite becomes gradually indistinct. At a 19 mm. stage (Fig. 4, SA) the somite consists simply of a solid mass of cells, gradually thinning out into the general mesenchymatoustissue. At a 26 mm. stage no trace of the somite can be seen. While,, as Hoffmann says, no muscle fibres are formed by this somite, still, as both Miss Piatt, 91, and Neal, 98, obserTed, the cells proliferated into the cavity assume an elongate form.
 
 
 
The anterior somite undergoes an interesting change in its position relative to the first somite. Originally pressed against the anterior walls of both the 1st and 2nd somites it comes, at a 12.5 mm. stage (Fig. 1), to occupy a position lateral to somite I, and in the angle between that somite and somite II. This seems to be due to the great enlargement of the eye vesicle, and also to the forward growth of somite I. Soon, however, the outpocketing from the posterior end of somite I, which gives rise to the inferior oblique, appears, and this ultimatel}^ grows nearly around the anterior somite, so that this later somite occupies a deep depression in its wall (Figs. 2 and 3).
 
 
 
ISTeal, 98, p. 227, found at a 65 segment stage processes apparently extending from the ciliary ganglion to this somite. I have not been able to find such processes of whose nervous character I was certain. FiKST, OE Peemandibular Somite. — The epithelial walls of this somite give rise to four of the six muscles of the adult eye. It is therefore preeminently the eye-muscle somite of the head. Balfour stated that this cavity was cut off from the anterior end of the coelom by the formation of the first gill cleft. Marshall also held that it was cut off from tlie anterior end of the coelom, but that this took place independently of the formation of the gill cleft. In Scyllium and Pristiurus. Van Wijhe found that the cavity was never in other than potential connection with the primary coelom, arising independently of it
 
 
 
 
 
 
 
Arthur B. Lamb
 
 
 
 
 
 
 
189
 
 
 
 
 
 
 
S. Obl. Cil. S-.
 
 
 
 
 
 
 
C. St. Op. V.
 
 
 
 
 
 
 
SI.
 
 
 
 
 
 
 
from the undifferentiated mass of cells in which the notochord anteriorly ends.
 
 
 
In Acanthias, as was shown above in the history of the anterior somite, tlie down-growing infnndihulum divides the anterior prolongation of the archenteron into an anterior and posterior portion. From the anterior portion the anterior somite develops. The posterior por
 
 
 
 
 
 
 
FiG. 3. — Reconstruction of optic vesicle, head somites and nerves of au Acauthias embrj'o, 16 mm. total length. Right side, medial view.
 
 
 
 
 
 
 
SII.
 
 
 
 
 
V.
 
 
 
 
 
sir.
 
 
 
 
 
D.
 
 
 
 
 
I. Obl.
 
 
 
 
 
S. A
 
 
 
 
 
 
 
 
 
Inf.
 
 
 
 
 
Rec
 
 
 
 
 
 
 
tion, growing laterally, gives rise to the first somite of either side, while axially it forms the connecting stalk or canal so characteristic of this somite (Fig. 3, C. si.). At a stage when 31-22 somites are differentiated, both the somite and the connecting strand are solid. This is still the case at a 29-30 mm. stage, except that in the connecting stalk a small cavity is visible. At a slightly later stage the number of these median cavities has increased. Miss Piatt, gi, homologised these with the median cavities described by Dohrn^ go, in Torpedo.
 
 
 
 
 
 
 
190 The Development of the Ej-e Muscles in Acanthias
 
 
 
A considerable lumen can now be distinguished in the lateral somites. The median stalk is continuous at its middle with the notochord above and the alimentary canal behind. Hoffmann describes three processes arising at this stage (32 segments) from the 1st somite :
 
 
 
I. A process extending backwards and downwards, running close to, and j)arailel with, the visceral prolongation of the 2nd somite. He considered it probable that this process, although he had never seen a lumen in it, was comparable to the hollow process found by Zimmermann, 91, in Pristiurus, connecting the premandibular somite with the ventral coelom. Neal, 98, p. 201 note, while confirming the. presence of this " Zellstrang," was very certain that it was derived not from the mesoderm but from the neural crest, having followed the migration of neural crest cells ventrally into the mandibular arch. Neal also called attention to a similar strand of cells situated posterior to the visceral portion of the mandibular cavity. While I found that this strand of cells is apparently continuous with a slight outgrowth from the 1st somite, as figured by Hoffmann, this continuity seems to be more apparent than real. In the first place, I am able to confirm the presence of the posterior strand described by Neal. Further, I found this posterior strand not only continuous laterally with the anterior strand, but dorsally with neural crest cells. This is especially evident at a 32-33 somite stage. I therefore conclude with Neal that this " Zellstrang " is not a process of the somite, but is rather derived from the neural crest.
 
 
 
II. A process extending ventrally forward below the anterior cavity. A similar process was seen by A'^an Wijhe in Pristiurus and was homologised by him with the anterior head cavity of Galeus. Since the anterior cavity in Galeus is in all probability homologous with that in Acanthias, if Hoffmann's process in Acanthias is homologous with that found by Van Wijhe in Pristiurus, then, since both occur at once in Acanthias, the homology drawn by Van Wijhe could not 1)e maintained. Hoffmann believed this to be the case.
 
 
 
1 have been able to find but very slight evidences of this process. I am led therefore to doubt the homology drawn by Hoffmann between this process and the process described by Van Wijhe in Pristiurus. Consequently, I cannot on this ground take exception to the homology drawn by Van Wijhe between the process of the 1st somite in Pristiurus and the anterior somite in Galeus.
 
 
 
III. A process extending dorsally along the anterior surface of the 2nd somite. It is small in extent and transitory in appearance. I am inclined, here as before, to doubt the real continuity between this
 
 
 
 
 
 
 
Artluir B. Lamb
 
 
 
 
 
 
 
191
 
 
 
 
 
 
 
process and the somite, since it seems to me that it might equally well be derived from neural crest cells.
 
 
 
As the embryo develops, the median stalk is pushed away from the alimentary canal and around the end of the notochord by the intervening aorta. The notochord thus shifts its position from the dorsal wall of the median stalk to its posterior wall. Such is the compression
 
 
 
 
 
 
 
iE >■
 
 
 
 
 
 
 
o. s
 
 
 
 
 
vir.
 
 
 
 
 
o.
 
 
 
 
 
S. V.
 
 
 
 
 
s
 
 
 
 
 
Obi.
 
 
 
 
 
s.
 
 
 
 
 
Rec.
 
 
 
 
 
 
 
 
 
OC.
 
 
 
 
 
Int.
 
 
 
 
 
Eec.
 
 
 
 
 
 
 
 
 
Fig. 4. — Reconstruction of optic vesicle, ganglia and somites of an Acautbias embryo, 19 mm. total length. Right side, medial view. The Anlagen of the muscles lined obliquely.
 
 
 
that exists in this region that the median stalk is nearly severed at the point of contact. The irregularly placed median cavities of the stalk now fuse together and finally assume connection with the lumen of the somite on either side. The lumen of the median stalk persists until a late stage, when the walls become mesenchymatous and the cavity is obliterated. The stalk persists, however, as connecting strand until nearly a 30 mm. stage (C. st. in Figs. 1-7).
 
 
 
 
 
 
 
193 The Development of the Eye Muscles in Acanthias
 
 
 
The thickenings of the epithelial walls which give rise to the four muscles^ occur in general on the dorso-median walls. The first thickening appears on the more ventral end of the somite. This soon becomes a large outpocketing with thick walls (Fig. 3, I. Ohl.). It is the anlage of the muscle obliquus inferior. This therefore is, as Hoffmann pointed out, the first of the oculomotor muscles to be differentiated.
 
 
 
The next thickening to appear is located very near the abovementioned anlage of the inferior oblique. It gives rise to the muscle rectus inferior (Fig. 3, Inf. Bee). At a later stage thickeniugs appear on the more dorsal end of the somite. These do not become well marked and differentiated from one another until a somewhat late stage, about 20-22 mm. The more dorsal of these thickenings forms the muscle rectus superior; the more ventral, the muscle rectus interior.
 
 
 
It will be seen from the above that the muscles arise in two pairs, one at either end of the somite.
 
 
 
The outpocketing which is to form the inferior oblique soon becomes constricted off froin the somite. At a 19 mm. stage its lumen has nearly disappeared, and the muscle has assumed an elongate form (Fig. 3). The direction of the principal axis as well as the direction of its muscle fibres is longitudinal. In the adult the direction is also nearly longitudinal, but it will be seen from the series of reconstructions (Figs. 4, 6, 7, 8), that the originally anterior end has become posterior; i. e., the direction of the muscle is nearly reversed. This transformation is brought about by a revolution of the posterior end about the anterior end as a centre, combined with a general ventral shifting of the muscle. The adult condition is approximately reached at a 33 mm. stage (Fig. 8).
 
 
 
The thickening, which is to form the inferior rectus, and belonging to the same pair as the inferior oblique, at first extends parallel to that muscle and therefore in a longitudinal direction (Fig. 4, Inf. Eec). At a 26 mm. stage it has turned through approximately a right angle, and runs in a general dorso-ventral direction (Figs. 6, 7, Inf. Rec).
 
 
 
The thickenings which arise in the more anterior and dorsal end of the somite, and which give rise to the superior and internal recti, have only become clearly differentiated at a 25 mm. stage (Fig. 7). The internal rectus retains its nearly longitudinal direction; the superior rectus describes approximately a right angle about its posterior end. At a 33 mm. stage (Fig. 8) it has approximately reached its adult dorsoventral direction. Except where the muscle thickenings have been formed, the walls of the somite retain their single layered epithelial character until about a 27-30 mm. stage, when they become converted into loose mesenchyme and the outline of the somite is lost.
 
 
 
 
 
 
 
Arthur B. Lamb
 
 
 
 
 
 
 
193
 
 
 
 
 
 
 
The ninsculature arising from this somite is innervated by the ■oculomotor. This nerve is diiTerentiated at an 8 mm, stage. It arises, Neal, 98, from the ventral floor of the mid-brain, as processes from neuroblast cells in the ventral horn of this encephalomere. It extends backward to the ciliary ganglion and runs through it to the walls of the 1st somite. Neal, 98, p. 22 T, found, at a stage before the appearance of the oculomotor, processes extending from the ciliary ganglion to the somite, similar to those found in connection with the anterior
 
 
 
 
 
 
 
s c
 
 
 
 
 
 
 
 
 
Fig. 5. — Reconstruction of optic vesicle anrl derivatives of the mandibular and liyoid somites of tlie right side of an Acanthias embryo, 22 mm. long, viewed from medial side.
 
 
 
 
 
 
 
somite. As in that case, I have been unable to convince myself that the fibres Avhich seem to connect ganglion and somite are really nervous in character.
 
 
 
Secoxd, oe M.vxD[BUL.\ii SoMiTE. — This somite is the largest in the head, and is characterized by the possession of a visceral portion connecting it with the ventral ctelom.
 
 
 
Hoft'mann. 96, found this somite marked off by constrictions frcnii the general body cavity at a 20 somite stage. A contracted lumen was present. At this stage the floor of the brain is pressed closely down upon the notochord and somite, but as growth takes place it draws
 
 
 
 
 
 
 
194 The Development of the E3^e Muscles in Acanthias
 
 
 
away, and a considerable space is left beneath it. Into this space a proliferation of mesenchyme cells takes place from the anterior7median wall of the somite.
 
 
 
The walls of the somite at this stage are thin and single-layered^ except in the median side, where the cells are higher and a tendency towards the formation of two layers is evident. Later, the median wall becomes thicker, and the area of mesenchyme proliferation more definite. The cells derived from this outgrowth are spreading out around the walls of the somite. This outoTowth seems to me to be
 
 
 
 
 
 
 
 
 
Fig. 6. — Reconstruction of optic vesicle and derivatives of the premandibular somite of an Acauthias embryo, 2(5 mm. total length. Right half, medial view.
 
 
 
 
 
 
 
comparable to a sclerotome of a trunk sonute, both in position and in histological appearance.
 
 
 
At a 10 mm. stage the lateral, as well as the posterior median walls, have become very thin. This thinning out at posterior median wall continues, and soon the epithelial character of the bounding tissue is lost. This break, together with a constriction which gradually takes place, divides the somite at this point ultimately into a dorsal and a
 
 
 
 
 
 
 
Arthur B. Lamb 195
 
 
 
visceral part. At a 13 mm. stage the lumen of the one part is completely separated from that of the other. Until a late stage, however (24 mm.), the two parts are connected by strands of mesenchymatous tissue (Ifig. 5, MS.).
 
 
 
For the sake of clearness I will treat the further development of dorsal and visceral parts separately.
 
 
 
Dorsal Part, or the Myotome Proper. — At a fourteen mm. stage a large outpocketing with slightly thickened walls is apparent at the anterior end of this part of the somite. This outpocketing is the anlage of the muscle obliquus superior. At the base of this outpocketing, on the median side of the somite, a thickening of the epithelium is evident. This is the anlage of a rudimentary muscle first mentioned by Miss Piatt, and spoken of by her as " Muscle E." The walls of the remainder of the dorsal part are very thin.
 
 
 
Sixteen mm. stage (Kg. 3). The outpocketing giving rise to the superior oblique has become very thick-walled. " Muscle E is well developed. The direction of its principal axis, as well as of its fibres, is longitudinal. It is not straight, however, but its anterior end is curved outwards.
 
 
 
From this stage on, those parts of the walls not forming muscles rapidly degenerate, and the lumen of the somite is usurped by mesenchymatous tissue. A contracted lumen, however, persists until a late stage in the anlage of the superior oblique muscle. The general direction of this muscle and of the fibres which are now present in it, is longitudinal. The whole muscle migrates forward, the posterior end becoming attached, while the anterior end moves ventrally. The muscle consequently comes to extend in a dorso-ventral direction (Figs. 4, 5, 8, S. Ohl.). At a 19 mm. stage (Fig. 4) it is still longitudinal and still retains connection with " Muscle E " by a strand of connective tissue. This latter muscle has now reached its maximum development. The anterior end curves not only outward but upward as well, so that the direction of the muscle is approximately dorso-ventral. From now on this muscle undergoes degeneration. At a 26 mm. stage scarcely a trace of it remains.
 
 
 
Visceral Part. — At a 12 mm. stage the walls of this part, except "where they are continuous dorsally with the somite proper, are closely compressed right and left. At the ventral end the walls are very thick, and constitute the anlage of the muscle adductor maxillse.
 
 
 
At a 14 mm. stage an outpocketing appears at the dorsal end. This is the anlage of a rudimentary muscle which Miss Piatt, gi, first recognized. Hofl'mann, 96, considered this muscle identical with that de15
 
 
 
 
 
 
 
196
 
 
 
 
 
 
 
The Development of the Eye Muscles in Acanthias
 
 
 
 
 
 
 
scribed by Vetter^ 74, and designated by him, muscle levator labii superioris. He does not, however, give any reasons for this view. Miss Piatt was able to trace this muscle only until it came to occupy a position in close proximity to the inferior oblique eye muscle. She believed, however, that the muscle was permanent.
 
 
 
This outpocketing, as will be seen in the reconstructions, can readily be folloAved until a 26 mm. stage (Figs. 4, 5, 6, F). At a 28 mm. stage
 
 
 
 
 
 
 
 
 
 
 
 
 
O. S. V.
 
 
 
 
 
 
 
 
 
^'^'i; ■^
 
 
 
 
 
 
 
Int. Uec.
 
 
 
 
 
 
 
O f
 
 
 
■? o
 
 
 
 
 
 
 
Fig. 7. — Optic vesicle, nerves and developing eye muscles of an Acanthias embryo, 27 mm. total length. Right side, medial view. The oblique lines indicate the parts of the somites which are being converted into muscles.
 
 
 
 
 
 
 
its walls are thin and enclose an extensive lumen. At a 26 mm. stage its lumen has disappeared and its constituent cells have become elongate and apparently muscular. The muscle is situated at this stage, as Miss Piatt stated, close to the inferior oblique eye muscle. As will be seen in reconstructions, it is nearly continuous with the thickened ventral edge of the remainder of the visceral part. The cells of this
 
 
 
 
 
 
 
Arthur B. Lamb 197
 
 
 
thickened edge seem also muscuhir. At a 27 mm. stage I have been able to find only the very slightest remains of this muscle. Such a rapid degeneration seems, however, improbable, and requires, I feel, further investigation.
 
 
 
The superior oblique muscle derived from this somite is innervated in the adult by the trochlearis. This nerve, however, is the last cranial nerve to be differentiated, not appearing until a 21-22 mm. stage (Fig. 5). At a 16 mm. stage, as several investigators have shown, the small ramus ophthalmicus superficialis V. sends fibres to this somite. From this it might be inferred that motor impulses were originally transmitted to this somite by the 5th nerve. Neal considers this supposition untenable, since in embryos of but 19 mm. length, consequently before the appearance of the trochlearis, the ramus ophthalmicus superioris V. shows no connection with the muscle. While unwilling to contradict this latter statement and say that such a connection does exist, I should be even more unwilling, because of the very close proximity of muscle and nerve at 19-24 mm. stages, to say that such a connection does not exist.
 
 
 
Thikd, ok Htoid Somite. — This is the most posterior somite which contributes to the musculature of the eye, giving rise to the external rectus muscle. It is marked off from the rest of the body cavity merely by constrictions before and behind, at a time Avhen the 7th somite of Van Wijhe is completely separated. This emphasizes the progressive development which takes place both forward and backward from a point in the neck region. At this stage its form and position is very similar to that of the four succeeding head somites. At a 22 segment stage a contracted lumen is generally visible. ISTeal points out that at this stage the somite is in every way comparable with a trunk myotome. It is plainly dorsal in its topographical relations to the notochord, dorsal aorta and dorsal wall of the alimentary canal. Mesenchymatous cells are plainly proliferated from a well marked area on its median wall, forming an outgrowth comparable to the sclerotome of trunk somites. The somatic wall is plainly epithelial and there is a well marked myocoel. Finally, the somite is, as will be seen later, innervated by a nerve which is generally recognized as comparalile with the ventral root of a spinal nerve.
 
 
 
At a 28-30 segment stage this somite is completely separated from its neighbors. Its lumen has become well marked, while anteriorly the somite has assumed a bilobed appearance. At a 32 segment stage this lobation is especially evident. More marked growth now takes place in the middle and posterior part of the somite, and at the same time the walls there begin to lose their epithelial character.
 
 
 
 
 
 
 
198
 
 
 
 
 
 
 
The Development of the Eye Muscles in x\canthias
 
 
 
 
 
 
 
At a 10 mm. stage the lateral posterior end has become distinctly bilohed, and the more lateral of these lobes bears, in favorably orientated sections, a striking resemblance to the visceral portion of the 2nd somite. The disintegration of the epithelial walls of the main portion of the somite continues, while the more dorsal of the two anterior lobes has increased greatly in size. At a 13 mm. stage (Fig. 1) the main
 
 
 
 
 
 
 
fci > >
 
 
 
 
 
 
 
^ p; ►-;
 
 
 
 
 
 
 
O. S. V.
 
 
 
Tr. S. Ot)l.
 
 
 
Op. V. Int. Rec.
 
 
 
 
 
 
 
 
 
Abd. Ga. g. B. Rec.
 
 
 
 
 
 
 
Inf. Rec.
 
 
 
 
 
 
 
Fig. 8. — Optic vesicle, nerves and eye muscles of an Acautliias embryo, 33 mm. total length. Right side, medial view. The muscles have now nearly their definitive position.
 
 
 
 
 
 
 
portion of the somite, which is large and globular, is bounded merely by loose .connective tissue which now rapidly fills up the lumen of this part of the somite. There is therefore remaining only the anterior prolongation or dorsal lobe, which consists mainly of elements derived from the median wall. This prolongation now grows rapidly and extends directly forward as an elongate pointed process. Its median dorsal wall is thickened, especially at the posterior end. There, as well
 
 
 
 
 
 
 
Arthur B. Lamb
 
 
 
 
 
 
 
199
 
 
 
 
 
 
 
as in the anterior portion, cells are assuming an elongate form and a longitudinal direction. The posterior end is indistinct in outline, and cells are evidently dropping off into the mesenchyme. By this degeneration at its posterior end, by growth of the muscle as a whole, and especially by the outpushing at its anterior end (Fig. 9, E), the whole somite moves forward, so that while originally located some distance away, it comes to lie in close proximity to the eyeball (Figs. 4, 5, 7, 8, E. Bee. Fig. 9, a).
 
 
 
 
 
 
 
OP ve
 
 
 
 
 
 
 
 
 
& »8P*'S
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 9. — Part of transverse section of Acantbias embryo, 19-20 mm. total lengtb, showing tbe proliferation of the external rectus muscle (E) from the byoid (III) somite.
 
 
 
 
 
 
 
This somite is innervated by the abducens. This nerve is, as mentioned above, generally considered as homologous to the ventral motor root of spinal nerves. It arises (Neal, g8, p. 230) at a ten mm. stage from the tloor of the hind brain at a point opposite the ear vesicle as an outgrowth from neuroblast cells in the ventral wall of eneephalomere YII. The nerve gradually extends forward until it reaches its somite.
 
 
 
 
 
 
 
200 The Development of the Eye Muscles in Aeanthias
 
 
 
Conclusions.
 
 
 
Broadly considered, it will be seen that the necessary mechanical relations between eyeball and muscle is secured: (1) by a forward growth of processes from the 3nd and 3rd somites, and the development of muscle fibres in them; (2) by a S23reading out of the 1st somite around the eyeball and the development of muscles in its distal portions.
 
 
 
I wish to call attention to the fact, which so far as I know has not been noticed before, that the original direction of all the eye muscles together with " Muscle E " is longitudinal. This seems to me to represent an originally flexible condition of the head and to be an additional support for the homology of head and trunk somites.
 
 
 
Finally, it seems to me improbable that the present musculature of the eye in Aeanthias is the primitive one for several reasons: (1) The adult condition is reached only after the constituent muscles have undergone rather extensive alterations in form and transfer of position, (2) The muscles do not all arise equally early, nor do they reach their definitive condition at the same time. (3) Before some of the permanent eye muscles are formed, one muscle ("' Muscle P] "), which later disappears, reaches an advanced stage of development. This muscle, from its form and position, must either have once been functionally connected with the eye or with some structure now lost, and of which not even an embryonic rudiment is known. The same reasoning applies to the anterior somite though with diminished force, since it does not reach the same advanced stage of development.
 
 
 
If then the present musculature of the eye is not the primitive one, it becomes an interesting question to inquire if the embryonic development will indicate any stages in the phylogenetic development. Two such stages, it seems to me, are indicated. 1st, a stage where if any eye musculature existed it was furnished by the anterior somite. This is indicated first by the fact that this somite is the only one which from its topographical relations could move the eye; and second, the longitudinal direction and serial arrangement of the remaining muscle anlagen indicate a jointed condition of the head and consequently a functional activity on the part of these muscles which would preclude any connection with the eye.
 
 
 
2nd, a stage at which four muscles moved the eye. These were the superior oblique, the external rectus, the inferior oblique and " Muscle E." These four muscles were arranged radially. " Muscle E " and the inferior oblique opposed one another, the former pulling the back of the eye dorsally, the latter, ventrally. The superior oblique and the
 
 
 
 
 
 
 
Arthur B. Lamb 201
 
 
 
external rectus opposed one another, the former pulling the back of the eye forward; the latter, backwards. This stage is reached in ontogeny at a length of 21-22 mm. (Figs. 4, 5, 6). The four muscles then have the rectangular radial arrangement described above. They have all reached approximately the same degree of differentiation, which is far in advance of the three remaining eye muscles.
 
 
 
These speculations, based solely on ontogenetic evidence, require confirmatory phylogenetic evidence derived from a study of forms lower than Acanthias.
 
 
 
EEFERENCE LETTERS COMMON TO ALL FIGURES.
 
 
 
Altd., abducens nerve.
 
 
 
AM., anlage of muscle adductor mandibulEe.
 
 
 
A. v., auditory vesicle.
 
 
 
BC. I., first branchial cleft.
 
 
 
Cil. g., ciliary ganglion.
 
 
 
C. m,, stalk connecting the premandibular somites of the two sides.
 
 
 
U., Temporary muscle derived from the 'mandibular somite; in Fig. 9, external rectus muscle.
 
 
 
Ect., ectoderm of dorsal surface of head.
 
 
 
E. Rec, externuis rectus muscle.
 
 
 
F., temporary muscle derived from the mandibular somite.
 
 
 
Qa. ff., Gasserian ganglion.
 
 
 
/. Obi., inferior oblique muscle.
 
 
 
Inf. Rec, inferior rectus muscle.
 
 
 
Int. Rec, internal rectus muscle.
 
 
 
MB., mesenchyme connecting dorsal and visceral parts of somite II.
 
 
 
NC, notochord.
 
 
 
OC, oculomotor nerve.
 
 
 
Op. St., optic stalk.
 
 
 
Op. Ves., optic vesicle.
 
 
 
Op. v., ophthalmicus profundus branch of fifth nerve.
 
 
 
O. a. -v., ophthalmicus superficialis branch of fifth nerve.
 
 
 
0. S. VII., ophthalmicus superficialis branch of seventh nerve.
 
 
 
Ra. v., anterior root of fifth nerve.
 
 
 
Rp. v., posterior root of fifth nerve.
 
 
 
R. VII., root of seventh nerve.
 
 
 
S. A., anterior head somite.
 
 
 
»SV., 811., 811 1., first (premandibular), second (mandibular), and third (hyoid) somites.
 
 
 
8. Ohl., superior oblique muscle.
 
 
 
8. Rec, superior rectus muscle.
 
 
 
811. D. d: v., dorsal and ventral portions of somite II.
 
 
 
Tr., trochlearis nerve.
 
 
 
YII., seventh nerve.
 
 
 
All of the figures except 2 and 9 are drawn from vs^ax reconstructions. The regions ruled on figures 4 and 7 are those portions of the somites which are being transformed into muscles.
 
 
 
 
 
 
 
202 The Development of the Eye Muscles in Acanthias
 
 
 
BIBLIOGRAPHY.
 
 
 
DOHRN, Anton, 'go. — Studien zur Urgeschichte des Wirbelthierkorpers. XV.
 
 
 
Nene Grundlag-en zur Beurtheihing- der Metamerie des Kopfes.
 
 
 
Mittheil. Zool. Station, Neapel, Bd. IX, p. 330, 1890. HoFJFMANN, C. K., '96. — Beitrage zur Eutwicklungsgeschiclite der Selachii.
 
 
 
Morphol. Jahrbuch, XXIV, pp. 209-286, pis. ii-v, 1896. Marshall, A. M., '81. — On the head cavities and associated nerves in Elasnio branchs. Quarterly Jour. Micros. Sci., XXI, pp. 72-97, 2 pis., 1881. MiNOT, Charles S., '91. — On the morphology of the pineal region, based
 
 
 
upon its development in Acanthias. American Journal of Anatomy, i. pp. 81-98, 1901. Neal, H. v., '98. — The segmentation of the nervous system in Squalus acanthias. Bulletin Mus. Comp. Zool., Harvard College, XXXI, No. 7, pp.
 
 
 
147-294, 9 pis., 1898. Platt Julla. B., '91. — A contribution to the morphology of the vertebrate
 
 
 
head based on a studj^ of Acanthias vulgaris. Journal of Morphol.,
 
 
 
V, pp. 79-112, 3 pis., 1891. Platt, Julia B., 'gi^. — Further contribution to the morphology of the
 
 
 
vertebrate head. Anatom. Anzeiger, VI, pp. 251-265, 1891. Van Wijhe, J. W., '82. — Ueber die Mesodermseg-mente und die Entwickelung
 
 
 
der Nerven des Selachien Kopfes. Natuurk. Verh. Koninkl. Akad.
 
 
 
Amsterdam. Deel. XXII, pp. 50, 5 pis., 1882. Vetter, Benjamin, '74. — Untersuchungen zur vergleichenden Anatomic der
 
 
 
Kiemen- und Kiefermusculatur der Fische, Jena. Zeitschrift, VIII,
 
 
 
p. 405, 1874. ZiMMERMANN, K. W., 'gi. — l^cber die Metamerie des Wirbelthiereskopfs.
 
 
 
Verhandl. Anat. Gesellsch. V. (Anat. Anzeiger, VI, Ergtinzungs
 
 
 
Hefte), pp. 107-114, 1891.
 
 
 
 
 
 
 
A STATISTICAL STUDY OF THE ABDOMINAL AND BOEDERNERVES IN MAN.
 
 
 
BT
 
 
 
CHARLES RUSSELL BARDEEN, M. D.
 
 
 
From the Anatomical Laboratory of the Johns Hopkins University, Baltimore, 2Id.
 
 
 
With 8 Figures and 14 Tables.
 
 
 
The following imper presents the results of a study of the distribution of the main nerves of the abdomen and of the border region between the abdomen and thigh in man. The study was made in the dissecting rooms of the anatomical laboratory of the Johns Hopkins University^ The methods employed have been elsewhere described.'
 
 
 
Of the ventral branches of the twelve thoracic or intercostal nerves, the first six generally are confined in distribution to the thorax/ while . the last six are distributed in part to the abdominal walls. In addition, the first two lumbar nerves usually give rise to branches that are distributed to the distal margin of the thoracic wall, and to the skin at the junction of the abdomen and thigh. Considerable variation, however, exists in the origin and distribution of the nerves of the abdomen and of the border region. The extent of this variation in the main nerve tracts is shown in the tables given on pages 216 to 228. The following notes briefly explain these tables:
 
 
 
The most Anterior Thoracic Nerve, the Ventral Branch of WHICH Extends into the Abdominal Wall to Form the "First Abdominal Nerve." See Table I.
 
 
 
This table is based upon a very limited number of instances. The ventral branches of the intercostal nerves confined to the thorax emerge ventrally from between two successive costal cartilages. The ventral branches of the nerves distributed both to the abdomen and to the thorax pass below the costal margin of the thorax, then course forwards
 
 
 
, '(1) A statistical study of tbe variations in the formation and position of the lumbo-sacral plexus in man, Bardeen and Eltin^, Anatomischer Anzeiger, 1901, VoL XIX, p. 124. (2) Use of the material of the dissecting room for scientific purposes, Bardeen, Johns Hopkins Hospital Bulletin, 1901, Vol. Xll, p. 1.5.5.
 
 
 
2 With the exception of the fibres distributed by the first two or three to the arm.
 
 
 
 
 
 
 
204 Study of the Abdominal and Border-Nerves in Man
 
 
 
between the muscles of the abdominal wall, and finally are distributed in part to the rectus musculature and in part pass through the latter to be distributed to the skin. The last intercostal nerve confined to the thorax is distributed in part to the thoracic segment of the rectus knd in part to the overlying skin, while the ventral branches of the more anterior intercostal nerves pass through, the overlying structures directly to the subcutaneous tissue. It is probable that occasionally more than one thoracic segment of the rectus muscle is developed in man. Such, however, has not been the case in the subjects which I have examined. As shown in the table, in 10 instances the 6th nerve was the nerve distributed to the thoracic segment of the rectus, and in 6 instances the 7th. The first nerve passing below the costal margin before entering the rectus (that is to say, the first abdominal nerve) was in 10 instances the 7th, and in 6 instances the 8th nerve. A record was preserved of the race, sex, side of body, skeletal conditions, position of the lumbo-sacral plexus and of the border-nerves in the instances studied. No marked relation seems to exist between any of these factors and the variations noted in the table.
 
 
 
Eelations of the Abdominal Nerves to the Teansverse Tendons (Linea transversa, Inscription es tendinece) OF the Eectus AbdoimiNis Muscle. See Table II. — The transverse tendons of the rectus abdominis muscle in man correspond to the 7th, 8th, 9th, 10th, and 11th ribs.^ The relation between the transverse tendons and the costal cartilages is best seen in the transverse tendons corresponding to the seventh and eighth cartilages. In this region the bundles of fibres making up a segment of the rectus originate or terminate in part on the costal cartilage, in part in the corresponding tendon. The transverse tpndons corresponding to the eighth costal cartilage are often, to the ninth are usually, and to the tenth and eleventh are always some distance removed from the corresponding costal cartilage.
 
 
 
In the adult individual the transverse tendons are often to a greater or less extent obliterated, owing to the unequal growth of muscle-fibre bundles, some of which extend over more than one segment. Tlie transverse tendon corresponding to the 7th costal cartilage has been fairly distinct in all of the instances I have examined, but that corresponding to the 8th was absent in 9 out of 37 instances (24.3^); to the 9th in 4 out of 62 instances (6.5^); to the 10th in 9 out of 85 instances
 
 
 
3 See Mall, Devel. of the Ventral Abdominal Walls in Man, Journal of Morphology, Vol. XIV, p. 2, 1898.
 
 
 
 
 
 
 
Charles Eussell Bardeen 205
 
 
 
(10.6^); and to the 11th in 56 out of 79 instances (70. 9j^). In no instance have I seen a transverse tendon corresponding to the 13th rib.
 
 
 
In the simple conditions where the segments of the rectus are distinct and the transverse tendons well marked, the abdominal nerves after emerging from the intercostal spaces take a fairly direct course to the lateral margin of the rectus. Each nerve then pierces the rectus sheath, courses along the under surface of the latter muscle, and then gives off one or more cutaneous branches which usually emerge through the rectus in the vicinity of the transverse tendon corresponding to the rib by which the nerve is designated, and one or more muscular branches which are distributed to the segment of the rectus distal to that transverse tendon. The region where the rectus sheath is pierced by a given spinal nerve is usually posterior to the corresponding transverse tendon in case of the 7th and 8th nerves, and anterior in case of the 10th, 11th and 12th nerves. In a recent article in this journal * it has been shown that the primary ventral cutaneous branches of the more distal thoracic nerves are caught between the successive tips of those myotomes which give rise to the rectus musculature, and that in this way, the segmental arrangement of the nerves of the abdomen is early insured. In those instances in Avhich no transverse tendon is developed in the region between the tips of two myotomes, the tissue derived from each myotome is fused to a considerable extent, and the corresponding nerves are less definitely guided in their growth. We find, therefore, in the adult far greater irregularity in the course and in the distribution of the branches of such nerves. Often two or more nerve trunks, arise from a single intercostal nerv^e, and course forward to pierce the rectus sheath separately. See Fig. II. This was found to be the case with the 8th nerve, in 2 out of 37 instances; with the 9th in 3 out of 61; with the 10th in 6 out of 85; with the 11th in 18 out of 79 instances; and with the 12th in 17 out of the 56 instances in which the twelfth furnished no direct hypogastric branch. In 18 instances, out of the 74 in which a careful record was made of the branches of the 12th thoracic nerve, the nerve sent a ventral branch to the rectus, and a separate hypogastric branch to the skin of the abdomen. See Fig. Ill A.
 
 
 
Not infrequently an abdominal nerve will divide into two or more branches immediately before entering the rectus sheath. See Fig. I C.
 
 
 
In the majority of instances, as pointed out by Mall (op. cit.), the transverse tendon of the rectus corresponding to the 10th rib is attached
 
 
 
4 Bardeen and Lewis: Development of the limbs, body-walls and back in man. This journal, Vol. I, 1901, p. 1.
 
 
 
 
 
 
 
206
 
 
 
 
 
 
 
Study of the Abdominal and Border-Nerves in Man
 
 
 
 
 
 
 
on its median margin to the dense tissue surrounding the umbilicus. This occurred in. 73 out of 85 instances. In 13 out of the 85 instances (15.3;^), the transverse tendon corresponding to the 11th rib was intimately united to the tissue surrounding the umbilicus. See Fig. V. In 9 of these instances the transverse tendon corresjoonding to the 10th rib Avas absent, in 4, present.
 
 
 
I have discovered no close relation between the development of the 7th, 8th, 9th, 10th, and 11th nerves, and the transverse tendons corresponding to them on the one hand, and race, sex, side of body, skeletal conditions, position of the lumbo-sacral plexus, or distribution of the border-nerves, upon the other.
 
 
 
 
 
 
 
Okigin of the Most Distal Abdominal JSTerve En^tepjxg the Eectus Muscle. See Table III. — As may be seen from the table, the 20th spinal (12th thoracic) nerve is the most distal spinal nerve supplying the rectus muscle in the great majority of instances (9G out of 112 instances, 85.8^).
 
 
 
The nineteenth spinal nerve was the last nerve to furnish fibres to the rectus muscle in but two instances. In both of these the spinal column was shorter than normal, the plexus had an anterior position, and the border-nerves were of a proximal type.^
 
 
 
Frequency with 2i<hicJi the most distal nerve to the rectus ahdominis muscle arose in the types of plexus designated^ from the spinal nerves indicated in the column at the left.
 
 
 
 
 
 
 
 
 
 
 
Types of Plexus.
 
 
 
 
 
Spinal Nerves.
 
 
 
 
 
A
 
 
 
 
 
B
 
 
 
 
 
C
 
 
 
 
 
D
 
 
 
 
 
E
 
 
 
 
 
E
 
 
 
 
 
G
 
 
 
 
 
 
 
 
 
No.
 
 
 
of
 
 
 
inst.
 
 
 
 
 
%
 
 
 
 
 
No.
 
 
 
of
 
 
 
inst.
 
 
 
 
 
%
 
 
 
 
 
No.
 
 
 
of
 
 
 
inst.
 
 
 
 
 
%
 
 
 
 
 
No.
 
 
 
of
 
 
 
inst.
 
 
 
 
 
%
 
 
 
 
 
No.
 
 
 
of
 
 
 
inst.
 
 
 
 
 
i
 
 
 
 
 
No.
 
 
 
of
 
 
 
inst.
 
 
 
 
 
i
 
 
 
 
 
No.
 
 
 
of
 
 
 
inst.
 
 
 
 
 
%
 
 
 
 
 
XIX
 
 
 
 
 
1
 
 
 
 
 
100 1
 
 
 
 
 
7.7
 
 
 
 
 
6c
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
XX
 
 
 
 
 
 
 
 
 
 
 
 
 
1-3
 
 
 
 
 
92.3
 
 
 
 
 
33
 
 
 
 
 
100
 
 
 
 
 
30
 
 
 
 
 
90^
 
 
 
 
 
u
 
 
 
 
 
78.8
 
 
 
 
 
9
 
 
 
 
 
90
 
 
 
 
 
5
 
 
 
 
 
55.5
 
 
 
 
 
XXI
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
3
 
 
 
 
 
10^
 
 
 
 
 
4
 
 
 
 
 
22.2
 
 
 
 
 
1
 
 
 
 
 
10
 
 
 
 
 
4 '44.5
 
 
 
 
 
 
 
6 Owing to an unfortunate oversight, a number of the earlier charts, made at a time when especial care was not taken in the study of the abdominal nerves, were included in making up the column on the " last nerve to the rectus muscle" in the Tables 2-8, in the article by Bardeen and Elting, on "A statistical study of the variations in the formation and position of the lumbo-sacral plexus in man." Anatomischer Anzeiger, Vol. XIX, pp. 228-237, 1901. These earlier charts should have been excluded in making up this column. The following table is based upon charts which record with especial exactness the more distal nerves of the abdomen:
 
 
 
 
 
 
 
Charles Eiissell Bardeen 207
 
 
 
Of the 14 instances in which the 21st nerve furnished fihres to the rectus muscle, in 9 the vertebral column was apparently normal, and in 5 it was lengthened by an additional vertebra. In 2 instances the plexus was of the normal type, in 10, of the distal type, and in 2 instances no good record of the plexus was preserved. See Fig. V.
 
 
 
No marked relations were noted between these variations in the distal supply of the rectus muscle and sex, race or side of body. Both Huge " and Bolk ' have given interesting accounts of the relations of the distal abdominal and border-nerves in the anthropoid apes.
 
 
 
The Number of Spinal Neeves Contributing to the Nerve Supply OF THE Abdomen. See Table 77.— In connection with the abdominal nerves it is of interest to inquire how many spinal nerves contribute to the nerve supply of the abdomen. With the possible exception oc twigs furnished by the most distal nerve confined to the thorax to the most anterior portion of the transversalis abdominis muscle, the first nerve of supply of the abdomen is the most anterior intercostal nerve the ventral branch of which passes below the costal margin to enter the abdominal wall. Similarly, with the exception of twigs furnished now and then by the genito-crural nerves, the inguinal nerve is the most posterior nerve furnishing a nerve supply to the abdominal walls. Taking the 1st abdominal nerve as the anterior limit, and the inguinal nerve as the posterior limit, we find that in 10 out of 16 instances (62.5^), seven spinal nerves contributed to the supply of the abdominal wall, in 4 instances (25;^) six nerves, and in 2 instances (12.5;^) five nerves thus contributed.
 
 
 
The transversalis and internal oblique muscles are supplied by branches which spring from the main abdominal nerves during their course to the rectus, and from the ileo-hypogastric, ileo-inguinal, and sometimes from the genital nerve, during their course between these muscles.- The last nerve also furnishes fibres to the cremaster muscle. The muscular branches springing from these various nerves are irregular in origin and distribution, and give rise to a plexiform union between successive nerve trunks. Owing to the great irregularity of these secondary muscle branches no statistical data concerning them are furnished.
 
 
 
s T. Ruge : Verschiebungen in den Endgebieten der Nerven des Plexus lumbalis der Primaten. Morph. Jahrbuch X, 1893, p. 305.
 
 
 
■> Beitrag zur Neurologie der unteren Extremitat der Primaten. Morph. Jahrbuch XXV, 1898, p. 305.
 
 
 
 
 
 
 
208 Study of the Abdominal and Border-Nerves in Man
 
 
 
The lateral branches of the intercostal nerves furnish nerves to the external oblique muscle and cutaneous branches which vary much in distribution. I have seen, for instance, a combined nerve trunk, arising from the lateral branches of the 11th and 13th thoracic nerves, extend well into the pubic region. The charts furnish, however, insufficient data on which to base a statistical study of variations in the lateral cutaneous branches of the eleven more anterior intercostal nerves. On page 310 the relation of the lateral branch of the 13th intercostal nerve to the iliac region is considered.
 
 
 
The Vaeious Types of Disteibution of the Boeder-Neeves. See Table V, and Figs. 1-8. — Greater variation seems to exist in the origin of the border-nerves from spinal nerves than in the origin of the main abdominal nerves. With the exception of the genito-crurab nerves, however, the courses taken by the border-nerves are fairly definite and are well described in the standard text-books. We shall consider first the variation in origin of the border-nerves taken as a group, and then that of the individual border-nerves.
 
 
 
In Tal)le V, we have divided the sets of border-nerves found in the subjects studied into various types. The five main types are based upon the spinal nerves from Avhich the border-nerves arise. In Type I, all border-nerves arise from the 30th and 31st spinal nerves; in Type II, from the 31st; in Type III, from the 30tli, 31st and 32nd; in Type IV, from the 31st and 33nd; and in Type V, from the (30), 21st, 23nd and 23rd. Types I, III and IV are further subdivided into sub-types, according to the relation of the spinal nerves to the individual bordernerves. These relations are made clear by the table. It will be noted that the border-nerves most frequently arise from the 30th, 31st and 33nd spinal nerves, and next most frequently from the 31st and 33nd. Types III and IV A., the forms most commonly met with, correspond with the pictures given in most text-books to illustrate the normal type.
 
 
 
Kelations of Race, Sex and Side of Body to the Various Types OF Distribution of the Bordee-Nerves. See Table VI. — In Table VI are given the relation of race, sex and side of body to the various types of distribution of the border-nerves. It will be noted that no very marked relations of this nature seem to exist. A much larger number of instances than we have studied would be necessary before reliable deductions could be drawn as to the influence of these factors in determining variation in the distribution of the peripheral nerves under discussion.
 
 
 
 
 
 
 
Charles Kussell Bardeen 209
 
 
 
Eelative Distkibutiox oe the BoRDER-ISrERVES OX Each Side of THE Body. See Table VII. — There is considerable variation in the types of distribution exhibited by similar nerves on the two sides of the same body. In Table VII the relation of the types of distribution of the border-nerves on one side of the individual to those on the other are given. Each numeral in the body of the table indicates the number of instances in which the type of distribution indicated at the left of the table was found associated with the type of distribution indicated at the top of the column. Thus, in two instances, when Type I D. was found on the right side, Type I C. was found on the left. The heavy figures indicate that the type of distribution of the border-nerves was the same on each side of the body in the number of individuals denoted by the figure. Thus, Type I B. was found on each side of the body in two individuals. It will be noted that while slight variations in type of distribution is very common, marked variation is rare.
 
 
 
Eelatiox of Variations in the Spinal Column to the Various Types of Distribution of the Border-Nerves. See Table VIII. — This table shows that a close relation exists between the development of the spinal axis and the distribution of the border-nerves. Eeduction in the spinal axis is marked in extreme cases by the loss of a thoracic, lumbar or sacral vertebra. In less extreme instances there is a tendency for the twelfth thoracic vertebra to assume the lumbar type, for the fifth lumbar to assume the sacral type, and for the fifth sacral to assume the coccygeal type. Although occasionally the twelfth rib may be ill-developed witliout accompanying changes in the lumbar and sacral vertebra, as a rule a rudimentary 12th rib indicates a tendency to a shortening of the spinal axis, as outlined above. AYlien the spinal axis is reduced, the hip bones are attached to the spinal axis more anteriorly than is usual, although not necessarily to the 24th vertebra. This anterior position of the posterior limb accounts for the derivation of the border-nerves from a more anterior set of spinal nerves than normal. Types I A., B., C, D., E., II and III A. and B. On the other hand, types of border-nerves marked by a more distal origin than normal (IV B. & V.), are characterized by the great frequency with which they are associated with extension in the vertebral column, marked in extreme cases by the addition of an extra thoracic, lumbar, or sacral vertebra.
 
 
 
Types of Lumbo-Sacral Plexus Associated with the Various Types of Distribution of the Boeder-Nerves. See Table IX. — As
 
 
 
 
 
 
 
210 Study of the Abdominal and Border-Nerves in Man
 
 
 
might be expected from the intimate relations existing between the development of the spinal axis, the position of the posterior limb, and the type of distribution of the border-nerves, we find also that when the lumbo-sacral plexus has a more anterior position than usual, the bordernerves generally axise from an anterior set of spinal nerves; and that when the lumbo-sacral plexus has a posterior positidn, the border-nerves likewise arise from a distal set of spinal nerves/
 
 
 
Origin of the Hypogasteic Neeve. See Table X. — We shall now consider in turn the individual border-nerves, beginning with the hypogastric.
 
 
 
In 6 instances, 2^ of the number studied, the hypogastric nerve arose from the 19th and 20th spinal nerves; in 91 instances, 32^, from the 20th; in 98, 34^, from the 20th and 21st; and in 92, 32^, from the 21st. In 106 instances, 37^, the hypogastric nerve arose from the ventral trunk of the 20th spinal nerve; in 190 instances, 69^, from the 21st; in 9 instances, 3^, there were two hypogastric nerves.
 
 
 
In the table the term " dorsal origin " is used to indicate the separation of the hypogastric nerve from the main ventral trunk of the twelfth thoracic nerve near the spinal axis. See Fig. 1, A. The term " ventral origin " is used to indicate the separation of the hypogastric nerve from the main ventral trunk after the latter has extended well into the abdominal wall. See Fig. 1, C. The hypogastric has a dorsal origin from the twelfth nerve in 40 instances, and a ventral origin in 43 instances out of the 83 in which these conditions were tabulated distinctly.
 
 
 
Oeigin of the Iliac Neeves. See Table XI. — By the term " iliac ,nerve " is meant a nerve which passes over the crest of the ilium to be distributed on the lateral surface of the hip. Only those nerves have been called "iliac nerves" the distribution of which is entirely distal to the level of the iliac crest. In addition, the lateral branches of the more distal intercostal nerves often extend in their distribution from the region of the abdomen over the lateral region of the hip.
 
 
 
In 3 instances, 1^ distinct iliac nerves Avere given off from the 19th spinal (11th thoracic) nerve; in 110 instances, 40^, iliac nerves were derived directly from the 20th spinal nerve; and in 76 additional instances, 27f^, from the 21st, after the latter had received a branch of
 
 
 
8 The relation between the lumbo-sacral plexus and the development of the spinalaxis has been pointed out elsewhere. — Bardeen and Elting, op. cit. p. 203.
 
 
 
 
 
 
 
Charles Eussell Bardeen 311
 
 
 
communication from tiie 30th. The iliac nerve was derived from the 31st spinal nerve in 198 instances, 70.4^. Tlie mode of origin of the iliac nerve is indicated in the table. Most commonly (331 instances, 86;^) it arises as a branch from the hypogastric nerve as this passes near the iliac crest. In 43 instances, 15.3^', it arose as a branch of the main ventral trunk of the ■30th spinal nerve. Less frequently (31 instances, 7.5^) it passed as a separate trunk from the region of the spinal axis (dorsal origin) to the crest of the ilium. In only 33 instances, 8.3^ was an iliac nerve found to arise from the inguinal. The term ilioinguinal should be restricted to nerves of this character, which are comparatively rare. Two iliac branches are not infrequent.
 
 
 
Okigin of the Inguinal Neeve. — The inguinal nerve in the great majority of instances arises from the 31st spinal nerve (358 instances, 89.8;?;). In nearly half of these instances (110) fibres were also derived through a proximal communicating branch from the 30th spinal nerve. In 10 instances, 3.5^, it arose from the 30th spinal nerve, and in 19 instances, 6.6^, the place of the inguinal nerve was taken by the genital branch of the genito-crural. Most commonly (334 instances, 78^) the inguinal nerve takes a course to the iliac crest separate from that of the hypogastric. Not infrequently (36 instances, 13.5;^), however, these two nerves pass in a common trunk as far as the iliac crest, and infrequently (8 instances, 3.8;^) they pass in a common trunk to the region of the external ring, Avhence the hypogastric branch turns up over the abdomen, while the inguinal nerve takes its way to the region where scrotum and leg adjoin.
 
 
 
Okigin and Course of the Genito-ceueal Keeves. See Table XIII. — So great is the variety in the distribution of the genito-crural nerves, it would be necessary to describe nearly every subject examined in order to record the many different courses taken by these nerves. The main trunks of the abdominal nerves are kept fairly constant in distribution, owing to their relation to the rectus muscle. The hypogastric and inguinal nerves are kept within moderate bounds, owing to their course in channels between the transversalis and internal oblique muscles, and between the latter and the flat tendon of the external oblique, channels that are limited distally by the crest of the ilium and by Poupart's ligament. Not infrequently the inguinal nerve courses for some distance between the abdominal fascia and the transversalis muscle before piercing the latter and entering the channel offered between it and the internal oblique muscle. In the region of its attach 16
 
 
 
 
 
 
 
212 Study of the Abdominal and Border-Nerves in Man
 
 
 
ment to the ventral portion of the iliac crest the internal oblique muscle is often divided into two layers, and between the layers another channel is offered for the passage of the inguinal and hypogastric branches. But while the regions in which the hypogastric and inguinal nerves pass from the channel between the transversalis fascia and the transversalis muscle to that between the latter and the internal oblique muscles, and from this to the channel between the two layers of the internal oblique and thence to that between the internal oblique and the tendon of the external oblique, vary in different individuals, the general course of these nerves is fairly constant. On the other hand, there are no definite paths of guidance for the genito-crural nerves. They take an irregular course from the anterior region of the lumbo-sacral plexus through the psoas muscle and behind the transversalis fascia to the region of Poupart's ligament. Here the genital nerve pierces the transversalis and internal oblique muscles or their tendon enters the channel between the latter and the tendon of the external oblique, fuses here with the inguinal nerve, and is distributed in common with the branches of the latter nerve. The crural nerve, on the other hand, passes beloAv Poupart's ligament and supplies the skin of the leg near the region of its junction with the abdomen. Either or both nerves may give off branches to the external iliac and femoral arteries.
 
 
 
In origin, the genito-crural nerves vary no more than the otlier bordernerves. Thus, from Table Y it will be seen that the genito-crural nerves arise from the 21st nerve in 56 instances, 19;^ (in a certain number of these some fibres are derived from the 20th also); from the 21st and 22nd in 125 instances, 79;^; and in but 6 instances, 2^, from the (21st), 22nd and 23rd.
 
 
 
There is considerable variation in the number of nerves designated " genito-crural . Most commonly (in 154 instances out of 250 of which good records are preserved, 61.6^), the genital and crural branches are bound in a common trunk, which, at a variable distance above Poupart's ligament, divides into genital and crural branches. Not infrequently, in addition to such a trunk, there is an extra genital branch (16 instances, 6.4^), or an extra crural branch (25 instances, 10^). Occasionally no crural branch is found (3 instances, 1.2^); more often the genital branch is wanting (17 instances, 6.8;^). I have seen no instances in which both branches were wanting.
 
 
 
The regions of exit of the genital nerve into the path taken by the inguinal nerve and of the crural nerve into the superficial fascia of the thigh, vary greatly. The genital nerve may pass into the path of the inguinal not far from the anterior superior spine of the ilium (lateral
 
 
 
 
 
 
 
Charles Eussell Bardeen 213
 
 
 
region of emergence, 47 out of 121 instances, 38.9;^, see Fig. Y), or in the vicinity of the femoral nerve (middle region of emergence, 55 instances, -15.5^', see Fig. II), or near the pubic crest (median region, 19 instances, 15.6^, see Fig. IV. A.). The crural nerve may pass to the leg in corresponding regions (lateral emergence, 27 out of 133 instances, 20.3;?^, see Fig. I. C), middle emergence, 81 instances, 60. 9^ see Fig. I. A, median emergence, 25 instances, 18. 8^ see Fig. III. E.).
 
 
 
When the genital nerve passes into the path of the inguinal near the anterior superior spine, it assumes many of the characteristics of the inguinal nerve. The inguinal nerve may take a course to the extreme .ventral limit of the iliac crest before passing into the abdominal musculature. Between an inguinal nerve of this type and a genital nerve emerging in a lateral region, only an artificial distinction can be drawn. As the line of demarkation between the two, I have taken the anterior superior spine.
 
 
 
The crural nerve, when it emerges laterally, sIioavs a tendency to become more or less closely uiiited to the lateral cutaneous nerve of the thigh. Occasionally the crural nerve arises as a branch of the lateral cutaneous (9 instances out of 287, ,3.1^). In one instance only have I seen a genital nerve arising as a branch of the lateral cutaneous nerve of the thigh.
 
 
 
Origin of the Lateral Cutaxeous Nerve of the Thigh. See Tohle XIV. — In connection with the border-nerves it may be of interest to consider briefly the origin of the lateral cutaneous nerve in connection with the various types of distribution of the border-nerves. In the more " anterior " types of border-nerves, it Avill be noted that the lateral cutaneous nerve springs most frequently from the 21st and 22nd spinal nerves, while in more posterior types it springs from the (21st), 22nd and 23rd, or from the main trunk of the femoral nerve. This association is not, however, a constant one.
 
 
 
General Conclusions.
 
 
 
Variation in the abdominal and border-nerves may be due either to local conditions, which affect merely the nerves derived from a given spinal segment and their immediate neighbors, or it may be due to conditions which affect the whole distal region of the spinal axis, and the position of the limb relative to the axis. Eace, sex and side of body seem to have no specific influence in determining variation of either sort.
 
 
 
Variation in the abdominal uerves anterior to the twelfth intercostal
 
 
 
 
 
 
 
314 Study of the Abdominal and Border-Nerves in Man
 
 
 
seems, in the main at least;, to be due to local conditions. Not infrequently the nerves and musculature derived from a given spinal segment have a less extensive development than usual. In such instances, the musculature derived from this segment gives way in part to musculature derived from a neighboring segment, and the nerve belonging to the latter covers a territory nearly equal to twice the usual territory, while the nerve belonging to the less developed segment is much restricted in distribution. These conditions become clear in a study of the rectus muscle. Similar vaxiations in extent of the cutaneous territory covered by a given spinal nerve, are also frequent.
 
 
 
The border-nerves may exhibit individual variations, or they may be affected as a group. In the latter case, the variations are intimately associated with the length of the spinal axis and the position of the posterior limb. This association is shown in Table VIII. When the border-nerves spring from the 20th and 21st spinal nerves merely, the condition is, with very few exceptions, found associated with skeletal conditions, Avhich indicate a rediiction in the vertebral axis. In the three exceptions given under Type I, we may assume that the genitocrural nerves were locally affected, and that the condition in these three instances does not indicate an influence exhibited on the border-nerves as a whole. The association of the more distal types of distribution of the border-nerves with extension in the vertebral column is less marked, owing probably to the fact that the relation of the pelvic bones to the vertebrae and the form of the vertebrae were recorded only in the more extreme instances of extension to the amount of a full segment. The frequency and the extent of the variation in the spinal origin of the border-nerves and of the lumbo-sacral plexus, make it important that these factors should be taken into account in making up tables of nerve distribution, like the valuable tables of Head. The marked relation existing between these variations and variations in skeletal conditions is their most noteworthy feature.
 
 
 
In the valuable paper by Bolk, referred to above (p. 207), he points out that the lumbo-sacral plexus in the anthropoid apes is as a rule in an anterior, or " high," position as compared to man, and that distinct border-nerves are less well developed. In man, however, when the spinal axis is shorter than usual and the lumbo-sacral plexus has an anterior position, we do not, as a rule, find that the border-nerves are reduced in number, although they may arise from but a single spinal nerve. Peripheral courses for nerve development are developed somewhat independently of the relation of the position of the limb to the spinal axis.
 
 
 
 
 
 
 
Charles Eussell Bardeen 315
 
 
 
In an interesting communication by P. An eel and L. Sencert/ these authors take exception to the terms " anterior " and " posterior " or " pre-fixed " and " post-fixed " as applied to the various types of variation in origin found in the lumbo-sacral plexus and the neighboring nerves. It must be admitted that local variation in the place and extent of origin of the border-nerves, and the nerves of the limb is more frequent than is marked variation in the relation of all of these nerves as a group to the segmental spinal nerves. The correlation, however, often found between variation in origin of the border-nerves and the position of the lumbo-sacral plexus on the one hand and the development of the spinal axis upon the other, makes it seem well to retain the terms " anterior " and " posterior " in describing variation in origin of these nerves.
 
 
 
1 Contribution a I'etude dn plexus lumbaire cbez I'bomme, Bibliograpbie anatomique IX, 1901, p. 209.
 
 
 
 
 
 
 
216
 
 
 
 
 
 
 
Study of the Abdominal and Border-lSTerves in Man
 
 
 
 
 
 
 
TABLE I.
 
 
 
The Relations of the Ventral Branches of the Sixth, Seventh and Eighth Intercostal Nerves to the Rectus Muscle, Thorax and Abdomen.
 
 
 
 
 
 
 
Most anterior attachment of rectus is at
 
 
 
 
 
No. of instances.
 
 
 
 
 
1st intercostal nerve entering- abdominal
 
 
 
wall.
 
 
 
 
 
Nerve entering rectus from between two costal cartilages.
 
 
 
 
 
7th.
 
 
 
No. of
 
 
 
instances.
 
 
 
 
 
8th.
 
 
 
No. of
 
 
 
instances.
 
 
 
 
 
eth.
 
 
 
No. of instances.
 
 
 
 
 
7th.
 
 
 
No. of
 
 
 
instances.
 
 
 
 
 
4th costal cartilage
 
 
 
 
 
1
 
 
 
 
 
1
 
 
 
7 2
 
 
 
10
 
 
 
 
 
4 o
 
 
 
6
 
 
 
 
 
1
 
 
 
■ 7 o
 
 
 
10
 
 
 
 
 
 
 
 
 
5th " •'
 
 
 
 
 
7 8 2
 
 
 
 
 
 
 
 
 
6th " "
 
 
 
 
 
4
 
 
 
 
 
7th " "
 
 
 
 
 
2
 
 
 
 
 
Totals
 
 
 
 
 
18
 
 
 
 
 
6
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
TABLE II.
 
 
 
 
 
 
 
The Frequency of the Presence and Absence of Transverse Tendons {Linea transversa, Inscriptiones tendmece) Corresponding to the 8th, 9th, 10th and 11th Ribs, and the Relations of the Tenth and Eleventh Tendons to the Umbilicus.
 
 
 
 
 
 
 
 
 
 
 
Tendon Corresponding to the
 
 
 
 
 
 
 
 
 
8th rib.
 
 
 
 
 
9th rib.
 
 
 
 
 
10th rib.
 
 
 
 
 
11th rib.
 
 
 
 
 
'Fresent :
 
 
 
No of Instances
 
 
 
 
 
fig 75.7
 
 
 
9 24.3
 
 
 
 
 
58 93.5
 
 
 
4
 
 
 
6.5
 
 
 
 
 
76 89.4
 
 
 
9 10.6
 
 
 
72 84.7
 
 
 
 
 
33
 
 
 
 
 
Per cent
 
 
 
 
 
29.1
 
 
 
 
 
A bsent :
 
 
 
No. of Instances
 
 
 
 
 
56 70.9
 
 
 
 
 
Median Margin near Umbilicus :
 
 
 
No. of Instances
 
 
 
 
 
13
 
 
 
 
 
Per cent
 
 
 
 
 
15.3
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Charles Eussell Bardeen 217
 
 
 
 
 
 
 
TABLE III. Origin of the Most Distal Abdominal Nerve Entering the Rectus Muscle.
 
 
 
Nerve arose from the XIX spinal nerve in 2 instances 1-8^
 
 
 
Nerve arose from the XX spinal nerve and from a communicatiuii- branch from
 
 
 
the XIX in 7 instances 6.3
 
 
 
Nerve arose solely from the XX spinal nerve in 89 instances 79. .5
 
 
 
Nerve arose from the XX and XXI spinal nerves through anastomosis in 5
 
 
 
instances 4.5
 
 
 
Nerve arose solely from the XXI spinal nerve in 9 instances 8.0
 
 
 
 
 
 
 
TABLE IV. Number of Spinal Nerves Distributing Branches to the Abdominal Muscles. 7 nerves contribute :
 
 
 
1st abd. nerve from XV spinal, last nerve to rectus from XX spinal,
 
 
 
inguinal from XXI spinal 10 instances
 
 
 
6 nerves contribute :
 
 
 
1st abdominal from XVI spinal, last nerve to rectus from XX spinal,
 
 
 
inguinal from XXI spinal 4 "
 
 
 
5 nerves contribute :
 
 
 
1st abd. nerve from XVI spinal, last nerve to rectus from XX spinal,
 
 
 
inguinal from XX spinal 2 "
 
 
 
 
 
 
 
TABLE V. The Various Types op Distribution of the Border-nerves. (Total number of instances studied, 287. See Figures on opposite page. I. T?ie border-nerves arise from the 20th and 21st spinal nerves. 50 instances, 17^ of tbe total number.
 
 
 
A. Hypogastric and inguinal nerves arise from the 20th spinal nerve, the genito-crural from the 21st, after this has received a communicating branch from the 20th. 5 instances, 10^. See Fig. I A.
 
 
 
B. Like A, but with no communicating branch, 6 instances, 12^.
 
 
 
C. The hypogastric nerve arises from the 20th spinal, the inguinal and genitocrural nerves arise from the 21st, after this has received a communicating branch from the 20th. 21 instances, 42^. See Fig. I C.
 
 
 
D. Like C. but with no communicating branch. 11 instances, 22^.
 
 
 
E. The hypogastric nerve, arises from the 21st spinal, after this has received a communicating branch from the 20th. The inguinal and genito-crural nerves likewise arise from the 21st. 7 instances, 14^.
 
 
 
II. All border-nerves arise from t?ie 21st spinal nerve. 6 instances, 2^ of total mimber. See Fig. II.
 
 
 
III. The border-nerves arise f7'om the 20th, 21st, and 22nd spinal nerves. 139 instances,
 
 
 
4:9/^ of total number.
 
 
 
A. The hypogastric nerve arises from the 20th spinal, the inguinal from the 21st and from a proximal communicating branch from the 20th, the genitocrural from the 21st and 22nd. 27 instances, 19^. See Fig. Ill A.
 
 
 
B. Like A, but with no proximal communicating branch. 27 instances, 19^.
 
 
 
C. The hypogastric and inguinal nerves arise from the 21st spinal nerve and from a proximal communicating branch from the 20th, the genito-crural arises from the 21st and 23nd spinal nerves. 68 instances, 49<^. See Fig.
 
 
 
inc.
 
 
 
D. Like C, except that none of the fibres from the 20th spinal nerve go into the inguinal nerve. 8 instances, 6^.
 
 
 
E. Two hypogastric branches, one from the 20th and one from the 21st spinal nerves, inguinal from 21st, genito-crural from 21st and 22nd. 9 instances, 7^. See Fig. Ill E.
 
 
 
IV. The border-nerves arise from the 21st and 22nd spinal nerves. 86 instances, 30^ of
 
 
 
total number.
 
 
 
A. Hypogastric and inguinal nerves from the 21st, genito-crural from the 31st and 22nd. 78 instances, 91^. 'P^ ^<
 
 
 
B. Hypogastric and inguinal nerves from the 21st, genito-crural from the33nd. 8 instances, 9<^.
 
 
 
V. The border-nerves arise from the 21st, 22nd and 23rtZ spinal nerves.
 
 
 
Hypogastric from the 20th and 21st spinal nerves, inguinal from the 21st and 22nd, genito-crural from the 22nd-23rd. 6 instances, 3^ of total number.
 
 
 
DESCRIPTION OF THE FIGURES ON OPPOSITE PAGE.
 
 
 
These figures represent the distal abdominal and the border-nerves of various types in their relation to the abdominal wall, spinal column, skeleton of the limb and lumbo-sacral plexus. The ventral portion of the abdominal wall is shown turned back. The transversalis muscle is not represented. In the rectus muscle the transverse tendon corresponding to the tenth rib is shown in all figures except Fig. V. A transverse tendon corresponding to the eleventh rib is shown in Figures IV A and V. In each figure the hypogastric nerve is represented passing through the internal oblique muscle near its distal margin and at a point about half the distance between the anterior superior spine of the ilium and the distal extremity of the rectus. The inguinal nerve is shown coursing from the anterior superior spine of the ilium to the crest of the pubis. The genital and crural nerves pass from the pelvis in various regions, the genital branches in each instance becoming united to the inguinal nerve while tbe crural branches pass out to the region of the leg. The nerves of the limb arising from the lumbo-sacral plexus are represented diagrammatically in double outline. The lateral cutaneous nerve passes to a point near the anterior superior spine of the ilium, the femoral nerve passes to a point over the head of the femur, the obturator emerges through the obturator foramen, the sciatic nerve passes behind the ischium and the pudic nerve passes between the great and lesser ischio-sacral ligaments.
 
 
 
The twelfth rib is denoted by the appropriate numeral, except in Fig. I C, where the eleventh is thus designated.
 
 
 
(218)
 
 
 
 
 
 
 
 
 
^Cl^l^ii^^
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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