Book - The Development of the Albino Rat 5
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Huber GC. The Development of the Albino Rat (Mus norvegicus albinus). (1915) J. Morphology 26(2).
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Completion of Segmentation and Blastodermic Vesicle Formation
The material covering the end stages of segmentation and the early stages of blastodermic vesicle formation is listed in table 5.
|RECORD NUMBER||AGE||NUMBER OF OVA|
|64||4 days, 14 hrs.||5||Early stage of blastodermic vesicle formation|
|52||4 days, 15 hrs.||8||Morula, beginning of segmentation cavity, early|
|55||4 days, 16 hrs.||1||Early stage of blastodermic vesicle|
|68||4 days, 16 hrs.||4||Early stage of blastodermic vesicle|
|53||5 days||7||Early stage of blastodermic vesicle|
|56||5 days||5||Early stage of blastodermic vesicle|
Thus there are at hand 30 ova, showing late morula stages, the beginning of segmentation cavity formation and early stages of the blastodermic vesicle, falling in the latter half of the fifth day after the beginning of insemination. In all of the uteri from which this material was taken, the ova are spaced in the uterine horns about as in later stages of development; they lie free in the uterine lumen, are in the main ovoid in form, their long axis presenting no definite relation to the long axis of the uterine horn. In preparing this material for sectioning, it was the custom to cut an entire uterine horn into segments measuring about 1.0 cm. to 1.5 cm. in length. These segments were then embedded so as to admit cutting longitudinally and in a plane parallel to the plane of the mesometrium. Cut in this way, the majority of the ova were cut longitudinally or nearly so, others in an oblique plane, others again, crosswise. Since it is impossible to orient the ova prior to sectioning, the secui'ing of desirable sections is a matter of chance. The difficulty is further enhanced by reason of the fact that owing to shrinkage as a result of the action of the fixing fluid, the ova in the vesicle stage are apt to be more or less folded, so that even though the plane of section may be that desired, the resultant sections lose in value by reason of this folding.
It has been shown that in the albino rat, the ova pass from the oviduct to the uterine horn toward the end of the fourth day. During the first half of the fifth day, the migration of the ova from the oviduct to the uterine horn appears to be completed, so that by the second half of the fifth day the ova are spaced in the uterine horn about as after fixation to the uterine mucosa. As to the factor or factors which play a role in the descent of the ova through the uterine horn and their fairly regular spacing, my own material gives no data; these changes occurring, apparently, during the first half of the fifth day, covering which my material is lacking. Widakowich, who has given especial study to these questions, presents the following considerations: In the downward migration of the ova in the uterine horn, it cannot be assumed that the ova are capable of active movement nor can their motion be ascribed to the action of gravity. While peristaltic action may play a part, it is difficult to see how peristalsis could be so regulated as to space the ova fairly regularly within the uterine cavity. The presence of a ciliated epithelium in the human uterine cavity during the intermenstrual period suggested the presence of a ciliated epithelium in the uterine horn of the rat. After many preparations had been searched in vain for its presence, Widakowich found short cilia, not more than 2 n long in the epithelium lining the uterine cavity of a rat killed four days after copulation, and containing ova in the blastodermic vesicle stage. It would appear, therefore, that the uterine epithelium of the rat presents a ciliary border for only a relatively short time, and that the transportation of the ova within the uterus is effected by the cilia. Mandl also found, his material however not including the rat, that cilia are present in many animals on the epithelium lining the uterus only at certain periods, and perhaps only relatively short periods. While the presence of cilia may explain the migration of the ova in the uterine tube, Widakowich can offer no conclusions as concerns the regulatory mechanism by means of which the ova are spaced at fairly regular intervals in the lumen of the uterus. In none of my sections of the uteri of albino rats, obtained during the fifth day after insemination, have I been able to note the presence of cilia on the uterine epitheliiun, even when sections were studied under the oil immersion. After reading the account of Widakowich, their presence was looked for in all pertinent stages, but without success. Especially in rat No. 50, in which the ova were passing from the oviduct to the uterine horn was careful search made, but nothing like a distinct ciliary border, composed even of short cilia, was ascertained. In the left genital tract of this rat, as has been stated, three ova were found in the terminal part of the uterine end of the oviduct, one in the uterine lumen just distal to the mouth of the oviduct, and one a little over a centimeter from this opening. The latter was lodged in a shallow depression of the uterine nuicosa, as is characteristic for stages lying free in the lumen. The question as to whether this ovum was permanently lodged is difficult to answer. If this is assumed, it is further necessary to assume that the other ova would need to pass it to reach the more distal parts of the uterine lumen.
The literature contains no definite statements as concerns the reactions of the epithelium and mucosa of the uterus to the ova soon after their appearance in the uterine cavity. Widakowich summarizes the views by stating that "It is generally stated, that so long as the ova lie free, the uterus shows no changes." He himself notes that at this time the mucosa presents evidence of marked new formation of capillaries. Burckhard, who had at his disposal a large number of stages showing implantation of the ovum of the mouse, discusses at length the appearance presented by the uterus soon after the ova enter the same and the lodgment of the ova therein. This observer notes that in the non-gravid uterus of the mouse, the lumen lies more or less eccentric, and towards the mesometrial border.
The amnion is not smooth, but presents numerous radially arranged folds, certain of which are relatively deep. essentially the same characteristics pertain to the mucosa of the uterus, soon after the beginning of gravidity. As the ova pass from the oviduct to the lumen of the uterus they become lodged in certain of the mucosal folds, and generally in certain of the deeper ones to be found along the anti-mesometrial border. I find Burckhard's account of the form of the lumen of the uterine horn, of the structure of the mucosa in early stages of gravidity, and the lodgment of the ova, pertaining to the mouse, applies equally well to the albino rat. No reason can as yet be given as to why the ova are lodged in the mucosal folds in which they are found, and not in others. So far as may be ascertained from the sections, the particular mucosal folds in which the ova are found, do not differ in form and structure from neighboring folds. It is possible that by reconstruction of the epithelial lining of the entire uterine horn in pertinent stages, certain characteristics of foi'm and position might be revealed as possessed by certain mucosal folds which make them especially favorable for the lodgment of the descending ova. Such reconstructions, however, have not been made. Burckhard states that in the mouse, about the middle of the fifth day, after the ova have been in the uterine cavity for a number of hours, there may be observed the first changes in the uterine wall. The changes consist primarily in a flattening of the uterine epithelium. In the immediate region where implantation is to occur, the lining epithelial cells present instead of a cylindric form, a cubic form. The area is sharply demarked from the surrounding epithelium, the transition of cubic to cylindric epithelium being marked by a sharp-lipped epithelial ledge. In my own material of the rat covering these stages, the uterine mucosa likewise presents shallow pits, in the immediate regions where the ova are lodged, lined by slightly flattened, cubic epithelium, very much as described by Burckhard for the mouse. Widakowich presents an excellent figure (fig. 2 of his contribution, rat four days after fertilization — 'Befruchtung') showing clearly the relations of the ova to the uterine wall. In this rat, the uterine epithelium presented a ciliary border, present even in the shallow pit lodging the ovum sketched. He argues from this that the shallow depression and the flattening of the epithelium are not a result of pressure exerted by the vesicle, as thought by Sobotta and Melissinos, but must be due to an active change in the epithelium itself. The mucosa underlying the shallow pits presents at this stage no change of structure. I am thus in accord with Widakowich when he states that he was not able to observe in the mucosa of the rat in the early stages of gravidity, the giant cells described by Disse as found in the uterine mucosa of Arvicola arvilis, in similar stages.
The form presented by the ova of the albino rat, in the late morula stages and the early stages of blastodermic vesicle, is ovoid, as may be seen from the figures to be presented. Widakowich is inclined to believe that the form of the blastodermic vesicle of the rat is in a measure dependent on the general form of the space in which it is lodged. He figures two vesicles (figs. 1-2) one of which is nearly spherical, the other of distinctly oval form. Duval (figs. 73-83) presents vesicles having o\oid, triangular, and spherical forms. Christiani's figures covering these stages, are too schematic to be of an}^ value in drawing conclusions. I fear Robinson's account is based on imperfectly fixed material. He states that toward the end of the fifth day, or the commencement of the sixth day, the longitudinal axis of the blastodermic vesicle is 125 tx long. Duritig the sixth day, that axis is diminished, first to 95 ^x, and then to 64 /x, after which it again increases, and at the commencement of the seventh day, it is 121 /x." Neither Fraser nor Selenka describes nor figures the stages here considered. In the mouse, according to the accounts of Melissinos, Burckhard, and Sobotta, the form of the blastodermic vesicle in early stages is spherical.
The more specific consideration of my own material I shall introduce with a discussion of three stages taken from the uterus of rat Xo. 52, killed 4 days, 15 hours after the beginning of insemination. In A, of figure 20, there is reproduced the middle one of seven sections of a late morula stage. This morula is of ovoid form, measuring 85 m in its long diameter, 54 ix in its broad diameter— that is, in plane of sections, and since it passes through seven sections of 10 thickness, measures approximately 70 /x in its third dimension. It consists of 24 cells, as estimated by counting the nuclei contained in its se\'cral sections. The cells vary in size as well as in shape. The nuclei are for the main of spherical form, presenting one or several large nucleoli and fine chromatin granules. This morula is found within a fold of the mucosa, each side of which presents a slight depression, lined by slightly flattened epithelium. This morula mass lies free in the lumen of the mucosal fold, and not in contact with the uterine epithelium. The outline and extent of the shallow depressions found in the opposing walls of the mucosal folds conform to shape and size of the morula mass contained, which appear as if slightly retracted as a result of fixation.
Fig. 20 Sections of morula mass and early stages of blastodermic vesicle of the albino rat. X 200. A, B, C, rat No. 52, 4 days and 15 hours. D and E, rat No. 68 4 days and 16 hours. A, late stage of morula; B, shows the very beginning of the formation of the segmentation cavity; C, D, E, early stages of blastodermic vesicle, in E, a distinct covering layer in the thicker portion or floor of the vesicle is evident.
In B, figure 20, is figured one of the sections of a series passing through a morula stage, comprising as estimated 30 cells and measuring 90 /z, by 60 ix, by approximately 50 m, in which the very beginning of the formation of the segmentation cavity is shown. Near one pole the outermost cells have separated slighth^ from the more deeply placed cells, so that an irregularly shaped cavity, eccentrically placed and passing clearly through two of a series of five sections of 10 ^ thickness, is evident. The small cleftlike cavity is bounded by the surrounding cells, the outline of which is distinct. So far as may be judged from the appearance noted as presented in the two sections in which this cavity is found, this arose as a single space and as a result of the separation of the enclosing cells.
In C, of figure 20, there is presented a slightly older stage showing the blastodermic vesicle formation and measuring 80 fx, by 50 ix, by approximately 50 m, comprising as is estimated, 34 to 36 cells. Unfortunately, the lower part of this vesicle is slightly folded as is shown in the lower left of the figure. The appearances presented in the sections are reproduced as faithfully as could be. Owing to the folding, a portion of the thin wall is cut tangentially. The more darkly colored curved line represents in reality the outer boundary of this portion of the vesicle. The segmentation cavity in this vesicle is distinctly larger than that shown in B of this figure. In the section reproduced the segmentation cavity is bounded for the greater part by four somewhat flattened cells, the increase in the size of the cavity being accompanied, it would seem, by a flattening of the enclosing cells.
In these three closely approximated stages, which, since they are taken from the same uterus are probably separated in time of development by only short intervals, the cells though varying in size and shape, show no essential or fundamental difference in structure, neither in cytoplasm nor nuclei; nor do they show any regularity in arrangement. Only few mitotic figures are to be observed ; none in the morula mass shown in A, and but two in each of the other two stages, shown in B and C. Judging from these preparations, one would be led to conclude that segmentation cavity formation in the albino rat is not associated nor accompanied by active cell proliferation. This point will be referred to again after the presentation of further material at hand. In slightly older stages of the blastodermic vesicle than here considered, the thicker portion of the vesicle is designated by Sobotta and others as its floor, which is directed toward the inesoiuctrial border of the uterine horn, wliile its thinner portion is designated as its roof, directed toward the antimesometrial border. Therefore, in shghtly ohler stages than thus far figured, the vesicle hes with its long axis approximately at right angle to the long axis of the uterine horn. In further description of the blastodermic vesicle, I shall use the term 'floor' and 'roof as here specified. In D and E of figure 20, there are reproduced typical sections of the two blastodermic vesicles taken from the uterus of rat No. 68, killed 4 days, 16 hours after insemination. Vesicle D measures 90 fi by 30 n by approximately 60 m. and is of distinctly ovoid form and slightly compressed. This vesicle is found lying free in a long but narrow fold of the mucosa, both sides of which are slightly molded in conformity with the form of the vesicle. The long axis of the vesicle is still parallel to the long axis of the uterine horn. The roof of the vesicle appears as if slightly contracted, though when traced through the series of six sections it does not appear folded. The roof is composed of only a few cells, perhaps seven in all. The segmentation cavity presents a regular outline. This vesicle supports the contention of Widakowich, that the form of the blastodermic vesicle of the rat is dependent in a measure on the form of the space in which it is found. Vesicle E, of figure 20, measuring 85 ^ by 45 n by approximately 40 /x, presents a roof that is slightly folded and shows an early stage in segmentation cavity formation. A figure of the vesicle is included since it represents more clearly than any other blastodermic vesicle of the albino rat in my possession, a differentiation of a layer of surface cells in the mass constituting its floor. This is a question to be more fully considered in further discussion. In all the measurements of blastodermic vesicles thus far given, even in those given for the morula mass shown in A, figure 20, it is evident that one of the short diameters is appreciabh' shorter than the other. The vesicles are not only of ovoid form, but slightly flattened, so that even when not folded, the form of the vesicle as seen in section, even when cut parallel to the long axis of the respective vesicles, is dependent in a measure on the plane of the section, whether parallel to the longer or the shorter of the two cross diameters. This ma^^ be seen from the series of drawings made of a blastodermic vesicle cut cross-wise, taken from the uterus of rat No. 68, from which were also taken the two vesicles shown in D and E of figure 20. This series of figures is shown in figure 21, in which are reproduced in serial order the seven successive cross sections into which the vesicle was cut. It measures 65 n by 38 /x by approximately 70 ,u, and is found at the bottom of a mucosal fold, found at the mesometrial border, and is resting with one side on the epithelial lining of a shallow pit, the other wall of this mucosal fold, also showing a shallow pit, is slightly retracted. From a study of this series of sections, I feel certain that the plane of section is cross and not oblique to the long axis of the vesicle. The roof of this vesicle passes through three sections, A, B and C. The segmentation cavity has thus a depth of less than 30 fx. The overlapping of the cells surrounding the segmentation cavity is to be noted, especially as seen in B of this figure. This arrangement of the cells may explain how the cavity may be enlarged without a material increase in the number of the enclosing cells — in part, by a flattening out of the cells, in part by a rearrangement of the relations of the cells. In the figures of the sections passing through the floor of this vesicle, D to G, attention is drawn to the size, form and relations of the cells and to the fact that there is no distinct covering layer. In this series of sections, there are shown in all 'An nuclei, two of which arc in a laic diasler phase. By excluding"; the nuclei that appear to he cut in two, appearing thus in two successive sections, T estimate that the hlastodennic vesicle is made up of only about ;^0 to 32 cells.
Fig. 21 A complete series of cross-sections of an early stage of blastodermic vesicle of the albino rat. X 200. Rat No. 68, 4 days and 16 hours. A to C, sections through roof of vesicle, showing segmentation cavity; D to G, sections through floor of vesicle.
In figure 22, there are reproduced typical sections of four blastodermic vesicles taken from the uterus of rat No. 5:^, killed five days after the beginning of insemination. This uterus contained in all, eight vesicles, one of which was distinctly pathologic. The four vesicles selected for reproduction and discussion present each certain characteristics worthy of consideration. ■ Vesicle A, which presents an early stage of segmentation cavity formation is of interest owing to the number of mitotic divisions it contains. As a rule, I have noticed only a few mitoses at this stage. In this vesicle, which measures 90 n by 55 m by approximately 40 ju, there are no less than five mitoses to be noted, three of which are in cells forming the roof of the small segmentation cavity, and are included in the section figured. This is the only vesicle in my possession in which an increase in the size of the segmentation cavity is accompanied b}" active mitoses in the cells bounding it. The vesicle lies free in the uterine lumen, one wall of which is only slightly pitted. In B of figure 22 is reproduced a section of a blastodermic vesicle, measuring 90 m by 55 /x by approximately 45 fx. It is evident that this vesicle passes through five sections of 10 /x thickness, thougli one of the end sections, the fifth section of the series, seems to have fallen out during manipulations of staining and mounting, since the preceding, or fourth section does not quite complete the series. This vesicle lies free in the lumen of the uterus, and there is evident only a shallow pit in the mucosa juxtaposed. In this vesicle the cells forming the roof of the segmentation cavity are relatively numerous, and are not markedly flattened, and in one an early mitotic phase is recognized. Here again cell proliferation appears to have accompanied increase in size of segmentation cavity.
Fig. 22 Sections of early stages of blastodermic vesicle of the albino rat, X 2G0. Rat Xo. 53, 5 days.
The vesicle shown in C of figure 22, measuring 130 n by 30 ^ by approximately 40 m, lies free in a long, narrow fold of the uterine mucosa, in close proximity to a shallow mucosal pit, lined by cubic epithelium; the pit conforming in shape and extent to the form of the side of the vesicle presented to it. Therefore, it would seem that the form of the vesicle as seen in sections of the fixed material is essentially the same as that obtained in vivo. The two vesicles, typical sections of which are shown in B and C of this figure, are almost in identically the same phase of development, although their form as seen in sections differs markedly. The ])lasticity of the living blastodermic vesicles is no doubt such that their form is in a great measure dependent on the shape of the mucosal fold in which they are lodged. In D of figure 22, there is reproduced a section of a blastodermic vesicle which points to a stage of development which is slightly more advanced than that shown in previousfigures. The vesicle measures 100 ix by 70 ^t by approximately 50 IX. The roof enclosing the segmentation ca\'ity is slightly folded; a portion of its wall is thus cut tangentially, as shown in the lower left of the figure. The segmentation cavity is distinctly larger than that shown in the preceding figures, and is bounded by a relatively large number of cells, fourteen in that portion of the roof sketched in this figure, one of which is in a mitotic phase. The mass of cells constituting the floor appears as slightly compressed, in consequence of a slight intravesicular pressure which aided in the enlarging of the segmentation cavity.
The cells i'()i-iirni<>; the roof of the sc^niciilalioii (';i\'i1>' do not appear so distinctly flattened as is the case in certain of the vesicles fifrured in figures 20 and 21. It would appear, therefore, that at least two factors are operative in the increase of size of the segmentation cavity after its anlage — a flattening out and consequent increase of the exposed surfaces of the enclosing cells, and secondly, a cell proliferation; and it would appear that both of these factors may be operative from the time of the beginning of segmentation cavity foinialiou.
Early stages in the blastodermic vesicle formation in the albino rat have been previously described by Robinson, Christiani, Duval, and Widakowich; Selenka's youngest stage is slightly older than any discussed by me. My own observations are wholly in accord with those of Widakowich in so far as his account covers early stages of blastodermic vesicle formation. He discusses and figures, however, only two vesicles, obtained four days after fertilization — 'l^efruchtung,' in each of which the segmentation cavity presents a smooth and regular outline and is of appreciable size. The observations of the other ob.servers who have considered these stages will be discussed in connection with a very brief presentation of much more comprehensive observations on the mouse in similar stages of development. Of these latter, those of Sobotta ('03) are based on abundant and apparently well fixed material. Sobotta begins his discussion with the consideration of three ova taken from the same mouse, the second half of the fourth day after fertilization, each of which shows beginning of segmentation cavity formation, one of which was cut in longitudinal axis and is figured in his figure 1. This ovum is interpreted as showing that the segmentation cavity arises not as a single space, but as a number of disconnected spaces, which later become confluent and form a single space.
A similar observation was made by Yan Beneden on the bat, a fact which Sobotta uses to support his contention that the mouse ova studied by him were of normal structure. Melissinos gives a number of figures showing early stages in the formation of the segmentation cavity in the mouse. His figures 21 and 22 (66 hours) are not unlike ni}' own figures shown in B of figure 20 and A of figure 22. According to this observer, the segmentation cavity arises as a single space, due to an accumulation of fluid secreted by the cells of the morula. This secretion is evidenced by globule-like droplets which are shown in his drawings as adhering to certain of the cells bounding the segmentation cavity. In my own preparations of the albino rat, I find no evidence which would lead to the supposition that the segmentation cavity does not begin as a single space nor do I find any evidence of secretory globules as described bj^ ]\Ielissinos. Selenka has described quite fully two blastodermic vesicles of the mouse, lying free in the uterine lumen. His account of their structure, supported by two somewhat diagrammatic figures, is as follows : The wall of the vesicle is formed by a layer of covering cells — 'Deckzellen' — constituting a covering layer — Deckschicht or Rauber's laj^er." The space enclosed by this layer, for about one-third to one-half of its extent, contains the 'formative cells,' for the remainder it contains fluid. The covering cells and formative cells are said to be separated by a sharp line. The formative cells are in all parts separable into two fundamental germ layers. An inner layer, bordering the cavity and constituting the entoderm, is said to be composed of cells possessing more deeply staining nuclei, irregular outline, with tongue-like processes which extend into the cavity, and a granular protoplasm; further, of cells which are more clear, more peripherally placed, and which constitute the ectoderm. Each of these fundamental germ layers consists of a single layer of irregularly- formed cells.
According to the observations of Selenka, therefore, the floor of the vesicle consists of three layers of cells; an outer covering layer — ^'Deckschicht' or 'Rauber's layer' — an inner layer of entodermal cells, and an intermediate layer of ectodermal cells. Jenkinson's account reads as follows: There are present (1) an outer layer, one cell deep, of trophoblast, which is continuous over (2) an inner mass which becomes differentiated into the embryonic epiblast and the hypoblast, and which is quite distinct from the overlying trophoblast, as my specimens invariably show." In Jenkinson's figures 1 and 2, giving early stages of the blastodermic vesicle, there is not shown a differentiation of the inner mass into ortodornial and cntodonnal fclls; tlic outor layci-. cox'orin^ lay(M'. HaulxM-'s rolls, or t lophohlasl, is cloarly dilTcrentiated tVoiu the iimci' mass by a distinct space. I)ii\;il has recognized in early stap;es of ))last()derniic vesicle formation of the mouse and rat, in the thicker part of the vesicle entodermal and ectodermal cells. The former are of irrefj;ular form, possess granular protoplasm and are said to possess tjic piopcrty of ameboid movement. The remaining; cells are recognised as ectoderm; a distinct covering layer is not recognized. In Christiani's figures (rat), which are, however, so diagrammatic as to be of little value and are evidently drawn from poorly fixed material, entodermal cells, ectodermal cells, and covering cells may be recognized as per legends. Melissinos f mouse), while not describing definitely a peripheral or covering layer, states that outer cells of the thicker pole, like the cells enclosing the segmentation cavity, stain less deeply than do the more centrally placed cells. In earlier stages of vesicle formation, neither in figures nor in text as given by this observer, do I find reference to a differentiation into ectodermal and entodermal cells. The observations of Selenka, Duval, Kobinson, Jenkinson, and others, bearing on the structure of the blastodermic vesicle of the mouse and the rat in early stages of development have been so thoroughly and critically reviewed by Sobotta that an extended discussion has here been deemed unnecessary. It may here suffice to say that while Sobotta's observations were made and his discussions based on ova obtained from the mouse, my own observations made on the albino rat are in agreement with his and support the contention that in early stages of blastodermic vesicle formation a differentiation of the thicker part or the floor of the vesicle into a covering, Rauber's cell, or trophoblast layer, and a further differentiation into ectodermal and entodermal cells, is not to be made: we differ in our accounts of the beginning of the formation of the segmentation cavity. An outer or covering layer is suggested in certain of my own preparations, most clearly in that sketched in E of figure 20. However, a uniform difference in structure and reaction to staining reagents of the outer layer of cells is not present in my own preparations. None of my own preparations gives evidence of such an early differentiation of ectoderm and entoderm as given by Selenka, Duval, Christiani, and others. Cells of irregular outline with tongue-like projections, such as figured by Selenka and Duval I have not observed. The cells constituting the floor or the thick part of the vesicle all present essentially the same structure, while the segmentation cavity, as soon as it presents appreciable size, shows a smooth and regular outline. In figures 1 and 2, of Widakowich's communication, excellent figures of early stages of blastodermic vesicles of the albino rat, there is presented no evidence of a trophoblast layer nor a differentiation of ectodermal and entodermal cells.
My own figures, 20 to 22, were drawn with the aid of camera lucida at a magnification of 1000 diameters and with the use of an intense Welsbach light. They are reduced five times in reproduction. With the exceptions of cell outlines, which as sketched do not in the preparations fall in the same optical plane, and are sketched more sharply than is perhaps warranted, the figures portray quite accurately the structural appearances presented, so far as may be with the use of a single color.
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