Paper - Transitory cavities in the corpus striatum of the human embryo (1915)

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Essick CR. Transitory cavities in the corpus striatum of the human embryo. (1915) Contrib. Embryol., Carnegie Inst. Wash. 2: 95-108.

Online Editor Note 
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This historic 1915 paper by Essick is an early description of the transitory cavities in the corpus striatum of the human embryo.

Modern Notes: neural

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Transitory Cavities in the Corpus Striatum of the Human Embryo

Contributions to Embryology, No. 6.

By Charles R. Essick. With three plates.


The human embryo, during the seventh week of development, possesses two bilaterally symmetrical cavities in the substance of the corpus striatum. Since one lies nearer the midline than the other, I have called them cavum mediale corporis striati and cavum laterale corporis striati. The former is deeply placed in the striate body, while the latter occurs nearer the surface of the brain, at times separated from the pia mater by a layer of nervous tissue only a single cell in thickness. In all of the embryos in the collection of the Carnegie Institution of Washington, ranging from 15 to 20 mm., crown-rump measurement, one or both o: these cavities are present, while above these measurements they appear with diminishing frequency; so that in all of the specimens under 15 mm. (measurement in formalin) and all over 24 mm. evidences of them are lacking.

Undoubtedly the ephemeral character of these cavities and their similarity to artifacts account for the fact that they have heretofore escaped notice. The study of human embryological material can not always be followed in specimens that have been fixed alive ; more often one must be content with tissue that has begun to macerate. It is very natural, then, to regard a rent in tissue as a fault in technique, especially in the central nervous sys- tem, where shrinkage is one of the most difficult things to avoid in preparing and mounting serial sections.

The Cell-Content of the Striate Cavities

Inasmuch as the identification of both mesial and lateral cavities is based on their contents, it may be well to describe the peculiar cells, foreign to the nervous tissue, which occupy both cavities in large numbers whenever found in the striate body. They must be regarded as the best proof of the normal occurrence of spaces in the human brain. From the moment one of these striate cavities makes its appearance as an irregular break in the continuity of the nervous tissue, until its complete disappearance, it is inhabited by large amceboid cells having the power of phagocytosis. Study of the whole embryo and its membranes shows that these cells are not peculiar to the cavities; for the same cellular elements may be seen in the mesenchymal spaces, especially when of loose texture. Enormous numbers may be found in the younger chorionic villi; along the umbilical cord many wander near the surface; multitudes are present everywhere in the loose mesenchyme around the central nervous system, especially near the base of the fore-brain; they occur with less frequency along the aorta and in the mesentery.

These widely distributed cells have been figured by many observers— usually in con- nection with the formation of the blood. Hofbauer, Grosser, and Minot have illustrated the cells in the chorionic villi of human embryos of the first months of development. Hof- bauer (5, p. 28) first called attention to specific round cells appearing in the human placenta toward the end of the fourth week of pregnancy. Describing in detail these "vakuolaren Zellen," he called attention to the many points of similarity to the plasma cell— i. c, the eccentric nucleus with its rich chromatin network and its surrounding clear zone.

According to Grosser (3, p. 224) : "Regel-massig finden sich in Zottenstroma bei Eiern des ersten Schwanger-schaftsmonate, grosse, protoplasma-reiche Zellen, auch die in letzter Zeit wieder Hofbauer aufmerksam gemacht hat, deren Bedeutung aber noch unklar ist."

These two authors are at a loss to explain the existence of the cells, while Minot (8, p. 498 ff.) regards them as erythrocytes which have wandered into the mesenchyma, and, remaining there, have swollen by imbibition and are undergoing degeneration by vacuoliza- tion of their protoplasm. In figure 361 this author has illustrated "degenerating blood-cells" in the chorion of embryo 350 — an embryo included in this study. Such an interpretation finds little or no support from my observations. The vacuolization of the protoplasm sug- gests degeneration, but such an hypothesis is overwhelmed by many other factors pointing to a very active life — i. e., mitosis, phagocytosis of foreign material, evidences of amoeboid movement, and lack of hemoglobin. Then, too, his figure 361 gives a false impression of the relative size of a normal erythrocyte and these large phagocytic cells (cf. figs. 13, 15, 18, 22, and 31). The former may easily be accommodated in the body of the latter, and only the largest red cell that one can find shows any approach to the size of the phagocyte. Most observers have regarded these colorless cells as the ancestors of the blood elements, at least whenever they are found in the body of the embryo itself. O. van der Stricht (11) includes under the class of phagocytes "leucoblasts" which correspond exactly to these cells, so widely distributed through the whole ovum. Saxer (10) derives them from the common vascular and blood anlagen, which he differentiates from connective-tissue elements. From these primary "Wanderzellen" of Saxer descend a series of cell-forms, including giant cells, wandering cells of I, II, and III orders, erythrocytes, and colorless cells. The Wander- zellen persist into adult life and continue their blood-forming powers in the bone marrow and adenoid tissues.

Maximow (6) holds that the embryonic ancestor of this cell is the mesenchyme out of whose network it is gradually separated to become a free-moving cell. With the exception of the extremities and gill-arches, he finds these "Wanderzellen" everywhere in the loose mesenchyme, "allermeisten sich in Kopfmesenchym in der Nahe der Gehirnwand, vornehm- lich an ihrer ventrolateralen Seite befinden."

For a long time, morphologically similar cells have been recognized in the adult as playing an important role in inflammation. It has recently been shown that these probably have no relationship to the normal blood elements. Metchnikoff (7) has given the cells of this class the name of "macrophage." This term has been adopted here because of its application by recent observers' to a group of cells normally inhabiting the connective tissues, serous cavities, and hematopoietic organs. The remarkable peculiarities of the adult cells characterize them even in the very young embryo. This will be seen from the following description, for although they are wholly unlike the young and incompletely differ- entiated tissue in which they are found, they correspond with striking exactness to those of the adult. A very clear physiological relationship between the widely distributed adult cells has been demonstrated by Dr. Evans (2) by means of vital stains; his results have led me to include these cells in the same class because of their morphological characteristics.

While Hofbauer's use of stains is hardly analogous to their introduction as vital dyes, it is interesting to note that by teasing living chorionic villi in solutions of neutral red, he found (p. 124), "Wahrend alles (darunter auch das Syncytium) ungefarbt blieb oder einen leichten Stich in Rosa annahm, treten die ' vakuolaren' Zellen der Zotte tief saturiert gefarbt und beherschen die Bildflache." Plato (9) showed that cells engaged in phagocytosis are particularly prone to stand out brilliantly in supravital stains because of the deep tingcing of their phagocytized contents. Such evidence would account for Hofbauer's finding.

Several characteristics, easily recognized, permit the inclusion in the same tissue class of the large mononuclear cells found so extensively distributed in the membranes of the embryo, in the umbilical cord, in the mesenchymal spaces, and in the cavities in the corpus striatum.

The average cell-body of the younger embryo measures from 10 to 13/t, while in the older specimens it ranges from 10 to 19/x. This increase in size accompanies roughly the growth of the embryo, although there are always smaller cells belonging to this group (figs. 19, 25, 30). Thus it may be seen that the largest red cell is several times smaller than the largest macrophage, and the former may be easily accommodated inside the latter. Comparison of the measurement of many cells from chorion and cavum corporis striati shows them to be of the same size, when one is fortunate enough to get the membranes and embryo mounted on the same slide; even when mounted separately there is very little discrepancy in favor of cells in either position.

The shape of the cell depends largely on its environment. When free it tends to assume a spherical form (figs. 13, 17, 18, 20, 21), but in places where it can find support it sends out long processes which are undoubtedly pseudopodia (figs. 15, 22). To obtain speci- mens of cells with extended pseudopodia the fixation must be prompt; failure to procure such pictures of moving cells is best explained on this basis. The shape is often irregularly oval, due to the ingestion of foreign bodies, particularly erythrocytes (figs. 26, 27). At times the macrophages grow in clumps of from 20 to 50, forming a mulberry-like mass. This may occur in either medial or lateral cavity of the corpus striatum, and is well illus- trated in figure 12 for the cavum laterale. Here the cells have becope flattened by the pressure of their neighbors and in section seem to be forming a loose membrane the elements of which are polygonal in outline.

The cells have a limiting membrane, very thin but definite; although it is almost impossible to defend the view that the phenomenon illustrated in figure 12 is not an instance of protoplasmic bridges connecting several cell-bodies. Large numbers of cells are extremely vermcose (figs. 23, 25, 26). Examination of the cavities where these cells are found reveals a great deal of debris simulating protoplasm. This is undoubtedly coagulated proteid which has accepted the counterstain and has precipitated on the surface of the macrophages. Figures 13, 18, 19, and 20 represent more faithfully the delicate cell-mem- brane when the cell has withdrawn all of its pseudopodia.

The protoplasm is finely granular and is delicately arranged throughout the cell-body in anastomosing strands which give an appearance of lacework. These trabecular vary greatly in thickness and surround vacuoles within the cytoplasm. In the younger cells these vacuoles are usually small, but at times they may be larger than the cell-nucleus itself. Where the protoplasmic strands reach the cell-membrane they reinforce it in an irregularly shaped mesh, such as may be seen in the illustrations of cells from chorion or brain. This appearance is brought about by the extreme vacuolization which extends throughout the whole cell, and as a result the finely granular protoplasm is heaped up in the interstices between the vacuoles and the cell-wall. Great numbers of macrophages may be found with foreign material included within their cell-bodies; the power of phagocytosis is one of the most interesting phenomena exhibited by these cells. Although cells containing ingested matter are rarely found in the chorionic villi, the cells everywhere in the body of the embryo may contain phagocytized matter. The erythrocyte forms the foreign material most frequently recognized within the phagocytes. An extravasation of red corpuscles in the young embryo has occurred in most of the material studied; this is probably due to the fact that it was subjected to high pressure in abortion. The presence of large numbers of erythrocytes free in the tissue-spaces is not unfortunate, since they have stimulated these macrophages to unusual activities whenever it occurred in their neighborhood. Embryo 406 furnishes many examples of red cells ingested by these large mononuclears in the cavi- ties of the corpus striatum. Figure 26 is particularly interesting, as it illustrates a remark- able phenomenon, namely, the division of a cell with a large mass of foreign material in its cytoplasm. One of the asters is found on the edge of the cell, while the other' lies beneath the ingested nucleated red blood-cell, and could not be drawn without confusing the picture. Figure 27 shows the beginning digestion of the erythrocyte. Here the large macro- phage from the cavum laterale corporis striati contains two smooth spheroidal bodies, stained decidedly pink, yet not as deeply colored as the erythrocyte contained in the dividing cell (fig. 26). Erythrosin has a marked affinity for cells containing hemoglobin, and it here reveals these inclusions as two homogeneous globules. There is no evidence of a nucleus in either of these two hemoglobin-containing fragments. Embryo 74 contains cells which show the last stages of erythrocyte digestion (fig. 28). Here the macrophages contain a few granules of brown pigment— not very unlike the large phagocytic cells known as "Herzfehlerzellen," in the sputum of patients suffering from chronic passive congestion of the lungs. It is very certain that long after the embryonic heart has stopped beating and erythrocytes have been forced out of the capillaries, these huge amoeboid cells continue to be actively engaged in the ingestion of foreign bodies. Even up to the time of fixation, apparently, these cells are functioning, if mitotic division may be used as an index of a living cell. It is easy to understand how some of these red cells may be reduced to brown pigment granules when one considers the length of time which often elapses between the separation of the placenta from the uterine wall and the fixation of the specimen. Careful, prolonged search has been made to determine the presence of nervous tissue in the bodies of the phagocytic cells inhabiting the corpora striata. In no instance was it possible to identify with certainty any of the inclusions as neuroblasts.

Most of the cells contain a single nucleus, for the most part eccentrically placed. It is large, varying from 4 to 6m in the younger embryo and from 5 to 8m in the older, and has a distinct nuclear membrane. The rich chromatin network with its many enlargements is beautifully shown in the specimens stained with hemotoxylin. Many of the cells have nucleoli attaining at times a diameter of 2 M . These are then the center of a radiation of chromatin threads. The nucleus may be irregular in shape, as shown in figures 12, 13, and 28, but in most instances it has a smooth contour. The irregular nuclei belong to cells which have begun to show signs of degeneration. As they grow older they gradually lose their power of staining and finally disappear. The eccentricity of the nucleus is a most striking peculiarity. One commonly sees the nucleus in direct apposition to the cell- membrane. Perhaps the only instance of a lack of similarity between the macrophages of the embryo and those of the chorionic villi is to be found in the method of regeneration.

Everywhere in the mesenchymal spaces, and especially in the cavities in the corpus striatum, it is not difficult to find mitotic figures in these phagocytes (figs. 24, 25, 26); in contrast to this one very rarely comes upon a dividing macrophage in the chorionic villi. Figure 16 is an illustration of such an instance. It must be noted that all of the charac- teristics described for the protoplasm are retained during mitosis— loose texture, vacuoliza- tion, and engorgement with foreign particles. Cells with two nuclei have been observed in the brain cavities, and the peculiar protoplasmic strands which sometimes bridge the gap between them are illustrated in figure 29. The size of the nuclei, as well as the presence of a nucleolus, in only one of them, makes it probable that we are dealing with direct division in these cells, although there is no indication of a constriction of the cell-body. An oddly shaped nucleus has been pictured in figure 30 ; here the form is concavo-convex, with the convex surface near the cell-membrane. This illustration is inadequate in so far as it gives one the impression of a kidney-shaped nucleus.

Unfortunately, not all of the material at my disposal has been exhausted in an effort to establish a life-history for these macrophages. Many puzzling questions have been presented by the distribution of the cells; yet a few general conclusions may be drawn from these observations. With few exceptions, in the youngest human embryos the preservation has precluded a careful cytological study; and often it is with reluctance that one admits the failure to discover these cells in positions where one expects to find them.

The chorionic villi in the very youngest ova seem to be devoid of the large vacuolated cells first noted by Hofbauer. Embryos of 2 mm. (No. 391 in this collection) possess a few of these cells sparsely scattered about the villi and a few in the chorion itself, but none in the body stalk nor in the embryo. In the larger specimens parts of the villi are entirely free from these independent cells, which are found in large numbers in other portions of the villi, chorion, and even the body stalk (No. 186, 3.5 mm.; No. 164, 3.5 mm., and No. 463, 3.9 mm.), but the mesenchyme around the aorta or brain shows none of the typical large vacuolated cells so common in the membranes. After the embryo has passed 5 mm. these cells may be seen in the mediastinum and around the brain; the ease with which they are found in the loose mesenchyme increases with the growth of the embryo. As noted by Hofbauer, they gradually disappear from the villi as the mesenchyme takes on a more fibrillar character. Hence in the older portions of the chorionic villi — i. e., those in which the larger vessels and denser framework exist — relatively few of these phagocytic cells may be found, even in the younger embryos. In the later stages they disappear from the pla- centa entirely.

Certain embryos contain a much larger number of macrophages in their placentae than others of a similar stage of development. This peculiarity, as we shall see, holds true for the cavities in the corpus striatum, where there is the same tendency to extreme variation in the number of the cells present. The presence of large numbers of macrophages in one locality is not necessarily accompanied by a similar increase in the other; thus in embryo 350 the placenta (fig. 14) contains very many cells, while the cavum mediale corporis striati con- tains only a few (fig. 2) ; in embryo 22, on the other hand, the conditions are exactly reversed.

In order to determine the origin of these cells more accurate methods of investigation than the mere study of routine serial sections must be employed. Hofbauer's attention was directed to their mode of origin by Marchand; these workers believed them to be descendants of the connective-tissue elements. A section taken from embryo 800 might be given in support of their view. The character of the cells of the layer of Langhans is not very unlike that of the younger macrophages, and it is possible that the macrophages of the villi may arise from that layer directly. The cells, when in the body of the embryo proper, apparently preserve their specificity and give rise to younger generations by mitotic division.

These cells, then, whose presence in the striate cavities argues strongly for the functional existence of bilateral spaces, should be considered as of the type of extra-vascular phago- cytes. They correspond in morphology to the Hofbauer cells of the young chorionic villus, to the Wanderzellen of Maximow and Saxer, and to the macrophages of later writers.

The Occurrence of the two Striate Cavities

In searching through the literature for illustrations of these cavities onty one could be found which unquestionably had reproduced either of the spaces in the corpus striatum. His (4, p. 124, fig. 83) has published a photomicrograph of the fore-brain of a 16 mm. human embryo Se, in which the two halves of the brain were not sectioned symmetrically, so that different levels appear on the two sides of the median line. Fortunately, on the left side the cavum laterale corporis striati occurred in the plane of section. It is roughly piriform, the pointed extremity being directed toward the cerebral ventricle, while the blunt end approaches the surface of the striate body. This illustrationof the lateral cavity is very similar in its position and size to those illustrated in figure 7, which is photographed from embryo 409, a specimen of exactly the same length as Se. The lateral cavity usually develops before the medial, so that it is safe to predict that the His embryo would show another cavity more deeply placed in the striate body if the plane of section had not been so oblique.

The accompanying list of embryological material from the Carnegie Institution of Washington includes all the specimens from 15 mm. to 23 mm. (crown-rump length) which permit of histological study. With but one exception, evidences of one or both sets of cavities have been found in the corpus striatum.



< i own-rump




length in mm.


length in mm.


length in mm.


























15 5
























The cavities in the corpus striatum fall into two groups — one appearing very near the lateral surface of the brain, the other occupying a medial position within its substance. Generally each is but a single cavity, and all of the smaller diverticula communicate with the main cavity. Either, however, may be represented by multiple cavities wholly uncon- nected with one another, yet so close as to be separated only by a thin partition of nervous tissue. As it lies deep in the striate body, the medial cavity tends to be more smooth-walled than the lateral. The latter is frequently traversed by numerous blood-vessels having a thin layer of neuroblasts adhering to their walls; small strands of nerve-cells stretch like spider-webs from one wall to another. In addition to the cavum mediale and cavum laterale an intermediate group is present in one specimen, situated midway between the two. In all of the specimens studied the medial group appears in younger embryos than does the lateral, and disappears first in some of them. There are embryos of 24 mm. in which both cavities are still well developed.

No direct communication between these cavities and the vascular system could be made out. In none of the four injected embryos included in this study (No. 390, 15.5 mm. ; No. 424, 17.2 mm.; No. 460, 21 mm.; No. 382, 23 mm.) was there any tendency for the injected mass to flow out into these spaces, although vessels containing granules of injection coursed directly through the cavity. In one instance (embryo 460) a small amount of India ink backed into the cavum laterale from an extensive subpial extravasation. When erythrocytes have been found in these spaces the cavity has shared in a generalized extrava- sation of formed elements throughout the entire head.

The Cavum Mediale Corporis Striati

If one neglect embryo 144, on account of its being given its measurement after it was mounted on the slide, whereas all of the other specimens were measured before embedding, we find that the cavum mediale makes its earliest appearance in embryos of 15 mm. (No. 7 1 !) of this series). Deep in the substance of the striate body, in close proximity to a blood- vessel, may be found a slight separation of the nervous tissue. Although not unlike an artifact, it exists in exactly the same place on the two halves of the brain — in other words, it is bilaterally symmetrical. In the transverse sections of this embryo it appears, under low magnification, as a less dense area 150m long and 65^ broad. A single 4C> section includes the major portion of the entire space, and it is terminated abruptly in the sections on each side of it. Closer observation reveals strands of neuroblasts bridging the gap, here a single cell stretching across the cleft, there a group of young neuroblasts extending from one wall to the other. In the irregular fissure thus produced are found many macrophages, their cell-bodies almost filling the space formed by the separation of nerve-cells. Unfortu- nately, a careful cytological study is not possible, owing to the thickness of the section and the dense stain. No formed elements could be made out in the bodies of these large phago- cytes, although it seems likely that an extensive destruction of nervous tissue is taking place. The macrophages are not found outside of the space {i. e., among the cells making up the adjacent nervous tissue), but lie among the nerve-cells which bridge the newly forming cavity. Were it not for the presence of these foreign cells, the earliest beginnings of the cavum mediale could not be easily determined from the material at hand, since this cavity differs in no other respect from a host of similar spaces found about the blood-vessels in the brain of this embryo, produced by too rapid interchange of fluids in the preparation for serial sections.

The further elaboration of the mesial cavity is furnished by embryo 423 (15.2 mm.), embryo 350 (15 mm.), and embryo 406 (16 mm.). In all of these specimens the tissue of the central nervous system is still intact in the region of the future lateral cavity. The cleft, which was barely demonstrable in No. 719 (15 mm.), has enlarged so that a complete separation of the nervous tissue has resulted. The vessel which was noted in the youngest embryo now lies exposed in the wall of the cavity, or even courses through it unsupported save for a few nerve-cells. Already the appearances of rupture of the nervous tissue have almost entirely disappeared and the protoplasmic strands which bridged the gap at its for- mation are wanting. Though not perfectly smooth in contour, as figure 2 illustrates, it has the appearance of being punched out of the section, so that healthy neuroblasts end abruptly at the boundary of the space. Occasionally one finds strands of cells bridging the mesial cavity at all periods of its existence. Such bridges are, then, comparatively large and are composed of many nerve-cells. The tissue making up the walls of the cavity is not in any sense a membrane, but appears to be composed of normal looking, undeveloped nerve-cells showing the same unbounded relation to one another as is found in the brain elsewhere. The edge of the space does not show the condensation of protoplasm such as is seen on the surface of the brain, where the cells form the external limiting membrane.

From the time of their first appearance, the cells contained in these cavities vary con- siderably in number as one compares different specimens. At times almost the entire mesial cavity is taken by macrophages which lie in close apposition to one another (fig. 8) . For no apparent reason, the cell-content of the mesial cavity (and the same is true for the lateral cavity) varies considerably, but in general one finds a greater number of macrophages compared to the volume of the cavity in the mesial than in the lateral cavity.

Very quickly the mesial cavity reaches its maximum size. Thus in embryo 74 (16 mm.) the dimensions are roughly 195/* by 300m by 416m, but these dimensions are not con- stant in other specimens. There seems to be no relationship between the length of the embryo and the volume of the mesial cavity. Its method of disappearance seems to be by gradual contraction of its walls, so that there appears to be a tendency for the older speci- mens to have smaller mesial cavities. Thus in embryo 460 (21 mm.) the cavum mediale has vanished entirely from the left side and is but 96m hi its greatest dimension in the right corpus striatum. Neither the shape nor complexity of the space is characteristic. Usually, how- ever, the cavity, with its smooth walls, assumes a spherical, ovoid, or lentiform shape. Many times diverticula extend from the main cavity, so that what appear to be multiple cavities in a single section, when traced through the series, turn out to communicate with one another. Figure 2 from embryo 350 shows such a condition. Finally there may be a duplication of these mesial cavities, so that two or more unconnected cavities may be found where normally only one appears. When this occurs each cavity is smaller than usual, but such a variation is not necessarily repeated on the opposite side of the brain. These cavities have but one distinguishing mark — namely, their position. As far as could be determined from series cut in the three planes, this space occupies a middle position in the striate body and is about as far from the cerebral ventricle as the surface of the brain. In figure 11 a geometrical projection gives its relation to the brain as well as to the lateral cavity. In the embryos of this collection no remnants of a mesial cavity appear when the length of 24 mm. has been exceeded.

The Cavum Laterale Corporis Striati

Farther laterally in the corpus striatum a second splitting of the nervous tissue makes its appearance soon after the medial one is evident. This space, which I have called the cavum laterale corporis striati, could not be made out in specimens 719, 350, 406, and 423. Embryo 144 1 (14 mm.) furnishes the earliest observed stage in the formation of the cavum laterale. An illustration (fig. 3) of the left side of this embryo gives a clear idea of its position in the brain. At the time of its appearance it occupies a deep position in the striate body just beneath the deeply stained ependymal zone. The separation of the ner- vous elements measures 270m by 26m, and may be traced through four sections — i. e., 160m of tissue. Its fellow on the other side of the brain has the same measurements. A drawing (fig. 4) of this less dense region brings out characters quite similar to those already described for the beginning mesial cavity. A narrow, elongated slit appears among the young nerve-cells which bridge over it with numerous thin filaments of protoplasm. In places the cell-body with its nucleus seems to be suspended midway between the diverging walls, as if undecided as to which it will follow. To the right, a small blood-vessel [bv) extends from one wall to the other. Moving among the strands of nervous tissue may be seen several large macrophages (ma) with their typical eccentric nuclei.

Like the cavum mediale, the cavum laterale reaches its full development very soon after its appearance. Its growth proceeds at the expense of the tissue lying between it and the surface of the brain, and usually halts only after a single layer of nerve-cells is interposed between it and the pia mater. All the transitional stages between the earliest evidences of a splitting of nervous tissue and such an appearance as is shown in the photomicrograph (fig. 5) have been found in the series of embryos here studied. The space does not always extend so close to the surface as this specimen would indicate. In every instance

'Measured from sections after mounting.

where good histological pictures were present there were no evidences of an anatomical opening under or through the pia mater. By studying the drawing (fig. 6) of a higher magnification the character of the walls and the behavior of the nervous tissue surrounding the cavity are more clearly made out. Roughly hour-glass in section, one finds the cavity spreading out just under the surface of the brain and having a narrow constriction where it adjoins the ependymal zone. Other embryos show an even more exaggerated narrowing than embryo 431 (18.5 mm.). Everywhere the contour is rough and broken by projecting masses of nerve-cells. Especially near the surface of the brain numerous delicate anasto- mosing processes of nerve-cells traverse the space. Five blood-vessels (br) are included in this section, coursing from one wall to another. The characteristic phagocytic cells (ma) are present here in relatively larger numbers than in most of the lateral cavities. It is unusual to find macrophages so numerous as to form a morula-mass, except in the cavum mediale. Single cells are wandering among the protoplasmic strands along the edges of the cavity. A large amount of debris is present, floating about in the space, whether merely coagulated proteid or degenerated formed elements could not be determined. This cavity was the largest seen, measuring 50G> in depth from the brain surface and 350^ in average diameter. Like the mesial, the lateral cavity varies considerably in shape, volume, and contents. The size does not increase pari passu with the growth of the embryo, except within the wide limits of its appearance and disappearance, both of which are remarkably sharp. In volume the lateral cavity is many times greater than the mesial, and owing to the irregu- larity of its walls has no characteristic shape. The diverticula are usually large and the lateral cavity is but seldom represented by several independent spaces. Here one usually finds delicate strands of nerve-cells stretching for a considerable distance across the cavity in a manner that suggests the growing embryonic nerve-cells on artificial media. This lends a roughened surface to the wall as contrasted with the smoother cavum mediale. Although the lateral cavities in embryo 431 (fig. 6) have many more macrophages per unit volume than the medial cavities of embryo 350 (fig. 2), in the main one finds but few cells in any one section of the lateral cavity.

For observing the relations of the cavities to one another a fortunate section through embryo 409 (16 mm.) was photographed with both low and high power magnification (figs. 7, 8, 9). The transverse section includes all four cavities, two on each side (fig. 7), and between the cavum mediale and laterale appears the dark mass of neuroblasts con- stituting the earliest nuclear anlagen in the striate body. Comparison of the magnified photomicrographs of. the right mesial (fig. 8) and right lateral cavity (fig. 9) reveals the characteristics of both spaces as found in the study of the entire series of embryos. Most striking, perhaps, is the macrophage-content, In the mesial cavity almost the enl ire space is occupied by these phagocytes, while it is with difficulty that one makes out the five macrophages hidden among the delicate protoplasmic network making up the boundaries of the lateral space. The cavum laterale, as it lies just beneath the surface of the brain, presents a group of unconnected areas in cross-section, an appearance due to the processes sent out by the nerve-cells lying more or less within the cavity itself. Traced serially, t hey all communicate, forming a complex honeycombed system. In marked contrast to this is the smooth-walled cavum mediale, the rounded mesial wall of which is made up of embryonic nerve-cells arranged along the contour as regularly as if retained by a limiting membrane. The lateral boundary of this space is made up of a large engorged vessel (bv) which lies exposed in the cavity wall. It is very common to find a vessel standing out in relief in the mesial cavity, and rarely even passing through it,

The Disappearance of the Cavities

The inoccupation of both cavities by nervous tissue and their consequent disappearance is apparently as rapid as are their appearance and growth. Embryo 460 (21 mm.) shows an extremely small cavity in the medial position on the right side and a complete absence on the left. In no instance could evidence of the mesial cavity be found after it had ceased to exist as a break in the nervous tissue, i. e., in embryos greater than 24 mm. crown- rump length. An extremely interesting picture is furnished by embryo 453 (23 mm.), inasmuch as it constitutes a stage immediately following the replacement of nervous tissue in the cavum laterale. Close to the surface of this brain, where in younger stages one is wont to find the lateral cavities of the striate body, a considerable collection of macrophages lies among the nerve-cells. There is no longer any break in the nervous tissue, but, in a very circumscribed area, these phagocytes may be made out where the walls fused with one another. Large vacuolated cells are present in symmetrical positions on both sides of the brain and are undoubtedly beginning to migrate from the nervous tissue. The mesenchyme without the brain contains many of these same cells, which are present in greater numbers near the site of the former cavities. It is not improbable that the macro- phages which we see here distributed among the nerve-cells would have crept out to join their brothers outside of the external limiting membrane.

Three specimens included in this list are worthy of notice, inasmuch as they constitute the extreme variations which have occurred. In the first, embryo 240, although its meas- urement is recorded as 20 mm., no sign of either space can be made out. The tissue is fairly well preserved and mounted. We are most likely dealing with a precocious or delayed development of the cavity. On the other hand, we may have a condition analogous to that obtaining for the pig. In this animal but two laboratory specimens, measuring 19 mm., showed an undoubted cavity in the corpus striatum (the mesial one) containing macrophages. Many serial sections of pig embryos from 10 mm. to 25 mm. were searched for evidences of either cavity, but in order to separate possible artifacts — which often occur in the central nervous system — from a true cavity, a typical vacuolated cell with eccentric nucleus was sought within the cleft. In all probability the existence of this cavity is of extremely short duration in this animal.

Embryos 128 (20 mm.) and 455 (24 mm.) showed a most remarkable degree of vacuoli- zation of the striate body. The former embryo has a complicated cavity in the corpus striatum, which lies just under the ependymal zone between the mesial and lateral cavities. I have called this the cavum intermedium corporis striati, but it is evidently an anomaly. It is unconnected with either the medial or lateral cavity, which are well developed. The latter specimen shows a remarkable reduplication of the medial cavity. The region occu- pied by the medial group is very much larger, being extended laterally in the ependymal zone. There are so many small and independent spaces in the nervous tissue that it would be tedious to count them. In most of these cavities are found one or more macrophages, and their presence eliminates artifacts due to rapid dehydration.

Discussion of the Occurrence of these Cavities

One naturally inquires as to the role of these sudden breaks in the continuity of the nervous tissue coincident with the appearance of a foreign cell which proves to he a common element distributed in various parts of the embryonic body and young chorion, and which is in all respects the counterpart of the macrophage of the adult. Equally mysteriously, after persisting during the time in which the embryo grows '.) mm., there is just as abrupt an obliteration of these spaces and a complete disappearance of the phagocytic cells from the corpus striatum.

Before suggesting a' probable reason for their existence, the possibility of an artifact must first be dismissed, since, in its early beginning, each cavity has the appearance of a shrinkage space. A large cavity having been observed occupying the center of the corpus striatum of a chick which has been incubated for 10(H hours, a specimen of similar age was carefully carried through graded alcohol in order to obviate as much shrinkage as possible. This chick was 121 hours old and measured 13 mm. in 40 per cent alcohol. It was fixed in modified Bouin's fluid, passed through 1 to 2 per cent grades of alcohol, and cut into 15m sections in paraffin. Figure 10 is a photomicrograph from this specimen, and the degree of shrinkage may be judged by examining the retina. There is only the slightest tendency toward separation of the layers— and this is perhaps the most difficult organ to maintain perfectly without previously opening the eyeball to facilitate free interchange of fluids. Not one of the blood-vessels in the brain shows the slightest perivascular space, but each has a fully distended lumen. The medial cavity, illustrated on both sides, lies between the ependymal and nuclear zones. Contained within these complicated spaces are cells which are undoubtedly the macrophages, and which exist in very small numbers compared to the volume of the cavity. In all respects, except for the superficial position, the cavum corporis striati in the chick has the characteristics of the cavum laterale of the human embryo; i. e., relatively few cells, roughened walls, and large, complicated lumen. There is no other cavity nearer the surface in either of the two specimens studied. No attempt has been made to follow the development in the chick, yet here is an excellent opportunity to compare the behavior of the macrophages in a vitally stained embryo.

Concerning the part played by these cavities, several possibilities have suggested them- selves, but none are without objection. First, the spaces may be places where macrophages are formed during the short period of embryonic growth, just as the macrophage formation in the fiver of the adult is limited to the endothelium. Glia cells have been described by Alzheimer (1) as forming macrophages ("Kornchenzellen") in the adult, and we may be dealing with such a transformation here. No such transition forms giving weight to this view were observed. If these spaces serve as foci of regeneration, then the size of the cavum laterale seems to be out of all proportion to the number of cells contained in it .

More likely we are dealing with an actual tissue destruction in the embryo of this early age. Both right cavities have been illustrated topographically (fig. 11). It will be noticed that this is the region about which a great shifting of tissue must take place to form the island of Reil and the temporal lobe. This mass movement might result in the destruction of earlier connections, and the macrophages are present simply to take away the debris formed by the rapid shifting of one portion of the brain over another. Two facts. however, are out of accord with this hypothesis ; the formation of the Sylvian fissure has its inception in embryos of the third month and then progresses over a very long period; in other places, where extreme sliding of tissue, like the pontine formation, is present, one never finds this extreme grade of destruction.

A destruction due to other causes is possible; for example, the imperfect circulation of perivascular cerebro-spinal fluid which may be forming in larger quantities than can be absorbed. A dual source for cerebro-spinal fluid has been shown by Weed (12) as being derived from the choroid plexus and the nervous tissue via the perivascular spaces. Unpub- lished work, undertaken by him in the Anatomical Laboratory of the Johns Hopkins University, shows many interesting facts concerning the circulation of cerebro-spinal fluid in the pig embryo.

The various inconsistencies, as well as the lack of uniformity in structure, could be explained on the assumption of a slow accumulation of fluid about the vessels of the corpus striatum for a short time and its disappearance as soon as adequate pathways in the menin- ges are established. Such an hypothesis of an accumulation of fluid in the striate body gains support from many of these recorded findings. The invariable association of per- forating blood-vessels with the cavities; the development of the spaces before the meningeal differentiation has proceeded far; the presence of coagulated protein-like contents; the bridging of the spaces by tissue strands resembling those of the adult perivascular canals — these are factors of possible consequence in this suggested origin. From this viewpoint, the accumulation of the intrinsic fluid of the nervous tissue resulted in the initial tissue destruction; further distention with more extensive tissue destruction and invasion of macrophages occurred; then, with a possible drainage outward of this fluid accumulation, the pressure was relieved and the space rapidly filled with the nervous tissue.


The observations here presented indicate that there occur in the corpus striatum of the human embryo between 15 and 20 mm. in length, two bilaterally symmetrical cavities. The presence in these cavities of cells morphologically similar to the macrophages of the adult and to the Hofbauer cells of the embryonic chorion argues strongly in favor of a func- tional significance. The lateral of these cavities is larger and less completely filled with macrophages than is the mesial. No satisfactory explanation of their presence can be definitely advanced.


1. Alzheimer, A. Beitrage zur Kcnntnis der pathologischen Neuroglia und ihrer Beziehungen zu den Abbauvorgangen im Nervengewebe. Histol. u. histopat.hol. Arbeit, lid. Grosshirnrinde (Nissl- Alzheimer), Jena, 1910, in, 401-562, 8 pi.

2. Evans, H. M. The macrophages of mammals. American Journ. Phys., 1915, xxxvn, 1-16.

3. Grosser, O. Vergleichende Anatomie und Entwicklungsgeschichte der Eihaute und Plazenta. 8°, Leipzig and Wien., W. Braumliller, 1909.

4. His, W. Die Entwicklung des menschlichen Gehirns wahrend der ersten Monate. 8°, Leipzig, S. Hirzel, 1904.

5. Hofbauer, J. Grundziige einer Biologie der menschlichen Plazenta. 8°, Leipzig and Wien., W. Brau- mliller, 1905.

6. Maximow, A. Untersuchungen iiber Blut und Bindegewebe. Arch. f. mikr. Anat., Bonn, 1909, lxxiii, 444-561, 5 pi.

7. Metchnikoff, E. Lecons sur la pathologie comparee de l'inflammation. 8°, Paris, G. Masson, 1892. Also, transl.: 8°, London, Kegan Paul, etc., 1893.




S. Minot, C. S. Die Entwicklung des Blutes. Handb.

d. Entwcklngsgesch. d. Menschen (Keibel and Mall), Leipzig, 1911, n, 483-517. Also, transl.: Man.

Human Embryol. (Keibel & Mall), Philadelphia, 1912, ii, 498-534. 9. Plato, J. Ueber die vitale Farbbarkeit der Phagocyten des Menschen und einiger Saugethicre mit Neutral- roth. Arch. f. mikr. Anat., Bonn, 1900, lvi, 868-917, 1 pi. Saxer, Fr. Die Entstehung der rothen und weissen

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(n. s., xxvi), 93-111, 6 pi.

1. Photomicrograph. Embryo 350 (/5 mm.). Cross-section 75 - I - 5. X 8 The cavum mediale corporis striali (c m) appears in symmetrical

positions on the two sides of the brain. That on the right side is given at a higher magnification in Figure 2.

2. Photomicrograph. Cavum mediale dextrum. X I44. The position is indicated in Figure I .

3. Photomicrograoh. Embryo I44 (14 mm.). Sagittal section 4-1-2. X 5. The earliest appearance of the cavum laterale sinistrum (c/) is indicated here and illustrated at a higher magnification in Figure 4.

4. Drawing of the cavum laterale corporis striati. X 500. Macrophages (m a) may be seen among the strands of protoplasm bridging the cavity, b v, blood-vessel The r>ns ; tion of this drawing is indicated in Figure 3. (Drawn by J. F. DiJinch.)

5 Photomicrograph. Embryo 43 ! (18.5 mm.). Sagittal section 12-1-1. X 7'<f. The very large cavum laterale sinistrum (c/) is shown in its relation to the nia mater and the ventricle of the brain A more highly magnified drawing is given in Figure 6. b Drawing of the cavum laterale sinistrum. X '90 A single layer of nerve-cells separates the space from the pia mater (p m). Large numbers of macrophages (m a), a detailed drawing of which is given in Figure 12. are present Rlood-vessels (b v) traverse the cavity. nded by a few nerve-cells. The position of this drawing is indicated in Figure 5. (Drawn hi/ J F. Didusch.)

7. Photomicrograph. Embryo 409 ( /6 mm.). Cross-section 10-2-5. X 10. This figure illustrates the symmetrical positions occupied by the lateral (c /) and mesial (c m) cavities. Larger reproductions of them are shown in Figures 8 and 9.

8. Photomicrograph of the cavum mediale corporis striati. X 200. The smooth-walled space is parked with macrophages. An engorged blood-vessel (b v) forms its lateral boundary. The position of this illustration is indicated in Figure 7.

9. Photomicrograph of the cavum laterale corporis striati X200. Some of the nerve-cells are growing across the cavity. The position is shown in Figure 7.

10. Photomicrograph of a section through the corpus striatum of a chicle (121 hours incubation). X 7'<. The cavities (<: s) in the striate body are unusually large.

11. Geometrical projection of a model made at 50 diameters. Embryo 460 (21 mm.). X6J£. The cavum laterale (c / ) and cavum mediale (c m) are shown on the right side.

12. Macrophages indicated in Figure 6 X 1000. (Drawn by J. F. Vidusch.)

13. Photomicrograph o( macrophages in cavum mediale. X 1000.

14. Photomicrograph of chorionic villus of Embryo 350. X 410. 15-17. Macrophages from the chorionic villi. X 1000.

18-30. Macrophages from the cava corporis striati. X 1000.

31. Normal erythrocytes from a 15 mm. human embryo. X 1000.

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