TRANSITORY CAVITIES IN THE CORPVS STRIATUM. L05 



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, 



