TRANSITORY CAVITIES IN THE CORPUS STRIATUM. 103 



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. 



