TRANSITORY CAVITIES IN THE CORPUS STRIATUM. 107 



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. 



