104 TRANSITORY CAVITIES IN THE CORPUS STRIATUM. 



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. 



