( 5? ) 



reconstructed from cross-sections of 5f<, magnified 800 to 1600 times, 

 and afterwards reduced by means of photography. To tig. 5 I added 

 the cross-sections lying on the spots indicated with a, b, c, d, to 

 show the width and form of the cavity at t he different spots. In 

 fig. 8 I give the reproduction of a real median section through the 

 brain of a specimen of about the same age as the one the recon- 

 struction of which is given in fig. 7, to show how much they are 

 like each other. 



The reconstruction of fig. 5 shows, that even in very young larvae, 

 (larvae of 1.5 to 2 m.m. give about the same picture), in which the 

 brain is still larger in diametre than the spinal cord, there exists a 

 dorsal dilatation of the cerebral cavity, which may be compared 

 with the fourth ventricle of craniote embryos (fig. 5, 6, VQ). It 

 represents a dorsal dilatation of the central canal (fig. 5c) and is 

 connected with the anterior vesicle by a narrow part (tig. 56). In 

 all my specimens this connection of the ventriculus quartus with the 

 anterior vesicle could be stated with absolute certainty, contrary to 

 the well-known observations by Hatschek. Even in very young 

 larvae the connection was very conspicuous. In the caudal part of 

 the dorsal dilatation (fig. bd) the midpart of the narrow fissure-like 

 central canal is obliterated, so that this part of the fourth ventricle 

 is separated from the ventral central canal which remains open. In 

 older animals this obliteration proceeds craniad. The dorsal wall 

 of the fourth ventricle is very thin consisting of one layer of flattened 

 cells, but it is always visible even in very young larvae, if only 

 the specimens are well-preserved. 



In much older individuals, which passed through the meta- 

 morphosis long ago (fig. 6), the fourth ventricle is still very con- 

 spicuous and connected with the anterior vesicle by a narrow 

 dorsal canal. The dorsal wall is still thin and membranous. The 

 large dorsal ganglion-cells (vide my former paper) that are now 

 developed to a certain extent, are still only visible at both sides of 

 the median plane and do therefore not appear in the median section 

 through the brain. Afterwards this peculiar group of cells is developed 

 to such an extent (fig. 7, fig. 8), that they occupy the entire dorsal 

 part of this region of the central nervous system, and so appear also 

 in the median section. It is only then that the distinct fourth ventricle 

 becomes indistinct, irregular, flattened, alters its shape and even 

 disappears here and there. Then we find the peculiar irregular 

 dilatations of the central cavity, described by Kupffer as "quere 

 Schenkel" and "blasenförmige Erweiterungen". They are not seg- 

 mental, are only of secondary importance, and are not to be com- 



