536 H. F. Osboin 



the plate does not form the g-round substance to be traversed by 

 these fibres. On the other hand the fibres bridge the fissure which 

 is gradually closing in front of the terminal plate. Immediately 

 above the anterior commissure, on either side, are descending fibres 

 which represent the first stage of the fornix. These appear before 

 the anterior commissure crosses the median line. This stage cor- 

 responds closely to that figured by Mihalkovics, Taf. VII, fig. (50. 

 In the next stage the terminal plate has extended in front of the 

 anterior commissure, the fornix fibres are more numerous and at 

 their upper limit a few fibres are observed extending towards the 

 median line, these are the earliest callosal elements. At 49 mm, 

 which follows a considerable interval of development (fig. 28; , the 

 hippocampal sulcus is very deep and the terminal plate is much 

 more extensive. In its lower portion the anterior commissure, now 

 a compact bundle, extends laterally above the cerebral peduncles. 

 The columns of the fornix are well defined and between them in the 

 upper portion of the plate pass the fibres of the corpus callosum. 

 A careful study of these fibres shows that, like those of the anterior 

 commissure, they unite with each other in front of the terminal plate. ' 

 The callosal fibres disappear as they pass around the hi])pocampal 

 sulcus. Above this sulcus is an interval in the inner wall of the 

 ventricle in which no fibres can be observed, but in the roof of the 

 ventricle are the fibres of the corona radiata. This leads me to 

 doubt whether the fibres extend at an early stage from the corona 

 radiata into the corpus callosum, as stated by Mihalkovics. It seems 

 rather that this is a subsequent union. This stage (fig. 28) differs 

 considerably from that figured by Mihalkovics as the initial stage 

 of the corpus callosum. 



Taken altogether the above observations, incliuliug the late de- 

 velopment of the corpus callosum and its wide separation from the 

 anterior commissure in the terminal plate do not greatly tend to con- 

 firm the theory of the cerebral commissures which is advanced in 

 this i)aper. When however we study the embryology of the lower 

 mammalian brain, in this case that of a Marsupial, we find strong 

 corroborative evidence. The youngest pouch specimen of Macropus 

 major which I procured, fortunately gave me the first step in the 

 development of the corpus callosum. It measured 36 mm. The 

 terminal plate at this stage is of much less vertical diameter than 

 in the sheep's brain (comi)are figs. 24 and 28). The hippocampal 

 sulcus is slightly developed. The anterior commissure is already a 



