SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 375 



with the lamina supraneuroporica in the mid-line. This point 

 has been labelled m in numerous figures in preceding papers. 

 Since there is an early differentiation between the tela chorioi- 

 dea and the lamina supraneuroporica in the embryo, this point 

 is always easily recognizable in both embryos and adults 'of all 

 vertebrates. It is one of the fundamental landmarks in the tel- 

 encephalon. Its position has been noted by Elliot Smith ('97 b, 

 p. 52, fig. 23; '97 c, p. 235) and is indicated in figures 4, 5, 6, 7, 

 9, 35, 45, 55, 64 of this paper. This point may be named margo 

 posterior pallii. 



To complete the identification of lamina terminalis and lamina 

 supraneuroporica in reptiles and mammals it is necessary only 

 to determine the position of the recessus neuroporicus. 



The neuroporic recess is always situated rostral or dorsal to 

 the anterior commissure. Dorsal to the recess in cyclostomes 

 and selachians is the lamina supraneuroporica with its commis- 

 sures and gray matter as already mentioned. In selachians there 

 is found an external neuroporic recess ('11 a) in the form of a 

 deep pit or a slender canal, through which blood vessels enter 

 the brain near the upper border of the lamina terminalis. 



In all the embryos of reptiles and mammals which the writer 

 has examined there is found a ventricular recess rostral to the 

 anterior commissure, and above the recess is a thick lamina 

 which in advanced embryos contains a pallial commissure. These 

 relations were shown in a previous paper ('10 c, figs, 17, 18, 19, 

 20). It will be seen at once upon comparison with selachian 

 embryos that the relations are essentially the same. This recess 

 is not preserved in the adults of all reptiles and mammals, but 

 its position is readily determined, since it must lie between the 

 anterior and the pallial commissures. The recess which is found 

 in this position is the recessus triangularis of Schwalbe ('81). 

 Its significance will be discussed in the following paragraphs. 



The location of the recessus neuroporicus is of critical impor- 

 tance. Most authors who have recognized a neuroporic recess 

 in mammals have identified it with the angulus terminalis of 

 His and the recessus superior, the relations of which are so clearly 

 figured by Elliot Smith in monotreme, marsupial and mam- 

 malian brains. 



