482 'Journal of Comparative Neurology and Psychology. 



the primitive optic groove (Fig. 16), furnishes a pathway for the 

 optic tract fibers as they grow in from the retina to the optic centers 

 in the thalamus and tectum opticum. The ridge may therefore be 

 called the oijtic ridge. The primitive optic groove from this stage 

 on is to be seen just behind the transverse ridge occupied by the optic 

 chiasma and other decussations. Referring to the general descrip- 

 tion at the beginning of this article it will be seen that what I have 

 called the terminal pit in the early embryo is the same as that called 

 by His the optic recess, and that to which I have in all my previous 



Fig. 18. Squalus ac, late stage K of Balfour, medial aspect of a model of 

 the right half of the head, x 25. 



papers given the name of preoptic recess. The primitive optic 

 groove I have heretofore called the postoptic recess. These terms 

 have been used by some other authors (Mrs. Gage, Sterzi, and others), 

 but not by all. I wish to emphasize the necessity of recognizing 

 the two pits and applying to them clearly distinctive names, because 

 they are both related to the optic vesicle and because the postoptic 

 recess has heretofore been confused with the infundibulum. 



b. Remainder of the Floor of the Diencephalon. — As soon as the 

 head-bend of the brain tube appears, a broad depression of the floor 

 of the forebrain vesicle can be seen which corresponds to the future 



