Johnston, Forcbrain Vesicle ni Vertebrates. 



481 



This thickening corresponds to the anterior border of the neural plate 

 and the lower border of the neuropore. It is the terminal ridge 

 of the early embryo with open neural plate, Figs. 3, 4, 5, 6. From 

 this terminal thickening a ridge is seen in Fig. 16 running obliquely 

 caudo-laterad behind the optic vesicle and cutting it off from commu- 

 nication with the primitive optic groove. This is the beginning of 

 a change in the relations of the optic vesicle which the following 

 stages show completed. 



An embryo of 42 somites is shown in Fig. 17 drawn from a model 

 of the right half of the head and a corresponding model of an 

 embryo of Balfour's stage K is sho^\Ti in Fig. 18. In these it is 



Fig. 17. Squalus ac, 42 somites, medial aspect of a model of the right half 

 of the head. This is the same embryo as the one shown in Fig. 15. 



clear that the optic vesicle is no longer connected with the primitive 

 optic groove, but the two vesicles are now connected with one another 

 by the terminal pit at the lower border of the lamina terminalis. 

 This is the pit which remains connected with the hollow optic stalk 

 as long as that persists and is known in the adult as the recessus 

 opticus (His) ; better called recessus pra'opticus. 



In the latter part of Balfour's stage K the optic tract fibers begin 

 to appear in the chiasma. This lies immediately behind the terminal 

 pit and in front of the primitive optic groove (Figs. 22, A and B). 

 and therefore lies in the temiinal ridge. The lateral prolongation 

 of this ridge, which has been described as running obliquely across 



