No. 3-] THE VERTEBRATE HEAD. 565 



The results of all the embryological studies on the epiphysial 

 outgrowth between 1887 and 1893 were to settle on one point 

 of common agreement regarding its origin. Each investigator 

 successively found it to arise as a tubular outgrowth from the 

 roof of the thalamencephalon comparatively late in ontogeny, 

 after the parts of the brain are established. No earlier trace 

 of it had been found ; but there was a previous undiscovered 

 history, and it is now timely to make the inquiry, What is 

 the very beginning of the pineal sense-organs } 



2. Remote Origin of the Pineal Outgrowth. 



As already indicated, there are upon the cephalic plate ac- 

 cessory eye vesicles which have made their appearance while 

 the neural groove is widely open ; and in tracing their fate I 

 shall attempt to fill up that gap in the history of the pineal 

 outgrowth from the time these cup-like structures appear upon 

 the cephalic plate to the time the tubular outgrowth begins 

 from the thalamencephalon. 



As the walls of the neural groove grow upwards these cup- 

 like structures are carried up with them; and there comes a 

 time (Figs. 13, 31) when they may be seen from the outside 

 as rounded eminences, and from the inside as concave cups. 

 When the edges of the neural groove meet in the middle line 

 the cups are approximated, and come to form part of the thala- 

 mencephalon. 



While these changes have been going on, further differenti- 

 ations have been taking place in the walls of the brain. The 

 bulging: of the walls to form the mid-brain vesicle has come on 

 insidiously, and has taken a position behind the vesicles of 

 the paired eyes, in apparently the same position previously 

 occupied by the accessory vesicles. These transformations are 

 confusing, as the walls of the mid-brain resemble the accessory 

 optic vesicles grown larger. In an earlier published paper I 

 made the mistake of identifying the mid-brain with the acces- 

 sory optic vesicles; but it was merely an error of identification, 

 and did not affect the main contention of that article. 



In working out the details of the formation of pineal out- 

 growth in Squalus, it soon becomes apparent that the surface 



