344 Robert J. Terry. 



Post-velar Arch. This structure can no longer be said to exist, 

 its place being occupied by the velum. 



Paraphysis. There is no trace of a paraphyseal differentiation 

 of the fore-brain tela. The latter, in its posterior part, has, how- 

 ever, been changed from the simple dome-form of the larval 

 stages by the appearance of bilateral diverticula. In fig. 18, it 

 will be seen that the ventricle is continuous from side to side, but 

 that the tela is depressed in the mid-line to form a broad, shallow 

 groove opposite the stalk of the epiphysis. Fig. 19, which is 

 taken from a section at the level of the velar pedicle, shows a 

 median partition between the two wide diverticula of the fore- 

 brain ventricle. This septum extends from the bottom of the 

 groove, noticed in the previous figure, and contains, besides some 

 large blood-vessels going to and away from the velum, the stalk 

 of the epiphysis. 



II Discussion 



Morphological Divisions of the Fore-brain Roof 



Burckhardt ('94, a & b) recognized in types of all vertebrate 

 classes the presence and constant relations of the following struc- 

 tures : 



Paraphysis Epiphysis 



Velum Transversum Pars Intercalaris 



Zirbelpolster Posterior Commissure. 

 Superior Commissure 



The forecast of these structures in the embryonic brain has been 

 described by Minot ('01) who, as mentioned in the beginning of 

 this paper, found that the pineal region of Acanthias at an early 

 stage presented six constant divisions. The divisions, in the form 

 of arch-like evaginations and alternating depressions into the 

 ventricle, were named by this author according to their subse- 

 quent differentiation. Further observations upon the pineal 

 region of embryos of other animals led to the belief that they were 

 fundamental and that homologous parts might be found in all 

 vertebrates. 



