PARAPHYSIS AND PINEAL REGION IN REPTILIA 347 



mid brain. Everything else seems uncertain, especially the deri- 

 vation of each of the secondary subdivisions from the primary 

 neuromeres. These later segments or neuromeres should be 

 distinguished as secondary from those which are primary. There- 

 fore in the present state of our knowledge it seems to me that 

 these later segments or subdivisions should be distinguished from 

 those which appear earlier and are essentially primary neuro- 

 meres. 



Paraphysal arch and paraphysis 



Minot (71) showed three primary arches in the roof of the fore 

 brain of Acanthias and gave the name paraphysal arch to that 

 part of the roof of the telencephalon which lies immediately ceph- 

 alad to the velum and passes into the dorsal end of the lamina 

 terminalis. This section of the fore brain roof is early differen- 

 tiated, figs. 2, 3, 17 and 18, and forms a striking arch up to the time 

 of the appearance of the paraphysis and the telencephalic cho- 

 roid plexuses. The paraphysal arch has been called by Burck- 

 hardt (12) the lamina supra-neuroporica and the plexus inferio- 

 ris and plexus hemisphericum (telencephalic plexus) develop from 

 it immediately in front of and lateral to the paraphysis. The shape 

 and extent of the arch with its plexus development can be fol- 

 lowed in Burckhardt's diagrams (13). The increasing size of 

 the hemispheres soon tends to cover up this portion of the roof of 

 the telencephalon. The differentiation of the arch in the lizard 

 and the turtle can be also seen in figs. 2-10 and 17-21. Minot (71), 

 Dexter (20) and Warren (97) show the development of this arch 

 in Acanthias, chick and Necturus, and Johnson (53) in a series of 

 pig embryos. It is well marked at first in the sheep, Neumayer 

 (75). The arch appears distinctly as such in early stages only. 

 The paraphysis and telencephalic plexuses tend to absorb this 

 segment and when they are well developed no traces of it remain, 

 but the opening of the paraphysis and the origin of the telence- 

 phalic plexus both still lie between the velum and the dorsal end 

 of the lamina terminalis and together occupy all the space for- 

 merly occupied by the paraphysal arch. 



