86 JOHN WARREN 



tion and has the best specimen of a paraphysis of any in the 

 Collection. 



Figure 14 shows a median section of this part of the roof of 

 the forebrain. The velum, V, forms a deep fold and immediately^ 

 in front of it can be seen a well marked paraphysis, P, with a 

 wide opening and a large cavity. On the opposite side of the 

 velum the diencephahc roof bulges forward and overhangs the 

 paraphysis. In figure 15 there is a view of the external surface 

 of the roof of the forebrain in this same region seen from in 

 front and a little to the right as in figures 6 and 9. The para- 

 physis, P, overlies the telencephalic roof plate. Immediately 

 behind the velum, V, arise several outgrowths from the dien- 

 cephalon, the so-called postvelar tubules to be considered later. 

 It is easy to see that .in a series of sections at right angles to the 

 telencephalic roof plate these prolongations might be mistaken 

 for a paraphysis if that organ was actually absent, and it is not 

 easy in such a series to determine at once on what side of the 

 velum these outgrowths really belong until an actual model has 

 been made. 



Figure 16 represents the same model seen from the interior of 

 the brain to show the relative positions of the opening of the 

 paraphysis and the openings of the postvelar tubules with ref- 

 erence to the velum and the telencephahc and diencephalic 

 roof plates. The extent of the remainder of the diencephahc 

 roof and the position of the epiphysis and commissures are well 

 shown in Bailey's models of 28 and 32 mm. embryos ( (2) figs. 

 20 and 22). The next stage in which a paraphysis could be 

 shown was an embryo of 36 mm., again a specimen in excellent 

 condition. Between this specimen and the previous one the 

 collection contained several embryos of 28.8, 29, 30 and 31mm. 

 In an embryo of 31 mm., sagittal series, there was a very slight 

 elevation in the paraphysal arch close to the velum which might 

 be regarded as a mere rudiment of a paraphysis (fig. 17). In 

 the others there was no sign of a paraphysis. The telencephalic 

 roof plate formed a well defined paraphysal arch similar to the 

 condition shown by Bailey (2), figures 20 and 22. For misin- 

 terpretation of the paraphysis and velum see Warren (24), 



