4 Paraphysis and the Pineal Region in Necturus Maculatus 
These cells are, of course, continuous with those which form the brain 
wall in this region. The same is true of the epiphysis, but the walls seem 
thicker, as the organ has been cut somewhat obliquely. Close to the 
paraphysis two vessels can be seen, a larger one cephalad and a much 
smaller one caudad, Ves. ‘The vessels lie in intimate relation to this 
structure, and it is important to note their relation at this early stage, 
because as development progresses the relation between paraphysis and 
blood vessels becomes more and more intimate. 
Fig. 7 is a section of an embryo of 15 mm. The most striking feature 
here is the increase in size of the paraphysis, which has become a long 
tube with a lumen extending its entire length, and at its distal end a lat- 
eral diverticulum has appeared. The roof of the fore brain has now 
descended to such a degree that the opening of the paraphysis is on a level 
with the tip of the velum. The velum itself has lost its cephalic layer, 
and consists of one layer only, which, however, is much longer than the 
velum in Fig. 5. If Figs 4, 5, and 7 are compared it will be seen that 
Fic. 7. Embryo of 15 mm. Harvard Embryological Collection, Sagittal 
Series, No. 79, Sections 85 and 89, * 63 diams. 
the distal end of the paraphysis is practically at the same distance from 
the ectoderm in each case.. As the paraphysis has developed during those 
stages into a long tube, its growth must have occurred by a downward 
extension of the neighboring parts into the cavity of the fore brain. This 
is practically the same process described by Minot in Acanthias. It is 
also shown by the great increase in distance between the roof of the 
telencephalon and the ectoderm from Fig. 4 to Fig. 7. The opening of 
the paraphysis in Fig. 3 is nearly ona level with the base of the velum, 
and as the down growth of the parts takes place the opening of the para- 
physis and the paraphysal arch descend, apparently pushing the cephalic 
layer of the velum ahead of them. Therefore the single layer of the 
velum in Fig. 7 really corresponds to the original caudal laver, plus the 
cephalic layer, which has been forced down ahead of the opening of the 
paraphysis. 
In studying Fig. 7 it might seem as if the posterior wall of the para- 
physis corresponded to the cephalic laver of the velum. This, however, is 
