144 GEORGE E. NICHOLLS 
One other point must be mentioned here. In the ray the ac- 
tual position of the terminal sinus varies slightly, it was found, 
in different individuals. In the case of the specimen of Raia 
blanda figured (text-fig. 3) this terminal chamber extended 
downwards behind the extremity of the notochord, which is, I 
believe, the strictly primitive condition. It occurs, however, 
less frequently in this position than might be expected, and in 
many cases it lies altogether dorsal to the notochord, not always 
extending even to the posterior extremity of that structure. 
Whether there has been some mutilation in these cases or whether 
on the contrary there takes place, normally, a certain amount of 
resorption of the tissue of the terminal filament, I can not 
decide. 
In some teleosts I have found what are, almost certainly, 
stages in the disappearance of the postero-ventral (post-chordal) 
part of the neural tube. I find, moreover, that the corrugation 
of the hinder end of the filum terminale in small rays which I 
have described (12, p. 423) as so strongly suggestive of neuro- 
meric constriction, is likewise frequently met with in the vanish- 
ing vestiges of the filum terminale in the region of the disappear- 
ing tail in the recently metamorphosed anuran. 
While these facts suggest that the variation in position of the 
sinus terminalis of the ray may be due to some extent to the ab- 
sorption of tissue in this region,! the possibility of mutilation 
must not be ignored. The actual end of the tail of the ray is 
soft and not protected by spines, and specimens which have suf- 
fered quite considerable mutilation are by no means rare. The 
terminal sinus, too, in those specimens in which it lies wholly 
dorsal to the notochord (fig. 19) rarely shows that bulbous ex- 
pansion which is seen in examples in which the sinus terminalis 
has the postero-ventral position (fig. 20) but has quite a marked 
resemblance, in shape, to the secondary terminal sinus which I 
‘That an absorption of tissue in this region does occur in rays is suggested 
by Beard’s statements (’96, p. 55, footnote 2), that the young (Raia radiata) 
immediately prior to escape from the egg case are shorter by a centimeter or so 
than embryos a month younger. Some of my own specimens which were six 
inches or less in length must, almost certainly, have been quite newly escaped 
and the process of resorption was possibly incompleted. 
