578 ADAM SEDGWIOCK. 
of the origin of the first somite. It is true that at the time 
of the formation of the medullary plate the notochord stops 
some little distance short of the front end of the body, and 
there is a portion of the gut in front of it ; but this is only a 
temporary state of affairs, and is due to the fact that the front 
end of the notochord, which is developed from behind for- 
wards, is not yet formed: moreover the solid mass of endo- 
derm referred to by Kastschenko is present at the front end of 
the gut even at this stage. When the notochord has acquired 
its furthest anterior extension in Scyllium, just before 
Stage G, it terminates in a solid mass of cells, which is con- 
tinuous also with the front end of the gut. The notochord 
has hitherto during the whole of its growth been continuous 
in front with the endoderm, and its condition at the period 
referred to is merely a persistence of that continuity. Wyhe’s 
account of the anterior end of the notochord appears to me to 
be quite correct. 
When the notochord has acquired its utmost anterior 
extension there is no portion of the gut in front of it, but 
merely this solid mass of cells, with which both it and the gut, 
and afterwards the ectoderm of the buccal slit and pituitary 
body, are continuous, and which underlies the very front end 
of the medullary tube. If this mass of cells be regarded as 
partly consisting of the anterior end of the notochord still vn- 
differentiated, it may be said that the notochord reaches in 
Scyllium, at any rate, to the very front end of the neural 
tube; in other words, that Scyllium at this stage is truly 
cephalochordate in the sense that Amp hioxus is cephalo- 
chordate. 
The solid mass of cells in which the notochord and gut 
terminate becomes in Scyllium and Pristiurus very early, 
before Stage G, connected with the ventral ectoderm. Wyhe, 
who connects this fusion with the formation of the mouth, 
puts it down as taking place later in Stage H ; but I can posi- 
tively assert that in Scyllium and Pristiurus it is present 
before Stage G—before any trace of the cranial flexure has 
appeared. 
