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E. W. MACBRIDE. 
nearer the mid-ventral line than the cavity on the left side. 
Below the most ventral point of the latter the gut is pressed 
against the ectoderm, which here exhibits a plate of large 
cells; this plate marks the place where the perforation 
to form the mouth will take place. In the next section, 
shown in fig. 16 d, the enlargement of the anterior end of the 
gut to form the pharynx is cleaidy seen, and on the right side 
of this there is a pouch-like outgrowth, the rudiment of the 
club-shaped gland. This structure has been interpreted 
by Willey (36) as the fellow of the first gill-slit, and in this 
opinion I am prepai’ed to concur. The posterioi- somites 
which have resulted from the segmentation of the collar- 
cavity are seen to be completely divided into myotome (my.) 
above and splanchuocele (sph) below, but beneath and 
external to the right and left rudiments of the 
splanchuocele are to be seen the posterior pro- 
longations of the right and left collar-cavities (r. 
coll., 1. coll.). These extend just as far back and no 
further than the enlargement of the gut, which is 
the rudiment of the pharynx. In the next section, fig. 
16 e, the same features are observable, but here we find the first 
gill-slit about to open, and we see that as in all Vertebrata it 
is produced by the meeting of an endodermal outgrowth 
{hr. end.) and an ectodermal ingrowth [hr. ect.). 
Figs. 16 f and g are two sections from the most posteidor 
part of the series. In fig. 16/ we see the anal diverti- 
culum (a.) which we had already noted in a much younger 
embryo, but the anus is not as yet open. In fig. 16 y, two 
sections further back, we see the solid cord of cells uniting 
gut and nerve-cord (iie.), which is the representative of the 
neurenteric canal of earlier forms, and this acts as a 
“meristem” for the tail, which in Amphioxus is of rela- 
tively small dimensions. Since the division of the gut into 
two storeys is still near the hind end of the animal, in spite of 
the great growth in length which has taken place, we draw 
the conclusion that this growth must take place mainly in 
front of this region, and that the band of actively growing 
