1880. ] Development of Amphioxus lanceolatus. 89 
and it is hardly more than accomplished, and perhaps in many 
instances not completed, before the body folds close up entirely 
over the pharyngeal clefts, the left body-fold advancing towards 
the cartilaginous ring coincident with the advance and final 
disappearance of the pharyngeal mouth aperture and mouth welt; 
the first two or three pharyngeal clefts become pushed almost 
entirely over upon the right side of the pharynx; the ciliated. mouth 
cavity enlarges anteriorly, the dorsal and posterior edges growing 
downward and forward, and small fleshy prolongations appear 
upon the posterior edge ; and two or three small oval openings 
appear piercing the left side of the pharynx at points intermediate — 
to the already formed pharyngeal openings, in a line just posterior 
to the cartilaginous ring and beneath the level of the notochord. 
Fig. 1, Pl. 11, shows the appearance of the anterior portion of the 
animal at this time, with mouth tentacles beginning to show along 
the posterior edge of the cavity, and the oblong openings along 
the left side of the pharynx. 
These openings in the pharynx represent the rudiments of the 
left branchial fissures or arches. They increase in size until the first 
one is about one-half the width of the pharynx, when a small 
protuberance forms in the center of the upper or dorsal border, 
and grows downward towards the ventral edge, finally merging 
with it and dividing the opening into twd more or less elongated 
apertures, the branchial slits or clefts. Before this division is 
effected however, each of the other openings has given rise to a. 
central prolongation, which grows downward to finally unite with 
the lower border, as in the first case, and two or three new open- 
ings have pierced the pharynx, one between the first arch and 
the cartilaginous ring, and the other in a line with, and posterior 
to, those first formed in the pharyngeal wall. 
Of these new arches, the first one never becomes divided by os 
central bar, but each of the others is. divided in turn into two 
sections, as a new opening makes its appearance in the pharyngeal 
walls. As there appears to be no limit to this formation of arches, 
and of their division into clefts, in the growth of the individual, 
there is always to be seenin either young or adult, a posterior — 
round or oval aperture and just anterior to it one or more open- 
ings partially segmented by central prolongations, although some- : 
times the last aperture indicates the central division, by a curva- 
ture of the dorsal edge, — the new apes” is eE After — 
VOL. XIV.—NO. 2 
