386 LEFEVRE. [Vox. XIV. 
flattening of the cells composing the remaining portion of the 
vesicle. It is this thickened wall, originally on the right side, 
which forms the floor of the pharynx in that part of the vesicle 
which is concerned in the displacement. Very soon after the 
beginning of the change in position, a shallow longitudinal 
groove is found on the inner surface of the vesicle in the middle 
region of the bud, lying on the right side on a level with the 
lower border of the pericardial rudiment. This is the first 
appearance of the endostyle ; its position is shown at exd in 
Pl. XXX, Fig. 11, but at an earlier stage it is found much 
higher up. In this figure and the next one it is seen that the 
groove runs through about the middle of the thickened area, 
that is to say, above and below it there are equal portions of 
the thick wall, which will lie to its right and left when the 
change in position of the vesicle is fully accomplished. 
The groove rapidly extends anteriorly and posteriorly, and 
at the same time becomes deeper and broader. When it 
reaches its definitive position in the mid-ventral line, it 
stretches throughout the entire length of the branchial sac. 
It will not be necessary to speak of the differentiation of the 
endostyle into the various zones of cells which go to make it 
up, as these have been described by numerous authors, — Della 
Valle (3), Herdman (7), Lahille (17), and others. 
It is to be especially noticed in Figs. 10, 11, 20, and 24 that 
the positions of the pericardial rudiment and endostylar groove 
in reference to each other remain the same during the dis- 
placement of the vesicle. From this fact it is evident that the 
thick portion of the vesicle is carried down bodily, and that no 
interstitial growth takes place in this region during the process, 
else the distance between the pericardium and endostyle would 
not remain the same. It cannot be said that the pericardial 
rudiment might compensate by its own growth for any increase 
in extent of that part of the wall against which it lies, for it 
covers practically about the same area as long as it adheres to 
the vesicle. It would seem, therefore, that the change in posi- 
tion of the inner vesicle is brought about by the stretching out 
and flattening of the cells in all but the thick area, and that the 
latter is borne or rather pushed down toward the ventral side. 
