28 F. L. LANDACRE 
gill pocket and also extends posterior to this point (figs. 39, 40); 
so that it extends backward past the point of origin of the placode. 
The dorsal sensory line shown in figs. 39 and 40 extends one- 
half of the distance from the hyoid gill pocket to the auditory 
vesicle, while the thickened area on which the placode forms 
extends two-thirds of this distance. This series is unusual, 
since in later series I cannot find a dorsal sensory line extending 
posterior to the placode and lying dorsal to it. 
The form and position of the placode is shown’ in fig. 39. It 
consists of a well defined mass of cells projecting mesially and 
lying just ventral to the dorsal sensory line. Fig. 40 is taken 
four sections posterior to fig. 39 and shows the posterior exten- 
sion of both the sensory line and the thickening which represents 
the posterior extension of the epibranchial placode and is contin- 
uous with the preauditory placode. The anterior end of the gen- 
eral visceral VII is more definite in outline than in the preceding 
stage figured, and comes into contact at its anterior end with 
the placode, although there is no difficulty in separating the two 
structures owing to the definite outline of the placode. There 
is the doubt, however, as to the composition of this extreme 
anterior end of the general visceral ganglion mentioned above. 
The changes in an 8 mm. embryo are not marked, consisting 
chiefly in the increase in size of the mass of cells proliferated 
mesially in the placode. It should be borne in mind that the 
anterio-posterior extent of the placode is usually not over three 
or four sections (24u) thick, so that in transverse sections one 
passes from the endodermic prolongations of the hyoid pocket 
directly into the ectodermic prolongation of the placode.’ This 
thickness in the anterio-posterior extent of the placode seems 
to be due to its being apposed so closely to the posterior surface 
of the gill pocket and gives the placode when seen cut through its 
greatest transverse dimension, the appearance of being much 
larger than it is. 
In fig. 41 the endoderm of the gill pocket characterized by its 
pale color and large cells with definite walls abuts directly against 
the dark staining small celled ectoderm so that the boundary 
line is quite definite. In fig. 41 the continuity of the gill pocket 
