470 B. F. KINGSBURY 
relations the tubercle of the floor bears to foregut and stomo- 
daeum would correspond to it, is marked in figure 20 by the 
letter T. It also agrees with the tubercle of the floor in being 
caudal to the primitive infundibular recess. This slight elevation 
in the shark is clearly but the mechanical expression of the underly- 
ing ‘preaxial mesoderm.’ In the neural plate stages, figures 7, 
8, and 9, there is indicated, somewhat faintly it is true, a wedge- 
shaped area® tapering caudally from a rounded ‘apical lobe’ 
which from its photographic reproduction might easily be inter- 
preted as a typical ‘tubercle of the floor’ extending quite to the 
anterior edge of the neural plate. The comparison of the Schulte 
and Tilney figure 1, plate 27, with figure 10 of this paper, already 
called for in connection with the optic sulcus, might be again ap- 
pealed to as establishing the equivalence of this wedge-shaped 
area with the tubercle of Schulte and Tilney. However, examina- 
tion of transections shows that in such stages as those of figures 
9, 10, and 11 there is no or but slight elevation of the neural 
plate and no marked thickening of the neural plate which cor- 
responds to the area reproduced by the photograph. Plottings 
were made of mesoderm, entoderm, notochord, and neural 
plate showing their position and extent in the embryos of figures 
7 and 9, and these are reproduced here as figures 3 and 4, re- 
spectively. From their examination it is quite evident that the 
appearance seen in the surface examination of shark neural 
plates and which the camera reproduces is mainly an expression 
of the greater thickness of the underlying preaxial tissues where | 
entoderm, mesoderm and notochord are indistinguishably united 
—a region with whose significance Adelmann (’22) has more 
particularly to do. The progressive transformation of this 
region as seen in the median plane from these or comparable 
stages may be seen in the comparison of figures 17 to 21 (plate 2). 
5 This anterior medial wedge-shaped ‘area’ was noted and figured by both Locy 
and Neal. The former says regarding it (p. 551): ‘‘This has already been spoken 
of in Part I as a tongue-like process extending from the median anterior tip back- 
wards to two-thirds the length of the cephalic plate. It continues to be a promi- 
nent feature of the cephalic plate for some time. I can offer no suggestion as to 
its significance, outside the obvious suspicion that it may represent a proboscis 
of some kind, or that it may be related to the large notochord of this region.”’ 
