FUNDAMENTAL PLAN OF VERTEBRATE BRAIN 471 
The ‘apical lobe’ is intimately related to the ‘pre-oral’ extension 
of the foregut, and it is the great expansion of the more lateral 
region of the neural plate in this region that sets it off as an 
apical lobe. 
Figs. 3 and 4 Plottings from series cut in the transverse plane of Squalus 
acanthias embryos, 2.2 mm. and 2.6 mm. length, 6 and 8 somites, respectively. 
These are the embryos of figures 7 and 9, plate 1. In each, the notochord is indi- 
cated by cross-barring; the stipple areas on the right and left mark the extent 
of the entoderm and mesoderm, respectively; the two lines on the right bound the 
lateral wall of the archenteric cavity; the outline of the median ‘wedge-shaped 
area’ is shown in broken line, and closely corresponds to the territory of the pre- 
chordal plate at these stages. 
Whether the tubercle of the floor in the cat is a mechanical 
expression of a moulding of the neural plate upon the “blind 
extension of the foregut” cannot be determined from the figures 
of Schulte and Tilney, but it seems hardly probable. A me- 
chanical crowding in the forward growth seems more likely to 
be responsible for its presence. 
