FUNDAMENTAL PLAN OF VERTEBRATE BRAIN 467 
series cut in the sagittal plane (for the head) at important stages 
were prepared supplementing the series already available in the 
collection of the Department of Histology and Embryology, 
Cornell University, and those of Prof. 8. H. Gage which he 
kindly placed at my disposal. In addition, series were prepared 
for and by Mr. H. B. Adelmann for his investigation of the origin 
of head mesoderm. His studies have materially aided the pres- 
ent study and his paper will be referred to subsequently and due 
acknowledgment made. c) Both of these forms have been 
previously utilized in important studies of the early development 
of the brain and, while the literature in itself afforded no com- 
plete basis for conclusions, its existence was nevertheless helpful. 
d) The two forms belong to quite remote classes of vertebrates. 
OBSERVATIONS 
1. The extent of the brain plate and the primitive optic furrow 
Squalus acanthias. In addition to the qualifications above 
mentioned, the shark possesses the important feature of a sharply 
defined brain plate. The early development of the brain of the 
shark has been investigated by a number, particularly von 
Kupffer (’06), Locy (95), Neal (98), and Johnston (09). De- 
spite this fact, there was not furnished in the literature sufficiently 
exact information to permit without repeated examination an- 
swers to the questions proposed. 
To determine the first point at issue, i.e., the extent of the 
brain plate, the location of the retinal areas and the existence of 
a primitive optic furrow (Johnston), surface views of a close 
series of shark blastoderms were examined, photographed at a 
magnification of ten diameters, and sectioned, usually in the 
sagittal plane, and a careful medial plane reconstruction prepared. 
Certain of these surface views are reproduced as figures 6 to 16. 
Middle plane reconstructions from the embryos of figures 6, 11, 
and 13 are given on plate 2 as figures 17, 19 and 20, respectively, 
while plottings of neural plate, notochord, mesoderm and ento- 
derm from embryos of figures 7 and 9 are given as text-figures 3 
and 4. 
