BY JAS. P. HILL AND C. J. MARTIN. 
57 
middle region of the mid-brain up to a short distance in front of 
the tirst somite, the notochord lies below the keel of the medullary 
plate and is connected with the entoderm hy a thin cellular 
filament (fig. 6). Somewhat anterior to the first somite the 
notochord becomes entirely free from the entoderm, and continues 
in this condition to its posterior end where it passes into the head 
process of the primitive streak. The notochord increases in size 
somewhat after becoming entirely free from the entoderm. It is 
then distinctly rod-shaped, while anteriorly it is somewhat oval in 
section. 
The notochord is relatively of very small size in Platypus. 
Heape* has also noticed a similar condition in the Mole, and he 
regards it as due to the very early appearance of the nervous 
system. 
Mesoderm. 
The mesoderm is at this stage established as two lateral wings 
distinct from the ectoderm and entoderm except at certain regions 
in the axial line, viz. :— At the anterior flattened-out end of the 
notochord, in front of the blastopore in the region of the head 
process, and behind the blastopore in the region of the primitive 
streak. As already mentioned in the description of the surface 
view of the embryo, the mesoderm lying on either side of the 
anterior end of the embryo forms two sharply limited plates to 
which we have given the name of mesodermal head plates as dis- 
tinguished from the mesoderm of the rest of the body. 
The head plates of mesoderm (PI. ix., h.p. mes.) are lateral 
thickenings on either side of the future fore- and mid-brains, and 
show no signs of segmentation. Their outer contours are directly 
continuous with the forward continuation of the outer boundaries 
of the protovertebral zones of mesoderm. Their very distinct 
posterior boundary is not due to the entire disappearance of 
mesoderm at this point, but to a very marked thinning of the 
same. 
Quart. Journ. Micros. Science, Vol. xxvii. 1887, p. 139. 
