46 
ON A PLATYPUS EMBRYO, 
passes back for a distance of approximately -17 mm. below the 
anterior end of the medullary plate and thus separates the latter 
from the underlying proamnion. This sulcus we must suppose 
has arisen as in other forms by the forward growth of the anterior 
end of the medullary plate over this bilaminar portion of the 
blastoderm. 
Lateral and tail folds are not yet developed. 
The medullary plate is still flat with the exception that along 
its mesial line a definite groove (the "Riickenfurche") is developed. 
Medullary folds are absent throughout except in the region of the 
future fore-brain. 
In the head region the three future cerebral vesicles are 
indicated by widenings of the medullary plate. The first is 
separated from the second by a well marked constriction, while a 
less marked constriction situated somewhat anterior to the 
posterior margins of the mesodermal head plates separates the 
second from the third. 
The upgrowths of the medullary folds in the lateral regions of 
the fore-brain are very apparent in the photo-micrograph as two 
dark lines (PI. ix., md.f.). The continuation of these lines across 
the front of the medullary plate is produced by the thickening and 
duplication of the medullary plate at the head fold (PL xi., 
figs. 15 and 16 cp.). 
The medullary plate in the region of the hind-brain is especially 
characterised by the presence on each side of four oblong meta- 
merically arranged opaque masses extending from the outer edges 
of the medullary plate to within a short distance of the mesial 
line. These, as will be shown later, are local thickenings of the 
medullary plate, and are therefore true neuromeres. The neuro- 
meres are arranged in bilaterally symmetrical pairs, and adjacent 
ones are separated from each other by well marked transverse 
constrictions. The first pair (PL ix., n'.) are situated entirely in 
front of the auditory plates. They are narrow and transversely 
elongated. The second pair are not so distinct, and do not extend 
quite so far mesially as the first. The third pair are very distinct 
and somewhat broader than the first. They are directed slightly 
