BY JAS. P. HILL AND C. J. MARTIN. 
47 
backwards. The fourth pair are much less distinct than the 
others, and in surface view are not so sharply limited from the 
medullary plate. 
In the anterior region of the mid-brain, a pair of neuromeres 
(PI. ix., N.) occur, but they are much less distinct than those of the 
hind-brain, appearing merely as local thickenings of the medullary 
plate in that region, without any accompanying constrictions. 
Opposite the second and third neuromeres of the hind-brain 
there is on each side a somewhat triangular thickened patch of 
ectoderm — the auditory plate (PI. ix., and.). The mesoderm in 
the region of the auditory plates is very thin, hence they stand 
out very distinctly. The anterior margin of each plate is on a 
level with the front edge of the second neuromere; indeed the 
greater portion of the plate is situated opposite this neuromere. 
Each plate is roughly triangular in shape, with the base of the 
triangle adjoining the medullary plate, and with a deep bay in 
the middle of its posterior margin. 
The hind-brain region narrows gradually posteriorly and passes 
into the medullary plate of the future spinal cord. The medullary 
plate widens out towards the posterior end of the embryo into a 
well marked sinus rhomboidalis which completely surrounds the 
primitive streak. The primitive streak is just visible in the photo- 
micrograph (PI. ix., pr. s.) as a faint linear thickening enclosing 
a whitish axial line— the primitive groove — at the posterior 
end of the embryo. The anterior end of the primitive streak 
exhibits a distinct thickening, to one side of which the blastopore 
(PI. ix., bl.) is situated. This thickening is continued forwards 
as the head process of the mesoderm which passes into the hinder 
end of the notochord. The notochord is very distinct in the 
photo-micrograph as the longitudinal line running along the 
middle of the medullary plate. At its anterior end it broadens 
out, and terminates about the middle of the future fore-brain. 
The bilateral Anlagen of the heart are very noticeable in surface 
view (PL ix., h.a.) as two tubular-looking structures lying external 
to the auditory plates, and extending backwards from them along 
the outer edges of the forward extensions of the lateral zones of 
