BY JAS. P. HILL AND C. J. MARTIN. 
53 
appearance somewhat suggests a precocious formation of the 
secondary optic cup, but whether this is so or not can only be 
settled by the examination of older stages. We are convinced, 
however, from the examination of serial transverse sections 
through the region in question, that the appearance is not caused 
by any artificial folding. Longitudinal sections of our second 
embryo also exhibit a series of appearances contradicting such an 
interpretation. 
N e ur o m ere s. — -As already mentioned four distinct pairs of 
neuromeres are present in the hind-brain and a less marked pair 
in the anterior region of the mid-brain. The mid-brain neuro- 
meres are not so distinctly marked off from the medullary plate 
as those of the hind-brain. It is, however, obvious in longitudinal 
section as a distinct local thickening of the medullary plate (fig. 
16, N.) corresponding to the region marked 1ST in the photo-micro- 
graph. 
In longitudinal section the neuromeres exhibit the character- 
istic arc-shaped form originally described by Orr* in the Lizard 
( Anolis), and afterwards by McCluref in the Chick, in Amblystoma 
and Anolis, and by Waters f in the Cod. 
The neuromeres in the hind-brain of Platypus exhibit very 
closely the characters described by Orr for those of the hind-brain 
of the Lizard. It must, however, be remembered that in the 
embryo Lizard, to which Orr's description applies, the medullary 
canal is closed, while in the Platypus embryo the medullary plate 
is still flat, so that the outer surface of the medullary canal of the 
Lizard corresponds to the undersurface of the medullary plate in 
our embryo, and his inner surface to our upper surface. 
Each neuromere as seen in longitudinal section (fig. 16, N.) is 
formed by an arc-shaped bulging on the under side of the 
medullary plate. Adjacent neuromeres are separated from each 
other by well-marked constrictions on the under side of the 
Orr, Journ. Morphology, Vol. i, 1887, PI. XII. fig. 5. 
t McClure, Journ. Morphology, Vol iv. p. 35-56. 
% Waters, Quart. Journ. Micros. Science, Vol. xxxiii. p. 457-472. 
