54 
ON A PLATYPUS EMBRYO, 
medullary plate, while on the upper side, opposite the constric- 
tions, there are slight transverse ridges. 
The elongated cells of which each neuromere is composed are 
distinct from those of its neighbours. The cells are arranged 
radially from the upper surface of the neuromere, and their nuclei 
are slightly more numerous just below that surface (fig. 18). In 
the hind-brain of the lizard, on the other hand, according to Orr, 
the nuclei are more numerous towards the outer surface of the 
neuromere, i.e., towards the undersurface in the medullary plate 
stage. 
In transverse sections the first neuromere of the hind-brain 
appears as a thickening of the medullary plate with two bulgings 
on its ventral side — a smaller one situated near the middle of each 
half of the medullary plate, and a larger one at the outer edge of 
the plate [fig. 19 (2)]. The outer bulging projects considerably 
beyond the lateral margins of the medullary plate in the inter- 
neuromeric region. 
The second neuromere [fig. 19 (4)] is less marked than the first, 
but also possesses mesial and lateral bulgings. The third 
neuromere [fig. 19 (6)] is on surface view the most distinct of the 
four. It possesses a single large bulging at its outer edge. The 
fourth neuromere [fig. 19 (8)] is the least distinct; it possesses, 
like the first and second, two enlargements of which the lateral 
one is the larger (fig. 17). Immediately behind the fourth pair of 
neuromeres of the hind-brain the medullary plate is thickened, but 
the thickenings are not limited behind by constrictions, and for 
the present we leave it an open question whether these are to be 
regarded as a fifth pair of neuromeres or not. 
Neuromeres in the fore-brain were not observed. All that we 
can definitely say at present, then, is that in the head region of 
the Platypus embryo of this stage a single pair of neuromeres exist 
in the mid-brain and four distinct pairs in the hind-brain. As 
Locy* has observed in Squalus acanthias, and Ambly stoma, so in 
Platypus the neuromeric segmentation appears very early, indeed 
* Anat. Anz. ix. Bel. p. 393-415. 
