54 ON A PLATYPUS EMBRYO, 



medullary plate, while on the u^Dper 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 

 Ijeyond 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 1)ulging 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, l)ut 

 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 Ambiy stoma, so in 

 Platypus the neuromeric segmentation appears very early, indeed 



* Anat. Anz. ix. Bd. p. 393-415. 



