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 (PL 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 especialty 

 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., ?/.'.) 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 



