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 b}^ 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 eur m ere s. — As already mentioned four distinct pairs of 

 neuromeres are present in the hind-brain and a less marked paii- 

 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, hov/ever, obvious in longitudinal 

 section as a distinct local thickening of the medullary plate (fig. 

 16, N.) corresponding to the region marked N 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 Ambiystoma 

 and Anolis, and by Watersf 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, l^e remembered that in the 

 embryo Lizard, to which Orr's description applies, the medullary 

 €anal 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 emVnyo, 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 w^ell-marked constrictions on the under side of the 



* Orr, Journ. Morphology, Vol. i, 1887, PI. xii. fi^. 5. 



t McClure, Journ. Morphology, Vol iv. p. 35-56. 



X Waters. Quart. Journ. Micros. Science, Vol. xxxiii. p. 4S1AT2,. 



